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325
OR
GA
NIC
CH
EM
IST
RYEach year millions of dollars are lost because of crop damage by pests.
OutlineCHEMISTRY CONNECTION:
A Cautionary Tale: DDT andBiological Magnification
12.1 Alkenes and Alkynes:Structure and PhysicalProperties
12.2 Alkenes and Alkynes:Nomenclature
A MEDICAL PERSPECTIVE:Killer Alkynes in Nature
12.3 Geometric Isomers: AConsequence ofUnsaturation
12.4 Alkenes in Nature12.5 Reactions Involving
AlkenesHydrogenation: Addition
of H2 to an AlkeneHalogenation: Addition of
X2 to an AlkeneA HUMAN PERSPECTIVE:
Folklore, Science, andTechnology
Hydration: Addition ofH2O to an Alkene
Hydrohalogenation:Addition of HX to anAlkene
Addition Polymers ofAlkenes
A HUMAN PERSPECTIVE:Life without Polymers?
AN ENVIRONMENTAL PERSPECTIVE:Plastic Recycling
12.6 Aromatic HydrocarbonsStructure and PropertiesNomenclatureReactions Involving
BenzeneA HUMAN PERSPECTIVE:
Aromatic Compounds andCarcinogenesis
12.7 Heterocyclic AromaticCompounds
Summary of ReactionsSummaryKey TermsQuestions and ProblemsCritical Thinking Problems
12The UnsaturatedHydrocarbons:Alkenes, Alkynes, andAromatics
Learning Goals
1 Describe the physical properties of alkenes andalkynes.
2 Draw the structures and write the I.U.P.A.C.names for simple alkenes and alkynes.
3 Write the names and draw the structures ofsimple geometric isomers of alkenes.
4 Write equations predicting the products ofaddition reactions of alkenes: hydrogenation,halogenation, hydration, andhydrohalogenation.
5 Apply Markovnikov’s rule to predict the majorand minor products of the hydration andhydrohalogenation reactions of unsymmetricalalkenes.
6 Write equations representing the formation ofaddition polymers of alkenes.
7 Draw the structures and write the names ofcommon aromatic hydrocarbons.
8 Write equations for substitution reactionsinvolving benzene.
9 Describe heterocyclic aromatic compoundsand list several biological molecules in whichthey are found.
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Unsaturated hydrocarbons are those that contain at least one carbon-carbondouble or triple bond. They include the alkenes, alkynes, and aromatic com-pounds. All alkenes have at least one carbon-carbon double bond; all alkynes have
326 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-2
We have heard the warnings for years: Stop using non-biodegradable insecticides because they are killing many ani-mals other than their intended victims! Are these chemicals notspecifically targeted to poison insects? How then can they beconsidered a threat to humans and other animals?
DDT, a polyhalogenated hydrocarbon, was discovered inthe early 1940s by Paul Müller, a Swiss chemist. Müller showedthat DDT is a nerve poison that causes convulsions, paralysis,and eventually death in insects. From the 1940s until 1972,when it was banned in the United States, DDT was sprayed oncrops to kill insect pests, sprayed on people as a delousingagent, and sprayed in and on homes to destroy mosquitoes car-rying malaria. At first, DDT appeared to be a miraculous chem-ical, saving literally millions of lives and millions of dollars incrops. However, as time went by, more and more evidence of adark side of DDT use accumulated. Over time, the chemicalhad to be sprayed in greater and greater doses as the insectpopulations evolved to become more and more resistant to it.In 1962, Rachel Carson published her classic work, SilentSpring, which revealed that DDT was accumulating in the envi-ronment. In particular, high levels of DDT in birds interferedwith their calcium metabolism. As a result, the egg shells pro-duced by the birds were too thin to support development of thechick within. It was feared that in spring, when the air shouldhave been filled with bird song, there would be silence. This isthe “silent spring” referred to in the title of Carson’s book.
DDT is not biodegradable; furthermore, it is not water-soluble, but it is soluble in nonpolar solvents. Thus if DDT isingested by an animal, it will dissolve in fat tissue and accu-mulate there, rather than being excreted in the urine. WhenDDT is introduced into the food chain, which is inevitablewhen it is sprayed over vast areas of the country, the result is bi-ological magnification. This stepwise process begins when DDTapplied to crops is ingested by insects. The insects, in turn, areeaten by birds, and the birds are eaten by a hawk. We can imag-ine another food chain: Perhaps the insects are eaten by mice,which are in turn eaten by a fox, which is then eaten by an owl.Or to make it more personal, perhaps the grass is eaten by asteer, which then becomes your dinner. With each step up oneof these food chains, the concentration of DDT in the tissues be-comes higher and higher because it is not degraded, it is simplystored. Eventually, the concentration may reach toxic levels insome of the animals in the food chain.
Consider for a moment the series of events that occurred inBorneo in 1955. The World Health Organization elected to
spray DDT in Borneo because 90% of the inhabitants were in-fected with malaria. As a result of massive spraying, the mos-quitoes bearing the malaria parasite were killed. If this soundslike the proverbial happy ending, read on. This is just the be-ginning of the story. In addition to the mosquitoes, millions ofother household insects were killed. In tropical areas it is com-mon for small lizards to live in homes, eating insects foundthere. The lizards ate the dead and dying DDT-contaminatedinsects and were killed by the neurotoxic effects of DDT. Thehouse cats ate the lizards, and they, too, died. The number ofrats increased dramatically because there were no cats to con-trol the population. The rats and their fleas carried sylvaticplague, a form of bubonic plague. With more rats in contactwith humans came the threat of a bubonic plague epidemic.Happily, cats were parachuted into the affected areas of Borneo,and the epidemic was avoided.
The story has one further twist. Many of the islanders livedin homes with thatched roofs. The vegetation used to makethese roofs was the preferred food source for a caterpillar thatwas not affected by DDT. Normally, the wasp populationpreyed on these caterpillars and kept the population undercontrol. Unfortunately, the wasps were killed by the DDT. Thecaterpillars prospered, devouring the thatched roofs, which col-lapsed on the inhabitants.
Every good story has a moral, and this one is not difficult todecipher. The introduction of large amounts of any chemicalinto the environment, even to eradicate disease, has the poten-tial for long-term and far-reaching effects that may be very dif-ficult to predict. We must be cautious with our fragileenvironment. Our well-intentioned intervention all too oftenupsets the critical balance of nature, and in the end we inad-vertently do more harm than good.
A Cautionary Tale: DDT and Biological Magnification
Introduction
OCOOCOCl
ClA
ClOA
ClA
HA
ClA
DDT: Dichlorodiphenyltrichloroethane
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at least one carbon-carbon triple bond. Aromatic compounds are particularly sta-ble cyclic compounds and sometimes are depicted as having alternating single anddouble carbon-carbon bonds. This arrangement of alternating single and doublebonds is called a conjugated system of double bonds.
Many important biological molecules are characterized by the presence ofdouble bonds or a linear or cyclic conjugated system of double bonds (Figure 12.1).For instance, we classify fatty acids as either monounsaturated (having one doublebond), polyunsaturated (having two or more double bonds), or saturated (havingsingle bonds only). Vitamin A (retinol), a vitamin required for vision, contains aten-carbon conjugated hydrocarbon chain. Vitamin K, a vitamin required for bloodclotting, contains an aromatic ring.
12.1 Alkenes and Alkynes: Structure andPhysical Properties
Alkenes and alkynes are unsaturated hydrocarbons. The characteristic functionalgroup of an alkene is the carbon-carbon double bond. The functional group that
12.1 Alkenes and Alkynes: Structure and Physical Properties 327
12-3
Fatty acids are long hydrocarbon chainshaving a carboxyl group at the end. Thusby definition they are carboxylic acids. SeeChapters 15 and 18.
These vitamins are discussed in detail inAppendix E, Lipid-Soluble Vitamins.
Figure 12.1(a) Structural formula of the sixteen-carbon monounsaturated fatty acidpalmitoleic acid. (b) Condensed formulaof vitamin A, which is required for vision.Notice that the carbon chain of vitamin Ais a conjugated system of double bonds.(c) Line formula of vitamin A. In the lineformula, each line represents a carbon-carbon bond, each double line representsa carbon-carbon double bond. A carbonatom and the appropriate number ofhydrogen atoms are assumed to be atthe point where two lines meet. Thevertical lines are assumed to terminate ina methyl group. (d) Line formula ofvitamin K, a lipid-soluble vitamin requiredfor blood clotting. The six-member ringwith the circle represents a benzene ring.See Figure 12.6 for other representationsof the benzene ring.
HOCAH
AH
OCAH
AH
OCAH
AH
OCAH
AH
OCAH
AH
OCAH
AH C
DH
PCG
H
CAH
AH
OCAH
AH
OCAH
AH
OCAH
AH
OCAH
AH
OCAH
AH
OCA
OAH
OC
CH3
Palmitoleic acid
Vitamin K
OCH PACH3
P
O
CC
C
CH2
CH2
OHCO
O
H3C
CH3
O
CH3 CH3 CH3
CH3
H2C
H2C CHP
OH
OCACH3
PCHOCH
CH3
OCH
Vitamin A
H3C CH3
CH3
CH3
CH
Vitamin A
(a)
(b)
(c)
(d)
BO
O
H
H
O
Learning Goal
1
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characterizes the alkynes is the carbon-carbon triple bond. The following generalformulas compare the structures of alkanes, alkenes, and alkynes.
General formulas: Alkane Alkene AlkyneCnH2n�2 CnH2n CnH2n�2
Structural formulas:
Molecular formulas: C2H6 C2H4 C2H2
Condensed formulas: CH3CH3 H2CPCH2 HCqCH
These compounds have the same number of carbon atoms but differ in thenumber of hydrogen atoms, a feature of all alkanes, alkenes, and alkynes that con-tain the same number of carbon atoms. Alkenes contain two fewer hydrogens thanthe corresponding alkanes, and alkynes contain two fewer hydrogens than the cor-responding alkenes.
In alkanes the four bonds to the central carbon have tetrahedral geometry.When carbon is bonded by one double bond and two single bonds, as in ethene(an alkene), the molecule is planar, because all atoms lie in a single plane. Eachbond angle is approximately 120�. When two carbon atoms are bonded by a triplebond, as in ethyne (an alkyne), each bond angle is 180�. Thus, the molecule is lin-ear, and all atoms are positioned in a straight line (Figure 12.2).
The physical properties of alkenes, alkynes, and aromatic compounds are verysimilar to those of alkanes. They are nonpolar. As a result of the “like dissolves like”rule, they are not soluble in water but are very soluble in nonpolar solvents such asother hydrocarbons. They also have relatively low boiling points and melting points.
12.2 Alkenes and Alkynes: NomenclatureTo determine the name of an alkene or alkyne using the I.U.P.A.C. NomenclatureSystem, use the following simple rules:
• Name the parent compound using the longest continuous carbon chaincontaining the double bond (alkenes) or triple bond (alkynes).
• Replace the -ane ending of the alkane with the -ene ending for an alkene orthe -yne ending for an alkyne. For example:
CH3—CH3 CH2PCH2 CHqCH
Ethane Ethene Ethyne
CH3—CH2—CH3 CH2PCH—CH3 CHqC—CH3
Propane Propene Propyne
• Number the chain to give the lowest number for the first of the two carbonscontaining the double bond or triple bond. For example:
CH3CH2CHPCH2
4 3 2 1
1-Butene(not 3-butene)
CHqCOCH2CH2CH3
1 2 3 4 5
1-Pentyne(not 4-pentyne)
HOCOCOH
HA
Ethane(ethane)
HA
HA
HA
CPCH
HH
HG
G D
D
Ethene(ethylene)
HOCqCOH
Ethyne(acetylene)
328 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-4
Learning Goal
2
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• Name and number all groups bonded to the parent alkene or alkyne, andplace the name and number in front of the name of the parent compound.Remember that with alkenes and alkynes the double or triple bond takesprecedence over a halogen or alkyl group, as shown in the followingexamples:
CH3OCHPCOCH3
4 3 2 1
2-Chloro-2-butene
CH3CHOCqCOCH2CH3
1 2 3 4 5 6
ClA
2-Bromo-3-hexyne
BrA
12.2 Alkenes and Alkynes: Nomenclature 329
12-5
i f
f iH
H
C
Ethene Ethyne
C
Planar
H
Allbondanglesapproximately120�
Hi&@
iH
H
$CH H
HCC
Ethane
Tetrahedral
Allbondanglesapproximately109.5�
CC
Linear
Bondangles180�
HOOH
(a)
H
C&@
HH
GCC
C%∆
(¨f
{&
HH
HH
HH
H H
C%∆
H H
A long-chain alkane(pentane)
D
H
i C
DC
H C
H
CC
A long-chain alkene(1-pentene)
DHG
H H%C
( ¨H H( ¨
H H
C
H H$C
( ¨H HC
COH
A long-chain alkyne(1-pentyne)
C C
H H$C
Hi
(b)
(b)
Figure 12.2(a)Three-dimensional drawings and ball-and-stick models of ethane, ethene, andethyne. (b) Examples of typical long-chain hydrocarbons.
Remember, it is the position of thedouble bond, not the substituent, thatdetermines the numbering of thecarbon chain.
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330 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-6
Killer Alkynes in Nature
There are many examples of alkynes that are beneficial to hu-mans. Among these are parasalamide, a pain reliever, pargyline,
an antihypertensive, and 17-ethynylestradiol, a synthetic estro-gen that is used as an oral contraceptive.
17-Ethynylestradiol
O
CHC CH
3
OH
HO
ParagylineParasalamide
NH
C N CH H
2
2CH2CH3
O CH2C CH
O
C NCH2 CCH2
CH3
CH
Alkynes used for medicinal purposes.
But in addition to these medically useful alkynes, there arein nature a number that are toxic. Some are extremely toxic tomammals, including humans; others are toxic to fungi, fish, orinsects. All of these compounds are plant products that mayhelp protect the plant from destruction by predators.
Capillin is produced by the oriental wormwood plant. Re-search has shown that a dilute solution of capillin inhibits thegrowth of certain fungi. Since fungal growth can damage ordestroy a plant, the ability to make capillin may provide asurvival advantage to the plants. Perhaps it may one day bedeveloped to combat fungal infections in humans.
Ichthyotherol is a fast-acting poison commonly found inplants referred to as fish-poison plants. Ichthyotherol is a very
toxic polyacetylenic alcohol that inhibits energy production inthe mitochondria. Latin American native tribes use these plantsto coat the tips of the arrows used to catch fish. Althoughichthyotherol is poisonous to the fish, fish caught by thismethod are quite safe for human consumption.
An extract of the leaves of English ivy has been reported tohave antibacterial, analgesic, and sedative effects. The com-pound thought to be responsible for these characteristics, aswell as antifungal activity, is falcarinol. Falcarinol, isolated froma tree in Panama, also has been reported by the Molecular Tar-gets Drug Discovery Program, to have antitumor activity. Per-haps one day this compound, or a derivative of it, will be usefulin treating cancer in humans.
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12.2 Alkenes and Alkynes: Nomenclature 331
12-7
Cicutoxin has been described as the most lethal toxin nativeto North America. It is a neurotoxin that is produced by the wa-ter hemlock (Cicuta maculata), which is in the same family ofplants as parsley, celery, and carrots. Cicutoxin is present in allparts of the plants, but is most concentrated in the root. Eatinga portion as small as 2–3 cm2 can be fatal to adults. Cicutoxinacts directly on the nervous system. Signs and symptoms of ci-cutoxin poisoning include dilation of pupils, muscle twitching,rapid pulse and breathing, violent convulsions, coma, anddeath. Onset of symptoms is rapid and death may occur withintwo to three hours. No antidote exists for cicutoxin poisoning.The only treatment involves controlling convulsions andseizures in order to preserve normal heart and lung function.Fortunately, cicutoxin poisoning is a very rare occurrence. Oc-casionally animals may graze on the plants in the spring, re-sulting in death within fifteen minutes. Humans seldom comeinto contact with the water hemlock. The most recent cases
have involved individuals foraging for wild ginseng, or otherwild roots, and mistaking the water hemlock root for an edibleplant.
O
IchthyotherolCapillin
C C CH3C C C C C C CCH3 C C CH CH
OH
H CCH2 C C C CCH C
HO
HH2 ( )CH2 CH37CH C
Falcarinol
CH2 CH OH2 CH3 CH2
HCH C HCH C HCHCH C
HO
Cicutoxin
CH2
C C C CCH2
CH2
Alkynes that exhibit toxic activity.
Cicuta maculata, or water hemlock, produces the most deadly toxinindigenous to North America.
den69056_ch12.qxd 12/27/02 9:33 AM Page 331
EXAMPLE 12.2
EXAMPLE 12.1
• Alkenes having more than one double bond are called alkadienes (twodouble bonds) or alkatrienes (three double bonds), as seen in these examples:
CH3CHPCH—CHPCHCH3 CH2PCHCH2CHPCH2
2,4-Hexadiene 1,4-Pentadiene 3-Methyl-1,4-cyclohexadiene
Naming Alkenes and Alkynes Using the I.U.P.A.C. Nomenclature System
Name the following alkene and alkyne using I.U.P.A.C. nomenclature.
Solution
Longest chain containing the doublebond: octene
Position of double bond: 3-octene(not 5-octene)
Substituents: 3-methyl and 4-propyl
Name: 3-Methyl-4-propyl-3-octene
Longest chain containing the triplebond: hexyne
Position of triple bond: 3-hexyne(must be!)
Substituents: 2,2-dimethyl
Name: 2,2-Dimethyl-3-hexyne
Naming Cycloalkenes Using I.U.P.A.C. Nomenclature
Name the following cycloalkenes using I.U.P.A.C. nomenclature.
Solution
Parent chain: cyclohexene
Position of double bond: carbon-1 (carbons of thedouble bond are numbered 1 and 2)
Substituents: 4-chloro
Name: 4-Chlorocyclohexene
Parent chain: cyclopentene
Position of double bond: carbon-1
Substituent: 3-methyl
Name: 3-Methylcyclopentene
1 2
34
5
P
C
HA
HA
C
HA
HA
C
HA
CH3A
CCD
H HG
C
HA
HA
C
ClA
HA
C
HA
HA
C
HA
HA
P CCD
HHG1 2
3
45
6
ACH3
332 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-8
Alkenes with many double bonds areoften referred to as polyenes (poly—many enes—double bonds).
CH3CH2CH2CH2
8 7 6 5
CPC
CH3CH2CH2
CH2CH3
G D
GD
CH3
2 1
4 3
CH3CH2OCqCOCOCH3
6 5 4 3 2 1
CH3
A
CH3A
Learning Goal
2
Learning Goal
2
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Draw a complete structural formula for each of the following compounds:
a. 1-Bromo-3-hexyneb. 2-Butynec. Dichloroethyned. 9-Iodo-1-nonyne
Name the following compounds using the I.U.P.A.C. Nomenclature System:
a. CH3—CqC—CH2CH3
b. CH3CH2CHCHCH2CqCH| |Br Br
c.
Br CH3| |
CH3CH—CPC—CHCH3| |CH3 CH3
d.
CH2CH3|
CH3CH—CqC—CHCH3|Br
12.3 Geometric Isomers: A Consequence ofUnsaturation
The carbon-carbon double bond is rigid because of the shapes of the orbitals in-volved in its formation. The electrons of one of the two carbon-carbon bonds lie ina line between the two nuclei. This is called a sigma (�) bond. The second bond isformed between two p orbital electrons and is called a pi (�) bond. The two elec-trons of the � bond lie in the region above and below the two carbon atoms asshown in the following diagram.
Rotation around the double bond is restricted because the � bond would have tobe broken to allow rotation. Thus, the double bond is rigid.
In Section 11.3, we observed that the rotation around the carbon-carbon bondsof cycloalkanes was restricted. The consequence of the absence of free rotation wasthe formation of geometric or cis-trans isomers. The cis isomers of cycloalkanes hadsubstituent groups on the same side of the ring (L., cis, “on the same side”). Thetrans isomers of cycloalkanes had substituent groups located on opposite sides ofthe ring (L., trans, “across from”).
H
H
H
H
C C
π bond
σ bond
π bond
Q u e s t i o n 12.2
Q u e s t i o n 12.1
12.3 Geometric Isomers: A Consequence of Unsaturation 333
12-9
Restricted rotation around doublebonds is partially responsible for theconformation and hence the activity ofmany biological molecules that we willstudy later.
The electron charge cloud associatedwith the two electrons making up the �bond (in red) is concentrated betweenthe two nuclei. The electron chargecloud associated with the two electronsof the � bond (in blue) is concentratedin two regions above and below the �bond framework of the molecule.
Learning Goal
3
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EXAMPLE 12.4
EXAMPLE 12.3
In the alkenes, geometric isomers differ from one another by the location ofgroups on the same or opposite sides of the double bond. Because the double bondof the alkenes is also rigid and there is no free rotation around it, geometric iso-mers are formed when there are two different groups on each of the carbon atomsattached by the double bond. If both groups are on the same side of the doublebond, the molecule is a cis isomer. If the groups are on opposite sides of the doublebond, the molecule is a trans isomer.
Consider the two isomers of 1,2-dichloroethene:
If one of the two carbon atoms of the double bond has two identical sub-stituents, there are no cis-trans isomers for that molecule. Consider the example of1,1-dichloroethene:
Identifying cis and trans Isomers of Alkenes
Two isomers of 2-butene are shown below. Which is the cis isomer andwhich is the trans isomer?
Solution
As we saw with cycloalkanes, the prefixes cis and trans refer to theplacement of the substituents attached to a bond that cannot undergo freerotation. In the case of alkenes, it is the groups attached to the carbon-carbon double bond (in this example, the H and CH3 groups). When thegroups are on the same side of the double bond, as in the structure on theleft, the prefix cis is used. When the groups are on the opposite sides of thedouble bond, as in the structure on the right, trans is the appropriate prefix.
Naming cis and trans Compounds
Name the following geometric isomers.
HCPC
H3C
HG D
GD
CH3
HCPC
H3C
CH3G D
GD
H
cis-2-Butene trans-2-Butene
HCPC
H3C
HG D
GD
CH3
HCPC
H3C
CH3G D
GD
H
HCPC
H
ClG D
GD
Cl
1, 1-Dichloroethene
HCPC
Cl
HG D
GD
Cl
cis-1, 2-Dichloroethene
HCPC
Cl
ClG D
GD
H
trans-1, 2-Dichloroethene
334 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-10
Continued—
Learning Goal
3
Learning Goal
3
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EXAMPLE 12.5
Solution
The longest chain of carbon atoms in each of the following molecules ishighlighted in yellow. The chain must also contain the carbon-carbon doublebond. The location of functional groups relative to the double bond is usedin determining the appropriate prefix, cis or trans, to be used in namingeach of the molecules.
Parent chain: heptene
Position of double bond: 3-
Configuration: trans
Substituents: 3,4-dichloro
Name: trans-3,4-Dichloro-3-heptene
Parent chain: octene
Position of double bond: 3-
Configuration: cis
Substituents: 3,4-dimethyl
Name: cis-3,4-Dimethyl-3-octene
In each of the following pairs of molecules, identify the cis isomer and the transisomer.
a.
b.
Provide the complete I.U.P.A.C. name for each of the compounds in Question 12.3.
Identifying Geometric Isomers
Determine whether each of the following molecules can exist as cis-transisomers: (1) 1-pentene, (b) 3-ethyl-3-hexene, and (c) 3-methyl-2-pentene.
Solution
a. Examine the structure of 1-pentene,
HCPC
H
CH2CH2CH3G D
GD
H
Q u e s t i o n 12.4
BrCPC
H3C
CH3G D
GD
Br
BrCPC
H3C
BrG D
GD
CH3
HCPC
CH3CH2
HG D
GD
CH2CH3
HCPC
CH3CH2
CH2CH3G D
GD
H
Q u e s t i o n 12.3
CH3CH2
CPCCH3
CH2CH2CH2CH3G D
GD
CH3
Cl
CPC
CH3CH2CH2
CH2CH3G D
GD
Cl
12.3 Geometric Isomers: A Consequence of Unsaturation 335
12-11
EXAMPLE 12.4 —Continued
Continued—
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We see that carbon-1 is bonded to two hydrogen atoms, rather than to twodifferent substituents. In this case there can be no cis-trans isomers.
b. Examine the structure of 3-ethyl-3-hexene:
We see that one of the carbons of the carbon-carbon double bond is bondedto two ethyl groups. As in example (a), because this carbon is bonded totwo identical groups, there can be no cis or trans isomers of this compound.
c. Finally, examination of the structure of 3-methyl-2-pentene reveals thatboth a cis and trans isomer can be drawn.
Each of the carbon atoms involved in the double bond is attached to twodifferent groups. As a result, we can determine which is the cis isomer andwhich is the trans isomer based on the positions of the methyl groupsrelative to the double bond.
Draw condensed formulas for each of the following compounds:
a. cis-3-Octeneb. trans-5-Chloro-2-hexenec. trans-2,3-Dichloro-2-butene
Name each of the following compounds, using the I.U.P.A.C. system. Be sure toindicate cis or trans where applicable.
a. c.
b.
12.4 Alkenes in NatureFolklore tells us that placing a ripe banana among green tomatoes will speed upthe ripening process. In fact, this phenomenon has been demonstrated experimen-tally. The key to the reaction is ethene, the simplest alkene. Ethene, produced byripening fruit, is a plant growth substance. It is produced in the greatest abun-dance in areas of the plant where cell division is occurring. It is produced during
CH3CH2
CPC
CH3
CH2CH3G D
GD
H
CH3
CPC
H
HG D
GD
CH2CCH3
CH3A
CH3
A
CH3
CPC
H
CH3G D
GD
CH3
Q u e s t i o n 12.6
Q u e s t i o n 12.5
H3CCPC
CH3CH2
CH3G D
GD
H
H3CCPC
CH3CH2
HG D
GD
CH3
CH3CH2
CPC
CH3CH2
CH2CH3G D
GD
H
336 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-12
EXAMPLE 12.5 —Continued
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fruit ripening, during leaf fall and flower senescence, as well as under conditionsof stress, including wounding, heat, cold, or water stress, and disease.
There are a surprising number of polyenes, alkenes with several double bonds,found in nature. These molecules, which have wildly different properties andfunctions, are built from one or more five-carbon units called isoprene.
CH3|
CH2PC—CHPCH2
Isoprene
The molecules that are produced are called isoprenoids, or terpenes (Figure 12.3).Terpenes include the steroids; chlorophyll and carotenoid pigments that functionin photosynthesis, and the lipid soluble vitamins A, D, and K (Figure 12.1).
Many other terpenes are plant products familiar to us because of their distinc-tive aromas. Geraniol, the familiar scent of geraniums, is a molecule made up oftwo isoprene units. Purified from plant sources, geraniol is the active ingredient inseveral natural insect repellants. These can be applied directly to the skin to pro-vide four hours of protection against a variety of insects, including mosquitoes,ticks, and fire ants.
D-Limonene is the most abundant component of the oil extracted from the rindof citrus fruits. Because of its pleasing orange aroma, D-limonene is used as a flavorand fragrance additive in foods. However, the most rapidly expanding use of thecompound is as a solvent. In this role, D-limonene can be used in place of more toxicsolvents, such as mineral spirits, methyl ethyl ketone, acetone, toluene, and fluori-nated and chlorinated organic solvents. It can also be formulated as a water-basedcleaning product, such as Orange Glo, that can be used in place of more causticcleaning solutions. There is a form of limonene that is a molecular mirror image ofD-limonene. It is called L-limonene and has a pine or turpentine aroma.
The terpene myrcene is found in bayberry. It is used in perfumes and scentedcandles because it adds a refreshing, spicy aroma to them. Trace amounts ofmyrcene may be used as a flavor component in root beer.
Farnesol is a terpene found in roses, orange blossom, wild cyclamen, and lily ofthe valley. Cosmetics companies began to use farnesol in skin care products in theearly 1990s. It is claimed that farnesol smoothes wrinkles and increases skin elas-ticity. It is also thought to reduce skin aging by promoting regeneration of cells andactivation of the synthesis of molecules, such as collagen, that are required forhealthy skin.
Another terpene, retinol, is a form of vitamin A (Figure 12.1). It is able to pene-trate the outer layers of skin and stimulate the formation of collagen and elastin.This reduces wrinkles by creating skin that is firmer and smoother.
12.5 Reactions Involving AlkenesReactions of alkenes involve the carbon-carbon double bond. The key reaction ofthe double bond is the addition reaction. This involves the addition of two atomsor groups of atoms to a double bond. The major alkene addition reactions includeaddition of hydrogen (H2), halogens (Cl2 or Br2), water (HOH), or hydrogenhalides (HBr or HCl). A generalized addition reaction is:
R RG D
ROCOA
RA
A
CBC GD
R R
�AAB
RA
ROCOB
12.5 Reactions Involving Alkenes 337
12-13
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338 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-14
Figure 12.3Many plant products, familiar to usbecause of their distinctive aromas, areisoprenoids, which are alkenes havingseveral double bonds.
C
Geraniol(Rose and geraniums)
CHCH2
CH2
CH OH2
CH3
CH
CH3
C
CH3
C
Limonene(Oil of lemon and orange)
CH
C
CH2
CH2 H2
CH3
CH
CH2
C
CH3
Myrcene(Oil of bayberry)
C
CH
C
CH2
CH2 H2
CH2
CH
CH3
C
CH3
C
Farnesol(Lily of the valley)
CHCH2
CH2
C CH2
CH
H OH2
CH3
C
CH3
CH
CH3
C
CH3
CH2
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EXAMPLE 12.6
Note that the double bond is replaced by a single bond. The former double bondcarbons receive a new single bond to a new atom, producing either an alkane or asubstituted alkane. This involves breaking the � bond between the two carbons ofthe double bond and forming a new � bond to each of these carbons.
Hydrogenation: Addition of H2 to an AlkeneHydrogenation is the addition of a molecule of hydrogen (H2) to a carbon-carbondouble bond to give an alkane. In this reaction the double bond is broken, and twonew C—H single bonds result. Platinum, palladium, or nickel is needed as a cata-lyst to speed up the reaction. Heat and/or pressure may also be required.
Writing Equations for the Hydrogenation of Alkenes
Write a balanced equation showing the hydrogenation of (a) 1-pentene and(b) trans-2-pentene.
Solution
(a) Begin by drawing the structure of 1-pentene and of diatomic hydrogen(H2) and indicating the catalyst.
Knowing that one hydrogen atom will form a covalent bond with each ofthe carbon atoms of the carbon-carbon double bond, we can write theproduct and complete the equation.
(b) Begin by drawing the structure of trans-2-pentene and of diatomichydrogen (H2) and indicating the catalyst.
CPC
CH3CH2
CH3H
HG
G D
D
trans-2-Pentene
HOH
Hydrogen
�Ni
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOH
Hydrogen
�Ni
CH3CH2CH2OCOCOH
HA
HA
HA
HA
Pentane
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOH
Hydrogen
�Ni
R RG D
ROCOH
RA
A
CBC GD
R R
�HAH
RA
ROCOH Pt, Pd, or NiHeat or pressure
Alkene Hydrogen Alkane
12.5 Reactions Involving Alkenes 339
12-15
Recall that a catalyst itself undergoes nonet change in the course of a chemicalreaction (see Section 8.3).
Remember that the R in these generalformulas represents an alkyl group.
Note that the alkene is gaining twohydrogens. Thus, hydrogenation is areduction reaction (see Sections 9.5and 13.6).
Continued—
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Knowing that one hydrogen atom will form a covalent bond with each ofthe carbon atoms of the carbon-carbon double bond, we can write theproduct and complete the equation.
The trans isomer of 2-pentene was used in Example 12.6. Would the result be anydifferent if the cis isomer had been used?
Write balanced equations for the hydrogenation of 1-butene and cis-2-butene.
Hydrogenation is used in the food industry to produce margarine, which is amixture of hydrogenated vegetable oils (Figure 12.4). Vegetable oils are unsatu-rated, that is, they contain many double bonds and as a result have low meltingpoints and are liquid at room temperature. The hydrogenation of these doublebonds to single bonds increases the melting point of these oils and results in a fat,such as Crisco, that remains solid at room temperature. Through further processingthey may be converted to margarine, such as corn oil or sunflower oil margarines.
Halogenation: Addition of X2 to an AlkeneChlorine (Cl2) or bromine (Br2) can be added to a double bond. This reaction, calledhalogenation, proceeds readily and does not require a catalyst:
R RG D
ROCOX
RA
A
CBC GD
R R
�XAX
RA
ROCOX
Alkene Halogen Alkyl dihalide
Q u e s t i o n 12.8
Q u e s t i o n 12.7
CPC
CH3CH2
CH3H
HG
G D
D
trans-2-Pentene
HOH
Hydrogen
�Ni
CH3CH2OCOCOCH3
HA
HA
HA
HA
Pentane
340 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-16
EXAMPLE 12.6 —Continued
Saturated and unsaturated dietary fats arediscussed in Section 18.2.
H2, 200�C,25 psi,metal catalyst
CHPCH
CHPCH
CHP CH
(CH2)7 CH2CH3
O
COB
OO O(CH2)7O
(CH2)7 CH2CH3
O
COB
OO O(CH2)7O
(CH2)7CH3
O
COB
OO O(CH2)7O
An oil
(CH2)7CH2CH3 OCOB
OO O(CH2)7O
(CH2)7CH2CH3 OCOB
OO O(CH2)7O
(CH2)7CH2CH3 OCOB
OO O(CH2)7O
A fat
CH2
CH2
CH2O
CH2O
CH2O
O
O
O
O
O
O
O
O
O
O
O
O
CH CH
Figure 12.4The conversion of a typical oil to a fatinvolves hydrogenation. In this example,triolein (an oil) is converted to tristearin(a fat).
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12.5 Reactions Involving Alkenes 341
12-17
EXAMPLE 12.7Writing Equations for the Halogenation of Alkenes
Write a balanced equation showing (a) the chlorination of 1-pentene and(b) the bromination of trans-2-butene.
Solution
(a) Begin by drawing the structure of 1-pentene and of diatomic chlorine (Cl2).
Knowing that one chlorine atom will form a covalent bond with each of thecarbon atoms of the carbon-carbon double bond, we can write the productand complete the equation.
(b) Begin by drawing the structure of trans-2-butene and of diatomicbromine (Br2).
Knowing that one bromine atom will form a covalent bond with each of thecarbon atoms of the carbon-carbon double bond, we can write the productand complete the equation.
Below we see an equation representing the bromination of 1-pentene. Notice thatthe solution of reactants is red because of the presence of bromine. However, theproduct is colorless (Figure 12.5).
This bromination reaction can be used to show the presence of double bondsin an organic compound. The reaction mixture is red because of the presence ofdissolved bromine. If the red color is lost, the bromine has been consumed. Thusbromination has occurred, and the compound must have had a carbon-carbon
1-Pentene(colorless)
Bromine(red)
CH3CH2CH2CHCH2
BrA
BrA
1,2-Dibromopentane(colorless)
CH3CH2CH2CHPCH2 � Br2
CPC
H3C
CH3H
HG
G D
D
trans-2-Butene
BrOBr
Bromine
� CH3OCOCOCH3
HA
HA
BrA
BrA
2,3-Dibromobutane
CPC
H3C
CH3H
HG
G D
D
trans-2-Butene
BrOBr
Bromine
�
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
ClOCl
Chlorine
� CH3CH2CH2OCOCOH
HA
HA
ClA
ClA
1,2-Dichloropentane
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
ClOCl
Chlorine
�
Learning Goal
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double bond. The greater the amount of bromine that must be added to the reac-tion, the more unsaturated the compound is, that is, the greater the number ofcarbon-carbon double bonds.
Hydration: Addition of H2O to an AlkeneA water molecule can be added to an alkene. This reaction, termed hydration, re-quires a trace of acid (H�) as a catalyst. The product is an alcohol, as shown in thefollowing reaction:
The following equation shows the hydration of ethene to produce ethanol.
With alkenes in which the groups attached to the two carbons of the doublebond are different (unsymmetrical alkenes), two products are possible. For example:
HOCOCPCOH � HOOH HOCOCOOCOH � HOCOCOCOH
3 2 1 3 2 13 2 1
HA
HA
HA
HA
H�
HA
HA
OHA
HA
HA
HA
HA
HA
HA
HA
OHA
HA
Propene(propylene)
Major product2-Propanol(isopropyl alcohol)
Minor product1-Propanol(propyl alcohol)
H HG D
HOCOH
HA
A
CBC GD
H H
�HAOH
HA
HOCOOH
Ethene Water Ethanol(ethyl alcohol)
H�
R RG D
ROCOH
RA
A
CBC GD
R R
�HAOH
RA
ROCOOH
Alkene Water Alcohol
H�
342 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-18
Figure 12.5Bromination of an alkene. The solution ofreactants is red because of the presenceof bromine. When the bromine has beenused in the reaction, the solutionbecomes colorless.
Learning Goal
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When hydration of an unsymmetrical alkene, such as propene, is carried outin the laboratory, one product (2-propanol) is favored over the other. The Russianchemist Vladimir Markovnikov studied many such reactions and came up witha rule that can be used to predict the major product of such a reaction.Markovnikov’s rule tells us that the carbon of the carbon-carbon double bond thatoriginally has more hydrogen atoms receives the hydrogen atom being added tothe double bond. The remaining carbon forms a bond with the —OH. Simplystated, “the rich get richer”—the carbon with the most hydrogens gets the new oneas well. In the preceding example, carbon-1 has two C—H bonds originally, andcarbon-2 has only one. The major product, 2-propanol, results from the new C—Hbond forming on carbon-1 and the new C—OH bond on carbon-2.
Addition of water to a double bond is a reaction that we find in several bio-chemical pathways. For instance, the citric acid cycle is a key metabolic pathwayfor the complete oxidation of the sugar glucose and the release of the majority ofthe energy used by the body. It is also the source of starting materials for the syn-thesis of the biological molecules needed for life. The next-to-last reaction in thecitric acid cycle is the hydration of a molecule of fumarate to produce a moleculecalled malate.
12.5 Reactions Involving Alkenes 343
12-19
Folklore, Science, and Technology
For many years it was suspected that there existed a gas thatstimulated fruit ripening and had other effects on plants. Theancient Chinese observed that their fruit ripened more quicklyif incense was burned in the room. Early in this century, ship-pers realized that they could not store oranges and bananas onthe same ships because some “emanation” given off by the or-anges caused the bananas to ripen too early.
Puerto Rican pineapple growers and Philippine mangogrowers independently developed a traditional practice ofbuilding bonfires near their crops. They believed that thesmoke caused the plants to bloom synchronously.
In the mid–nineteenth century, streetlights were fueled withnatural gas. Occasionally the pipes leaked, releasing gas intothe atmosphere. On some of these occasions, the leaves fellfrom all the shade trees in the region surrounding the gas leak.
What is the gas responsible for these diverse effects onplants? In 1934, R. Gane demonstrated that the simple alkeneethylene was the “emanation” responsible for fruit ripening.More recently, it has been shown that ethylene induces andsynchronizes flowering in pineapples and mangos, inducessenescence (aging) and loss of leaves in trees, and effects a widevariety of other responses in various plants.
We can be grateful to ethylene for the fresh, unbruised fruitsthat we can purchase at the grocery store. These fruits arepicked when they are not yet ripe, while they are still firm.They then can be shipped great distances and gassed with eth-ylene when they reach their destination. Under the influence ofthe ethylene, the fruit ripens and is displayed in the store.
The history of the use of ethylene to bring fresh ripe fruitsand vegetables to markets thousands of miles from the farms isan interesting example of the scientific process and its applica-tion for the benefit of society. Scientists began with the curiousobservations of Chinese, Puerto Rican, and Filipino farmers.Through experimentation they came to understand the phe-nomenon that caused the observations. Finally, through tech-nology, scientists have made it possible to harness the power ofethylene so that grocers can “artificially” ripen the fruits andvegetables they sell to us.
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EXAMPLE 12.8
Hydration of a double bond also occurs in the �-oxidation pathway (see Section23.2). This pathway carries out the oxidation of dietary fatty acids. Like the citricacid cycle, �-oxidation harvests the energy of the food molecules to use as fuel forbody functions.
Writing Equations for the Hydration of Alkenes
Write an equation showing all the products of the hydration of 1-pentene.
Solution
Begin by drawing the structure of 1-pentene and of water and indicatingthe catalyst.
Markovnikov’s rule tells us that the carbon atom that is already bonded tothe greater number of hydrogen atoms is more likely to receive the hydrogenatom from the water molecule. The other carbon atom is more likely tobecome bonded to the hydroxyl group. Thus we can predict that the majorproduct of this reaction will be 2-pentanol and that the minor product will be1-pentanol. Now we write the equation showing the reactants and products:
Write a balanced equation for the hydration of each of the following alkenes.Predict the major product of each of the reactions.
a. CH3CHPCHCH3 c. CH3CH2CH2CHPCHCH2CH3
b. CH2PCHCH2CH2CHCH3 d. CH3CHClCHPCHCHClCH3|CH3
Write a balanced equation for the hydration of each of the following alkenes.Predict the major product of each of the reactions.
a. CH2PCHCH2CHPCH2 c. CH3CHBrCH2CHPCHCH2Cl
b. CH3CH2CH2CHPCHCH3 d. CH3CH2CH2CH2CH2CHPCHCH3
Q u e s t i o n 12.10
Q u e s t i o n 12.9
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOOH
Water
�H�
CH3CH2CH2OCOCOH � CH3CH2CH2OCOCOH
HA
HA
OHA
HA
HA
HA
HA
OHA
2-Pentanol(major product)
1-Pentanol(minor product)
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOOH
Water
�H�
C—H A
COO�A
Fumarase
Fumarate Malate
H—C � H2O
COO� A
HO—C—H A
COO�A
H—C—H
COO� A
A
344 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-20
We have seen that hydration of a doublebond requires a trace of acid as acatalyst. In the cell, this reaction iscatalyzed by an enzyme, or biologicalcatalyst, called fumarase.
Learning Goal
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Learning Goal
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EXAMPLE 12.9
Hydrohalogenation: Addition of HX to an AlkeneA hydrogen halide (HBr, HCl, or HI) also can be added to an alkene. The productof this reaction, called hydrohalogenation, is an alkyl halide:
This reaction also follows Markovnikov’s rule. That is, if HX is added to anunsymmetrical alkene, the hydrogen atom will be added preferentially to the car-bon atom that originally had the most hydrogen atoms. Consider the followingexample:
Writing Equations for the Hydrohalogenation of Alkenes
Write an equation showing all the products of the hydrohalogenation of 1-pentene with HCl.
Solution
Begin by drawing the structure of 1-pentene and of hydrochloric acid.
Markovnikov’s rule tells us that the carbon atom that is already bondedto the greater number of hydrogen atoms is more likely to receive thehydrogen atom of the hydrochloric acid molecule. The other carbon atom ismore likely to become bonded to the chlorine atom. Thus we can predict
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOCl
Hydrochloricacid
�
HOCOCPCOH � HOBr HOCOCOCOH � HOCOCOCOH
HA
HA
HA
HA
HA
HA
BrA
HA
HA
HA
HA
HA
HA
HA
BrA
HA
Propene Major product2-Bromopropane
Minor product1-Bromopropane
H HG D
HOCOH
HA
A
CBC GD
H H
�HABr
HA
HOCOBr
Ethene Hydrogen bromide Bromoethane
R RG D
ROCOH
RA
A
CBC GD
R R
�HAX
RA
ROCOX
Alkene Hydrogen halide Alkyl halide
12.5 Reactions Involving Alkenes 345
12-21
Continued—
Learning Goal
4
Learning Goal
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that the major product of this reaction will be 2-chloropentane and that theminor product will be 1-chloropentane. Now we write the equationshowing the reactants and products:
Predict the major product in each of the following reactions. Name the alkenereactant and the product, using I.U.P.A.C. nomenclature.
a.
b. CH3CH2CHPCH2 � H2O H� ?c. CH3CHPCHCH3 � Cl2 ?d. CH3CH2CH2CHPCH2 � HBr ?
Predict the major product in each of the following reactions. Name the alkenereactant and the product, using I.U.P.A.C. names.
a.
b.
c.
d.
Addition Polymers of AlkenesPolymers are macromolecules composed of repeating structural units calledmonomers. A polymer may be made up of several thousand monomers. Manycommercially important plastics and fibers are addition polymers made fromalkenes or substituted alkenes. They are called addition polymers because theyare made by the sequential addition of the alkene monomer. The general formulafor this addition reaction follows:
CH3OCOCHPCH2 � HCl ?
CH3
A
CH3A
CH3CPCHCH3 � Br2 ?
CH3
A
CH3OCPCHCH2CH2CH3 � H2O ?
CH3
A
H�
CPC
H
HCH3
CH3
G
G D
DH2 ?�
Ni
Q u e s t i o n 12.12
CPC
H
CH3CH3
HG
G D
DH2 ?�
Pd
Q u e s t i o n 12.11
CPC
CH3CH2CH2
HH
HG
G D
D
1-Pentene
HOCl
Hydrochloricacid
� CH3CH2CH2OCOCOH � CH3CH2CH2OCOCOH
HA
HA
ClA
HA
HA
HA
HA
ClA
2-Chloropentane(major product)
1-Chloropentane(minor product)
346 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-22
EXAMPLE 12.9 —Continued
Learning Goal
6
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The product of the reaction is generally represented in a simplified manner:
COC
RA
RA
AR
AR n
RA
RA
RA
RA
RA
RA
RA
RA
HeatPressure
Alkene monomerR � H, X, or an alkyl group
Addition polymer
RCPC
R
RG D
GD
R
n etc.OCOCOCOCOCOCOetc.
RA
RA
RA
RA
12.5 Reactions Involving Alkenes 347
12-23
Life without Polymers?
What do Nike Air-Sole shoes, Saturn automobiles, disposablediapers, tires, shampoo, and artificial joints and skin share incommon? These products and a great many other items we useevery day are composed of synthetic or natural polymers. In-deed, the field of polymer chemistry has come a long way sincethe 1920s and 1930s when DuPont chemists invented nylon andTeflon.
Consider the disposable diaper. The outer, waterproof layeris composed of polyethylene. The polymerization reaction thatproduces polyethylene is shown in Section 12.4. The diapershave elastic to prevent leaking. The elastic is made of a naturalpolymer, rubber. The monomer from which natural rubber isformed is 2-methyl-1,3-butadiene. The common name of thismonomer is isoprene. As we will see in coming chapters, iso-prene is an important monomer in the synthesis of many nat-ural polymers.
The diaper is filled with a synthetic polymer called poly(acrylicacid). This polymer has the remarkable ability to absorb manytimes its own weight in liquid. Polymers that have this abilityare called superabsorbers, but polymer chemists have no ideawhy they have this property! The acrylate monomer and re-sulting poly(acrylic acid) polymer are shown here:
Another example of a useful polymer is Gore-Tex. This amaz-ing polymer is made by stretching Teflon. Teflon is producedfrom the monomer tetrafluoroethene, as seen in the followingreaction:
Clothing made from this fabric is used to protect firefighters be-cause of its fire resistance. Because it also insulates, Gore-Texclothing is used by military forces and by many amateur ath-letes, for protection during strenuous activity in the cold. In ad-dition to its use in protective clothing, Gore-Tex has been usedin millions of medical procedures for sutures, synthetic bloodvessels, and tissue reconstruction.
To learn more about the fascinating topic of polymerchemistry, visit The Macrogalleria, www.psrc.usm.edu/macrog/index.html, an Internet site maintained by the Department ofPolymer Science of the University of Southern Mississippi.
nCH2PCOCHPCH2 CH2OCPCHOCH2 n
CH3A
CH3A
2-Methyl-1, 3,-butadiene(isoprene)
Rubber polymer
n CPC CH2OCH
nCPOA
Acrylate monomer Poly(acrylic acid)
OA
HA
H
HG D
H
GD
CPO
OA
HA
CPC
Tetrafluoroethene Teflon
F
FG D
F
GD
F
n COC
FA
FA
AF
AF n
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Polyethylene is a polymer made from the monomer ethylene (ethene):
It is used to make bottles, injection-molded toys and housewares, and wirecoverings.
Polypropylene is a plastic made from propylene (propene). It is used to makeindoor-outdoor carpeting, packaging materials, toys, and housewares. When
nCH2PCH2 CH2OCH2 n
Ethene(ethylene)
Polyethylene
348 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-24
Plastic Recycling
Plastics, first developed by British inventor Alexander Parkesin 1862, are amazing substances. Some serve as containers formany of our foods and drinks, keeping them fresh for long pe-riods of time. Other plastics serve as containers for detergentsand cleansers or are formed into pipes for our plumbing sys-tems. We have learned to make strong, clear sheets of plasticthat can be used as windows, and feather-light plastics that canbe used as packaging materials. In the United States alone,seventy-five billion pounds of plastics are produced each year.
But plastics, amazing in their versatility, are a mixed bless-ing. One characteristic that makes them so useful, their stabil-ity, has created an environmental problem. It may take forty tofifty years for plastics discarded into landfill sites to degrade.Concern that we could soon be knee-deep in plastic worldwidehas resulted in a creative new industry: plastic recycling.
Since there are so many types of plastics, it is necessary toidentify, sort, and recycle them separately. To help with thissorting process, manufacturers place recycling symbols on theirplastic wares. As you can see in the accompanying table, eachsymbol corresponds to a different type of plastic.
Polyethylene terephthalate, also known as PETE or simply#1, is a form of polyester often used to make bottles and jars tocontain food. When collected, it is ground up into flakes andformed into pellets. The most common use for recycled PETE isthe manufacture of polyester carpets. But it may also be spuninto a cotton-candy-like form that can be used as a fiber fillingfor pillows or sleeping bags. It may also be rolled into thinsheets or ribbons and used as tapes for VCRs or tape decks.Reuse to produce bottles and jars is also common.
HDPE, or #2, is high-density polyethylene. Originally usedfor milk and detergent bottles, recycled HDPE is used to pro-duce pipes, plastic lumber, trash cans, or bottles for storage ofmaterials other than food. LDPE, #4, is identical to HDPEchemically, but it is produced in a less-dense, more flexibleform. Originally used to produce plastic bags, recycled LDPE isalso used to make trash bags, grocery bags, and plastic tubingand lumber.
PVC, or #3, is one of the less commonly recycled plastics inthe United States, although it is actively recycled in Europe. Therecycled material is used to make non-food-bearing containers,shoe soles, flooring, sweaters, and pipes. Polypropylene, PP or
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propylene polymerizes, a methyl group is located on every other carbon of themain chain:
or
CH2OCHOCH2OCHOCH2OCH
CH3ACH3A
CH3A
nCH2PCH CH2OCH n
CH3ACH3A
12.5 Reactions Involving Alkenes 349
12-25
#5, is found in margarine tubs, fabrics, and carpets. Recycledpolypropylene has many uses, including fabrication of garden-ing implements.
You probably come into contact with polystyrene, PS or #6,almost every day. It is used to make foam egg cartons and meattrays, serving containers for fast food chains, CD “jewel boxes,”
and “peanuts” used as packing material. At the current time,polystyrene food containers are not recycled. PS from nonfoodproducts can be melted down and converted into pellets thatare used to manufacture office desktop accessories, hangers,video and audio cassette housings, and plastic trays used tohold plants.
Code Type Name Formula Description Examples
PETE Polyethylene Usually clear Soda bottles, peanutterephthalate —CH2—CH2—O—C C—O— or green, rigid butter jars, vegetable
oil bottles
HDPE High-density —CH2—CH2— Semirigid Milk and waterpolyethylene jugs, juice and bleach
bottlesPVC Polyvinyl —CH—CH2— Semirigid Detergent and
chloride cleanser bottles,pipes
LDPE Low-density —CH2—CH2— Flexible, Six-pack rings, polyethylene not crinkly bread bags, sandwich
bagsPP Polypropylene —CH—CH2— Semirigid Margarine tubs, straws,
screw-on lids
PS Polystyrene —CH—CH2— Often brittle Styrofoam, packing peanuts, egg cartons, foam cups
Other Multilayer N/A Squeezable Ketchup and syrup plastics bottles
O OBO
OB
|Cl
|CH3
O
1
2
3
4
5
6
7
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Polymers made from alkenes or substituted alkenes are simply very largealkanes or substituted alkanes. Like the alkanes, they are typically inert. Thischemical inertness makes these polymers ideal for making containers to holdjuices, chemicals, and fluids used medically. They are also used to make sutures,catheters, and other indwelling devices. A variety of polymers made from substi-tuted alkenes are listed in Table 12.1.
12.6 Aromatic HydrocarbonsIn the early part of the nineteenth century chemists began to discover organic com-pounds having chemical properties quite distinct from the alkanes, alkenes, andalkynes. They called these substances aromatic compounds because many of the firstexamples were isolated from the pleasant-smelling resins of tropical trees. The car-bon:hydrogen ratio of these compounds suggested a very high degree of unsatu-ration, similar to the alkenes and alkynes. Imagine, then, how puzzled these earlyorganic chemists must have been when they discovered that these compounds donot undergo the kinds of addition reactions common for the alkenes and alkynes.
We no longer define aromatic compounds as those having a pleasant aroma; infact, many do not. We now recognize aromatic hydrocarbons as those that exhibita much higher degree of chemical stability than their chemical composition wouldpredict. The most common group of aromatic compounds is based on the six-member aromatic ring, the benzene ring. The structure of the benzene ring is rep-resented in various ways in Figure 12.6.
No reactionC C
C C
C
C D
J
BA
M
G DH
DH HG
HG
� Br2
AH
AH
CH2PCH2 � Br2 CH2OCH2
BrA
BrA
350 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-26
Table 12.1 Some Important Addition Polymers of Alkenes
Monomer name Formula Polymer Uses
Styrene CH2PCH Polystyrene Styrofoam containers
Acrylonitrile CH2PCHCN Polyacrylonitrile Clothing (Orlon)
OB
Methyl methacrylate CH2PC(CH3)—COCH3 Polymethyl Basketballmethacrylate backboards(Plexiglas, Lucite)
Vinyl chloride CH2PCHCl Polyvinyl chloride Plastic pipe, (PVC) credit cards
Tetrafluoroethene CF2PCF2 Polytetrafluoro- Nonstickethylene (Teflon) surfaces
O
Learning Goal
7
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Structure and PropertiesThe benzene ring consists of six carbon atoms joined in a planar hexagonalarrangement. Each carbon atom is bonded to one hydrogen atom. FriedrichKekulè proposed a model for the structure of benzene in 1865. He proposed thatsingle and double bonds alternated around the ring (a conjugated system of dou-ble bonds). To explain why benzene did not decolorize bromine—in other words,didn’t react like an unsaturated compound—he suggested that the double and sin-gle bonds shift positions rapidly. We show this as a resonance model today.
The current model of the structure of benzene is based on the idea of overlap-ping orbitals. Each carbon is bonded to two others by sharing a pair of electrons (�bonds). Each carbon atom also shares a pair of electrons with a hydrogen atom.The remaining six electrons are located in p orbitals that are perpendicular to theplane of the ring. These p orbitals overlap laterally to form pi (�) orbitals that forma cloud of electrons above and below the ring. These � orbitals are shaped likedoughnuts, as shown in Figure 12.7.
Two symbols are commonly used to represent the benzene ring. The represen-tation in Figure 12.6b is the structure proposed by Kekulé. The structure in Figure12.6d represents the � clouds.
The equal sharing of the six electrons of the p orbitals results in a rigid, flat ringstructure, in contrast to the relatively flexible, nonaromatic cyclohexane ring. Themodel also explains the unusual chemical stability of benzene and its resistance toaddition reactions. The electrons of the � cloud are said to be delocalized. Thatmeans they have much more space and freedom of movement than they wouldhave if they were restricted to individual double bonds. Because electrons repelone another, the system is more stable when the electrons have more space to oc-cupy. As a result, benzene is unusually stable and resists addition reactions typicalof alkenes.
Benzene as a resonance hybrid
A
DH
DH HG
HG
H
AH
A
DH
DH HG
HG
H
AH
12.6 Aromatic Hydrocarbons 351
12-27
Figure 12.6Four ways to represent the benzenemolecule. Structure (b) is a simplifieddiagram of structure (a). Structure (d), asimplified diagram of structure (c), is themost commonly used representation.
H
C
(a)
(b)
(c)
(d)
H
H
HH
H
H H
H
HH
H
C
C
CC
C
C C
C
C C
C
Resonance models are described in Section 4.4.
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NomenclatureMost simple aromatic compounds are named as derivatives of benzene. Thus ben-zene is the parent compound, and the name of any atom or group bonded to ben-zene is used as a prefix, as in these examples:
Other members of this family have unique names based on history rather thanlogic:
When two groups are present on the ring, three possible orientations exist, andthey may be named by either the I.U.P.A.C. Nomenclature System or the commonsystem of nomenclature. If the groups or atoms are located on two adjacent carbons,
meta-Xylene
ACH3
meta-Cresol
AOH
para-Xylene
ACH3
para-Cresol
AOH
GCH3
GCH3 A
CH3
ACH3
Benzoic acid
ACOOHBO
Benzaldehyde
ACOHBO
ortho-Xylene
ACH3
ortho-Cresol
AOH
DCH3 D
CH3
Toluene Phenol Aniline
ACH3 A
OHANH2
Anisole
AOCH3
Nitrobenzene Ethylbenzene Bromobenzene
ANO2 A
CH2CH3 ABr
352 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-28
Figure 12.7The current model of the bonding inbenzene.
C CH H
H H
HHC C
C C
π cloud
σ bond
C CHH
H H
H H
C CC C
p orbitals
Learning Goal
7
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EXAMPLE 12.11
ACH3 A
NO2
DCl A
CH2CH3
AOH
GNH2
EXAMPLE 12.10
they are referred to as ortho (o) in the common system or with the prefix 1,2- in theI.U.P.A.C. system. If they are on carbons separated by one carbon atom, they aretermed meta (m) in the common system or 1,3- in the I.U.P.A.C. system. Finally, if thesubstituents are on carbons separated by two carbon atoms, they are said to be para(p) in the common system or 1,4- in the I.U.P.A.C. system. The following examplesdemonstrate both of these systems:
If three or more groups are attached to the benzene ring, numbers must be usedto describe their location. The names of the substituents are given in alphabeticalorder.
Naming Derivatives of Benzene
Name the following compounds using the I.U.P.A.C. Nomenclature System.
a. b. c.
Solution
Parentcompound: toluene phenol anilineSubstituents: 2-chloro 4-nitro 3-ethylName: 2-Chlorotoluene 4-Nitrophenol 3-Ethylaniline
Naming Derivatives of Benzene
Name the following compounds using the common system of nomenclature.
a. b. c.
Solution
Parent compound: toluene phenol anilineSubstituents: ortho-chloro para-nitro meta-ethyl
Two groups 1,2 or ortho
AG
Two groups 1,3 or metaG � Any group
AG
DG
GG
Two groups 1,4 or para
AG
AG
12.6 Aromatic Hydrocarbons 353
12-29
ACH3 A
NO2
DCl A
CH2CH3
AOH
GNH2
Continued—
Learning Goal
7
Learning Goal
7
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Name: ortho-Chlorotoluene para-Nitrophenol meta-EthylanilineAbbreviatedName: o-Chlorotoluene p-Nitrophenol m-Ethylaniline
In I.U.P.A.C. nomenclature, the group—C6H5 derived by removing one hydro-gen from benzene, is called the phenyl group. An aromatic hydrocarbon with analiphatic side chain is named as a phenyl substituted hydrocarbon. For example:
One final special name that occurs frequently in aromatic compounds is thebenzyl group:
The use of this group name is illustrated by:
Draw each of the following compounds:
a. 1,3,5-Trichlorobenzeneb. ortho-Cresolc. 2,5-Dibromophenold. para-Dinitrobenzenee. 2-Nitroanilinef. meta-Nitrotoluene
Draw each of the following compounds:
a. 2,3-Dichlorotoluene d. o-Nitrotolueneb. 3-Bromoaniline e. p-Xylenec. 1-Bromo-3-ethylbenzene f. o-Dibromobenzene
Reactions Involving BenzeneAs we have noted, benzene does not readily undergo addition reactions. The typ-ical reactions of benzene are substitution reactions, in which a hydrogen atom isreplaced by another atom or group of atoms.
Q u e s t i o n 12.14
Q u e s t i o n 12.13
Benzyl chloride Benzyl alcohol
OCH2Cl OCH2OH
OCH2OorC6H5CH2O
ACH3CHCH2CH3 A
CH3CHCHPCH2
2-Phenylbutane 3-Phenyl-1-butene
354 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-30
EXAMPLE 12.11 —Continued
Learning Goal
8
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Benzene can react (by substitution) with Cl2 or Br2. These reactions require ei-ther iron or an iron halide as a catalyst. For example:
FeCl3C C
C C
C
C D
D
AA
G
G DH
DH HG
HG
� Cl2
HA
HA
Benzene
C C
C C
C
C D
D
AA
G
G DCl
DH HG
HG
� HCl
HA
HA
ChlorobenzeneChlorine
12.6 Aromatic Hydrocarbons 355
12-31
Aromatic Compounds and Carcinogenesis
We come into contact with many naturally occurring aro-matic compounds each day. Originally, the name aromatic wasgiven to these compounds because of the pleasant aromas ofsome members of this family. Indeed, many food flavoringsand fragrances that we enjoy contain aromatic rings. Examplesof other aromatic compounds include preservatives (such asBHT, butylated hydroxytoluene), insecticides (such as DDT),pharmaceutical drugs (such as aspirin), and toiletries.
The polynuclear aromatic hydrocarbons (PAH) are an impor-tant family of aromatic hydrocarbons that generally have toxiceffects. They have also been shown to be carcinogenic, that is,they cause cancer. PAH are formed from the joining of the ringsso that they share a common bond (edge). Three common ex-amples are shown:
The more complex members of this family (typically con-sisting of five or six rings at a minimum) are among the mostpotent carcinogens known. It has been shown that the carcino-genic nature of these chemicals results from their ability to bindto the nucleic acid (DNA) in cells. As we will see in Chapter 24,the ability of the DNA to guide the cell faithfully from genera-tion to generation is dependent on the proper expression of thegenetic information, a process called transcription, and the ac-curate copying or replication of the DNA. Accurate DNA repli-cation is essential so that every new cell inherits a completecopy of all the genetic information that is identical to that of theoriginal parent cell. If a mistake is made in the DNA replicationprocess, the result is an error, or mutation, in the new DNAmolecule. Some of these errors may cause the new cell to growout of control, resulting in cancer.
Polynuclear aromatic hydrocarbons are thought to causecancer by covalently binding to the DNA in cells and interfer-ing with the correct replication of the DNA. Some of the muta-tions that result may cause a cell to begin to divide in anuncontrolled fashion, giving rise to a cancerous tumor.
Benzopyrene (shown below) is found in tobacco smoke,smokestack effluents, charcoal-grilled meat, and automobileexhaust. It is one of the strongest carcinogens known. It is esti-mated that a wide variety of all cancers are caused by chemicalcarcinogens, such as PAH, in the environment.
Naphthalene
Anthracene
Phenanthrene Benzopyrene
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When a second equivalent of the halogen is added, three isomers—para, ortho, andmeta—are formed.
Benzene also reacts with sulfur trioxide by substitution. Concentrated sulfuricacid is required as the catalyst. Benzenesulfonic acid, a strong acid, is the product:
Benzene can also undergo nitration with concentrated nitric acid dissolved inconcentrated sulfuric acid. This reaction requires temperatures in the range of50–55�C.
12.7 Heterocyclic Aromatic CompoundsHeterocyclic aromatic compounds are those having at least one atom other thancarbon as part of the structure of the aromatic ring. The structures and commonnames of several heterocyclic aromatic compounds are shown:
All these compounds are more similar to benzene in stability and chemical behav-ior than they are to the alkenes. Many of these compounds are components of mol-ecules that have significant effects on biological systems. For instance, the purinesand pyrimidines are components of DNA (deoxyribonucleic acid) and RNA (ri-bonucleic acid). DNA and RNA are the molecules responsible for storing and ex-pressing the genetic information of an organism. The pyridine ring is found in
Imidazole Furan Pyrrole
H
NA
H
NA
N
O
Pyridine Pyrimidine Purine
N N
N
N
N
N
HA
N
Concentrated H2SO4
50–55°C
Benzene Nitrobenzene
� HNO3 ONO2 � H2O
Nitric acid
Concentrated H2SO4
Benzene Benzenesulfonic acid
� SO3 OSOOH � H2O
Sulfur trioxide
OB
OB
Benzene Bromobenzene
� Br2 OBr � HBr
Bromine
FeBr3
356 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-32
See the Chemistry Connection: TheNicotine Patch in Chapter 16.
Learning Goal
9
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nicotine, the addictive compound in tobacco. The pyrrole ring is a component ofthe porphyrin ring found in hemoglobin and chlorophyll.
The imidazole ring is a component of cimetidine, a drug used in the treatment ofstomach ulcers. The structure of cimetidine is shown below:
We will discuss a subset of the heterocyclic aromatic compounds, the heterocyclicamines, in Chapter 16.
Cimetidine
CH2SCH2CH2NHCNHCH3
NCNB
CH3DNG
H
N G
Porphyrin
Fe3�D
N
DN
”N
”N
Summary of Reactions 357
12-33
Summary of ReactionsAddition reactions of alkenes
Hydrogenation: Halogenation:
Hydration: Hydrohalogenation:
R RG D
ROCOH
RA
A
CBC GD
R R
�HAH
RA
ROCOH Pt, Pd, or NiHeat or pressure
Alkene Hydrogen Alkane
R RG D
ROCOX
RA
A
CBC GD
R R
�XAX
RA
ROCOX
Alkene Halogen Alkyl dihalide
R RG D
ROCOH
RA
A
CBC GD
R R
�HAOH
RA
ROCOOH
Alkene Water Alcohol
H�
R RG D
ROCOH
RA
A
CBC GD
R R
�HAX
RA
ROCOX
Alkene Hydrogen halide Alkyl halide
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358 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
12-34
Addition Polymers of Alkenes
Reactions of Benzene
Halogenation:
Sulfonation:
Nitration:
CPC
Alkene monomer Addition polymer
R
RG D
R
GD
R
n COC
RA
RA
AR
AR n
Summary
12.1 Alkenes and Alkynes: Structure andPhysical Properties
Alkenes and alkynes are unsaturated hydrocarbons. Alkenesare characterized by the presence of at least one carbon-carbon double bond and have the general molecular for-mula CnH2n. Alkynes are characterized by the presence ofat least one carbon-carbon triple bond and have the generalmolecular formula CnH2n�2. The physical properties of thealkenes and alkynes are similar to those of alkanes, buttheir chemical properties are quite different.
12.2 Alkenes and Alkynes: NomenclatureAlkenes and alkynes are named by identifying the parentcompound and replacing the -ane ending of the alkanewith -ene (for an alkene) or -yne (for an alkyne). The parentchain is numbered to give the lowest number to the first ofthe two carbons involved in the double bond (or triplebond). Finally, all groups are named and numbered.
12.3 Geometric Isomers: A Consequence ofUnsaturation
The carbon-carbon double bond is rigid. This allows theformation of geometric isomers, or isomers that differ from
one another depending on whether chemical groups are onthe same or opposite sides of the rigid double bonds. Whengroups are on the same side of a double bond, the prefix cisis used to describe the compound. When groups are on op-posite sides of a double bond, the prefix trans is used.
12.4 Alkenes in NatureAlkenes and polyenes (alkenes with several carbon-carbondouble bonds) are common in nature. Ethene, the simplestalkene, is a plant growth substance involved in fruit ripen-ing, senescence and leaf fall, and responses to environmen-tal stresses. Isoprenoids, or terpenes, are polyenes builtfrom one or more isoprene units. Isoprenoids includesteroids, chlorophyll and other photosynthetic pigments,and vitamins A, D, and K.
12.5 Reactions Involving AlkenesWhereas alkanes undergo substitution reactions, alkenes andalkynes undergo addition reactions. The principal additionreactions of the unsaturated hydrocarbons are halogenation,hydration, hydrohalogenation, and hydrogenation. Polymerscan be made from alkenes or substituted alkenes.
12.6 Aromatic HydrocarbonsAromatic hydrocarbons contain benzene rings. The ringscan be represented as having alternating double and single
Benzene Halobenzene
� X2 OX � HX
Halogen
FeX3
Concentrated H2SO4
Benzene
� SO3
Sulfur trioxide
Benzenesulfonic acid
OSOOH � H2O
OB
OB
Concentrated H2SO4
50–55°C
Benzene
� HNO3
Nitric acid
Nitrobenzene
ONO2 � H2O
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bonds. However, it is more accurate to portray � (sigma)bonds between carbons of the ring and a � (pi) cloud ofelectrons above and below the ring. Simple aromatic com-pounds are named as derivatives of benzene. Several mem-bers of this family have historical common names, such asaniline, phenol, and toluene. Aromatic compounds do notundergo addition reactions. The typical reactions of ben-zene are substitution reactions: halogenation, nitration, andsulfonation.
12.7 Heterocyclic Aromatic CompoundsHeterocyclic aromatic compounds are those having at leastone atom other than carbon as part of the structure of thearomatic ring. They are more similar to benzene in stabilityand chemical behavior than they are to the alkenes. Manyof these compounds are components of molecules thathave significant effects on biological systems, includingDNA, RNA, hemoglobin, and nicotine.
Key Terms
Questions and Problems
Alkenes and Alkynes: Structure and Physical Properties
12.15 Write the general formulas for alkanes, alkenes, and alkynes.12.16 What are the characteristic functional groups of alkenes and
alkynes?12.17 Describe the geometry of ethene.12.18 What are the bond angles in ethene?12.19 Describe the geometry of ethyne.12.20 What are the bond angles in ethyne?
Alkenes and Alkynes: Nomenclature and Geometric Isomers
12.21 Draw a condensed formula for each of the followingcompounds:a. 2-Methyl-2-hexeneb. trans-3-Heptenec. cis-1-Chloro-2-pentened. cis-2-Chloro-2-methyl-3-heptenee. trans-5-Bromo-2,6-dimethyl-3-octene
12.22 Draw a condensed formula for each of the followingcompounds:a. 2-Hexyneb. 4-Methyl-1-pentyne
c. 1-Chloro-4,4,5-trimethyl-2-heptyned. 2-Bromo-3-chloro-7,8-dimethyl-4-decyne
12.23 Name each of the following using the I.U.P.A.C.Nomenclature System:a. CH3CH2CHCHPCH2
|CH3
b. CH2CH2CH2CH2—Br|CH2CHPCH2
c. CH3CH2CHPCHCHCH2CH3|Br
d.
e. CH3CHCH2CHPCCH3| |CH3 CH3
f. Cl—CH2CHCqC—H|CH3
g. CH3CHCH2CH2CH2—CqC—H|Cl
h.
12.24 Draw each of the following compounds using condensedformulas:a. 1,3,5-Trifluoropentaneb. cis-2-Octenec. Dipropylacetylened. 3,3,5-Trimethyl-1-hexenee. 1-Bromo-3-chloro-1-heptyne
12.25 Of the following compounds, which can exist as cis-transgeometric isomers? Draw the two geometric isomers.a. 2,3-Dibromobutaneb. 2-Heptenec. 2,3-Dibromo-2-butened. Propenee. 1-Bromo-1-chloro-2-methylpropenef. 1,1-Dichloroetheneg. 1,2-Dibromoetheneh. 3-Ethyl-2-methyl-2-hexene
12.26 Which of the following alkenes would not exhibit cis-transgeometric isomerism?a.
b.
c.
d. CH3
CPCD
HD
CH2CH2CH3
G
CH3CH2G
CHCH3
CPCD
CH3
DCHCH2CH3
G
CH3CH2G
CH2CH3A
CH3
A
CH3
CPCD
CH3
DH
G
CH3G
CH3
CPCD
HD
CH2CH3
G
CH3G
ClD
BrG
OCH3CH3OCO
CH3A
CH3
A
Questions and Problems 359
12-35
addition polymer (12.5)addition reaction (12.5)alkene (12.1)alkyne (12.1)aromatic
hydrocarbon (12.6)geometric isomers (12.3)halogenation (12.5)heterocyclic aromatic
compound (12.7)
hydration (12.5)hydrogenation (12.5)hydrohalogenation (12.5)Markovnikov’s rule (12.5)monomer (12.5)phenyl group (12.6)polymer (12.5)substitution reaction (12.6)unsaturated hydrocarbon
(Intro)
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12.27 Which of the following structures have incorrect I.U.P.A.C.names? If incorrect, give the correct I.U.P.A.C. name.a.
b.
c.
d.
e.
12.28 Which of the following can exist as cis and trans isomers?a. H2CPCH2 d. ClBrCPCClBrb. CH3CHPCHCH3 e. (CH3)2CPC(CH3)2c. Cl2CPCBr2
12.29 Provide the I.U.P.A.C. name for each of the followingmolecules:a. CH2PCHCH2CH2CHPCHCH2CH2CH3b. CH2PCHCH2CHPCHCH2CHPCHCH3c. CH3CHPCHCH2CHPCHCH2CH3d. CH3CHPCHCHCHPCHCH3
|CH3
12.30 Provide the I.U.P.A.C. name for each of the followingmolecules:a. CH3CPCHCHCHPCCH3
| | |Br CH3 Br
b.
CH3 CH2CH3| |
CH2PCHCHCHPCHCHCH2CH3
c.
CH3 CH3| |
CH2PCHCCHPCHCH2CHPCHCHCH3|CH3
d.
CH2CH3 CH2CH3| |
CH3CHCHCHPCHCH2CHPCHCHCH3|CH3
Reactions Involving Alkenes
12.31 How could you distinguish between a sample of cyclohexaneand a sample of hexene (both C6H12) using a simple chemicaltest? (Hint: Refer to the subsection entitled “Halogenation:Addition of X2 to an Alkene.”)
12.32 Quantitatively, 1 mol of Br2 is consumed per mole of alkene,and 2 mol of Br2 are consumed per mole of alkyne. Howmany moles of Br2 would be consumed for 1 mol of each ofthe following:a. 2-Hexyneb. Cyclohexenec.
d.
12.33 Complete each of the following reactions by supplying themissing reactant or product(s) as indicated by a question mark:a. CH3CH2CHPCHCH2CH3 � ?
CH3CH2CH2CH2CH2CH3
b.
c.
d. 2CH3CH2CH2CH2CH2CH3 � ?O2 Heat ? � ?
(completecombustion)
e.
f.
12.34 Draw and name the product in each of the followingreactions:a. Cyclopentene � H2O (H�)b. Cyclopentene � HClc. Cyclopentene � H2d. Cyclopentene � HI
12.35 A hydrocarbon with a formula C5H10 decolorized Br2 andconsumed 1 mol of hydrogen upon hydrogenation. Draw allthe isomers of C5H10 that are possible based on the aboveinformation.
12.36 Triple bonds react in a manner analogous to that of doublebonds. The extra pair of electrons in the triple bond, however,generally allows 2 mol of a given reactant to add to the triplebond in contrast to 1 mol with the double bond. The “rich getricher” rule holds. Predict the major product in each of thefollowing reactions:a. Acetylene with 2 mol HClb. Propyne with 2 mol HBrc. 2-Butyne with 2 mol HI
12.37 Complete each of the following by supplying the missingproduct indicated by the question mark:
a. 2-Butene HBr
?
b. 3-Methyl-2-hexene HI
?c.
? HCl
? H2O, H�
OHA
? � � HClClO
? �
BrD
HG
CH2
BCH3OCOCH3 � ? CH3COOH
CH3
A
CH3A
—CqC—CH3
OCHPCH2
1-Chloro-5-methyl-2-hexene
H
CPCD
HD
CHCH2CH3
G
CICH2 G CH3A
trans-6-Chloro-3-heptene
CH2CHCH3
CPCD
HD
HG
CH3CH2G
ClA
CH3
ACH3CHCH2OCqCOCH2CHCH3
2-Ethyl-7-methyl-4-octyne
CH2CH3A
3-Ethyl-3-hexyne
CH2CH3
CPCD
CH3CH2
DH
G
CH3CH2G
CH3
ACH3CqCOCH2CHCH3
2-Methyl-4-hexyne
360 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
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12.38 Bromine is often used as a laboratory spot test forunsaturation in an aliphatic hydrocarbon. Bromine in CCl4 isred. When bromine reacts with an alkene or alkyne, the alkylhalide formed is colorless; hence a disappearance of the redcolor is a positive test for unsaturation. A student tested thecontents of two vials, A and B, both containing compoundswith a molecular formula, C6H12. Vial A decolorized bromine,but vial B did not. How may the results for vial B beexplained? What class of compound would account for this?
12.39 What is meant by the term polymer?12.40 What is meant by the term monomer?12.41 Write an equation representing the synthesis of Teflon from
tetrafluoroethene. (Hint: Refer to Table 12.1.)12.42 Write an equation representing the synthesis of polystyrene.
(Hint: Refer to Table 12.1.)12.43 Provide the I.U.P.A.C. name for each of the following
molecules. Write a balanced equation for the hydration of each.a. CH3CHPCHCH2CH3b. CH2CHPCH2
|Br
c.
12.44 Provide the I.U.P.A.C. name for each of the followingmolecules. Write a balanced equation for the hydration of each.a.
b. CH3CHPCHCH2CHPCHCH2CHPCHCH3
CH2CH3|
c. CH3CHPCHCCH3|CH3
12.45 Write an equation for the addition reaction that producedeach of the following molecules:
CH3|
a. CH2CH2CH2CHCH3|OH
b. CH3CH2CHCH2CH2CH3|Br
c.
d.
12.46 Write an equation for the addition reaction that producedeach of the following molecules:
OH|
a. CH3CH2CHCHCH3|CH2CH3
b. CH3CHCH2CH3|OH
c.
d.
12.47 Draw the structure of each of the following compounds andwrite a balanced equation for the complete hydration of each:a. 1,4-Hexadieneb. 2,4,6-Octatrienec. 1,3 Cyclohexadiened. 1,3,5-Cyclooctatriene
12.48 Draw the structure of each of the following compounds andwrite a balanced equation for the hydrobromination of each:a. 3-Methyl-1,4-hexadieneb. 4-Bromo-1,3-pentadienec. 3-Chloro-2,4-hexadiened. 3-Bromo-1,3-Cyclohexadiene
Aromatic Hydrocarbons
12.49 Draw the structure for each of the following compounds:a. 2,4-Dibromotolueneb. 1,2,4-Triethylbenzenec. Isopropylbenzened. 2-Bromo-5-chlorotoluene
12.50 Name each of the following compounds, using the I.U.P.A.C.system.a.
b. d.
c. e.
12.51 Draw each of the following compounds, using condensedformulas:a. meta-Cresolb. Propylbenzenec. 1,3,5-Trinitrobenzened. m-Chlorotoluene
12.52 Draw each of the following compounds, using condensedformulas:a. p-Xyleneb. Isopropylbenzenec. m-Nitroanisoled. p-Methylbenzaldehyde
ACH3
G DNO2O2N
ABr
ACH2CH3
ACH3
ABr
GCl
DCH3
ANO2
DNO2
ACH3
DCH3
Br
OHG
GD
OH
DBr
HO
CH3
G
G DCH3
OCH2CH3
OOH
Br
CH3
G
G
OCH2CH3
DCH3
CH3
G
Questions and Problems 361
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12.53 Describe the Kekulé model for the structure of benzene.12.54 Describe the current model for the structure of benzene.12.55 How does a substitution reaction differ from an addition
reaction?12.56 Give an example of a substitution reaction and of an addition
reaction.12.57 Write an equation showing the reaction of benzene with Cl2
and FeCl3.12.58 Write an equation showing the reaction of benzene with SO3.
Be sure to note the catalyst required.
Heterocyclic Aromatic Compounds
12.59 Draw the general structure of a pyrimidine.12.60 What biological molecules contain pyrimidine rings?12.61 Draw the general structure of a purine.12.62 What biological molecules contain purine rings?
Critical Thinking Problems
1. There is a plastic polymer called polyvinylidene difluoride(PVDF) that can be used to sense a baby’s breath and thus beused to prevent sudden infant death syndrome (SIDS). Thesecret is that this polymer can be specially processed so that itbecomes piezoelectric (produces an electrical current when it isphysically deformed) and pyroelectric (develops an electricalpotential when its temperature changes). When a PVDF film isplaced beside a sleeping baby, it will set off an alarm if the babystops breathing. The structure of this polymer is shown here:
Go to the library and investigate some of the other amazinguses of PVDF. Draw the structure of the alkene from which thiscompound is produced.
2. Isoprene is the repeating unit of the natural polymer rubber. Itis also the starting material for the synthesis of cholesterol andseveral of the lipid-soluble vitamins, including vitamin A andvitamin K. The structure of isoprene is seen below.
What is the I.U.P.A.C. name for isoprene?3. When polyacrylonitrile is burned, toxic gases are released. In
fact, in airplane fires, more passengers die from inhalation oftoxic fumes than from burns. Refer to Table 12.1 for thestructure of acrylonitrile. What toxic gas would you predict tobe the product of the combustion of these polymers?
4. If a molecule of polystyrene consists of 25,000 monomers, whatis the molar mass of the molecule?
5. A factory produces one million tons of polypropylene. Howmany moles of propene would be required to produce thisamount? What is the volume of this amount of propene at25�C and 1 atm?
CH2PCOCHPCH2
CH3A
OCOCOCOCO
FA
FA
HA
HA
FA
FA
HA
HA
362 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics
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