105 H 2 C CH 2 H 2 C CH 2 Br Br HC CH HC CH Br Br Br Br 2 Br 2 Br 2 5 Reactions of Alkenes and Alkynes CHAPTER SUMMARY Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons; the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source.
Transcript
105
H2C CH2 H2C CH2
Br Br
HC CHHC CH
Br
Br
Br
Br
2 Br2Br2
5
Reactions ofAlkenes and Alkynes
CHAPTER SUMMARY
Addition is the characteristic reaction of alkenes and alkynes.
Since the carbons of a double or triple bond do not have the maximum number
of attached atoms, they can add additional groups or atoms. Double bonds
undergo addition once and triple bonds can undergo addition twice. The
reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike
sigma bonds, pi-bonds are directed away from the carbons; the electrons are
loosely held, very accessible, and quite attractive electron-deficient species
(electrophiles) seeking an electron source.
Chapter 5 Reactions of Alkenes and Alkynes
106
5.1 Addition Reactions of Alkenes
A. General Reaction Equation for Addition to Alkenes
Alkenes add hydrogen halides, halogens (chlorine and
bromine), water (sulfuric acid catalyst), and hydrogen (metal catalyst).
One part of the adding reagent adds to each carbon of the double bond;
the double bond becomes a single bond during the process.
B. Mechanism of Electrophilic Addition
With the exception of hydrogenation, the addition reactions of alkenes
presented in this text occur by an electrophilic addition mechanism.
The electrophile (H+ or X+) attacks the electron-rich pi-bond of the
double bond. The pi electrons are used to form a single bond between the
carbon and attacking species; the other carbon becomes a carbocation.
The carbocation is then neutralized by halide ion or water; the addition is
complete. In bromination reactions, the bromine adds in a trans
fashion.
C. Orientation of Addition
When an unsymmetrical reagent adds to an unsymmetrical
alkene, two addition products are possible. When the electrophile bonds,
it can bond to either carbon of the carbon-carbon double bond to form two
different carbocations. The more stable carbocation is favored and the
addition product resulting from the more stable carbocation intermediate is
the predominant product.
The order of carbocation stability: 3o > 2o > 1o > methyl. A
tertiary carbocation has three bonded alkyl groups. Secondary
carbocations have two alkyl groups bonded directly to the carbocation
carbon and in primary carbocations there is only one. Since alkyl
groups are electron-releasing groups they stabilize the positive
carbocation. Tertiary carbocations have the greatest number of alkyl
groups and are the most stable.
Reactions in which one product predominates are termed
regioselective and those in which one is formed exclusively are
regiospecific. The electrophilic addition reactions in this chapter are
Reactions of Alkenes and Alkynes Chapter 5
107
usually regioselective and the rule for predicting the predominant product
is known as Markovnikov's rule.
5.2 Addition Reactions of Alkynes
A. General Reaction Equation for Addition to Alkynes
Alkynes add hydrogen, hydrogen halides, and halogens (chlorine
and bromine). They can add one mole of reagent to produce a double
bond or two moles to form a single bond.
B. Mechanism of Catalytic Hydrogenation
of Alkenes and Alkynes
Hydrogenation of alkenes and alkynes is accomplished in the
presence of a metal catalyst which attracts both the hydrogen and
hydrocarbon to its surface. As a result of the reactants being adsorbed
onto the same surface, the reaction occurs with cis addition.
C. Electrophilic Addition Mechanism for Alkynes
The mechanism of electrophilic addition to alkynes is the same as
with alkenes. Orientation of addition of unsymmetrical reagents to
unsymmetrical alkynes is determined by the stability of the intermediate
carbocation.
D. Addition of Water to Alkynes
Alkynes add water to form aldehydes and ketones.
5.3 Addition Polymers
A polymer is a giant molecule composed of a repeating structural unit
called a monomer. Addition polymers result from the addition of alkene
molecules to one another. The polymerization occurs by cationic, free-
radical, and anionic reaction mechanisms. Examples of addition polymers
include polyethylene, polystyrene, PVC, and Teflon.
Chapter 5 Reactions of Alkenes and Alkynes
108
A. Cationic Polymerization by Electrophilic Addition
In cationic polymerization, an electrophile (such as H+) adds to the
carbon-carbon double bond of a monomer to form the more stable
carbocation. The reaction conditions are such that there is relatively little
electrophile and corresponding carbocation neutralizing species. As a
result, the carbocation attacks the double bond of another monomer
molecule producing another carbocation that carries on the process until
the growing chain is eventually neutralized.
B. Polymerization by a Free-Radical Chain Reaction
In this mechanism of polymerization, a small amount of free radicals is
generated. These attack the carbon-carbon double bonds of monomer
molecules, bond to one carbon, and produce the more stable free radical;
this is the initiation step. Since few chains are initiated, the free radical
attacks yet another monomer, adds to the double bond, and forms another
free radical that, in turn, continues the process; this is propagation.
Eventually two developing free radical chains may bond together and
terminate the chain reaction.
CONNECTIONS 5.1 Serendipity in the Discovery of Polymers
CONNECTIONS 5.2 Recycling Plastics
5.4 Electrophilic Addition to Conjugated Dienes
Conjugated dienes are compounds in which two carbon-carbon double
bonds are separated by a single bond. Upon treatment with adding reagents,
conjugated dienes undergo 1,2-addition, in which the reagent adds to one of
the double bonds and 1,4-addition in which the reagent adds to the first and
fourth carbons with the remaining double bond shifting between carbons 2 and
3. This is caused by the formation of an allylic intermediate such as an allylic
carbocation. An allylic carbocation is one in which the carbocation carbon is
attached directly to a carbon-carbon double bond. Such a carbocation
engages in resonance allowing neutralization at the second and fourth carbons
of the original conjugated diene.
Reactions of Alkenes and Alkynes Chapter 5
109
Resonance forms are classical structures used to describe a more
complex system; they do not actually exist. The species is more accurately
described by a resonance hybrid which can be imagined as an average of
the resonance forms. Resonance always stabilizes a system. Each atom in a
resonance stabilized system has a p-orbital. Allylic carbocations are stabilized
by delocalization of the positive charge.
5.5 Resonance Stabilization of Reactive Intermediates
Allylic carbocations, free radicals, and carbanions are resonance
stabilized. In each case the stabilization is the result of delocalization of the
positive or negative charge or the free radical. Resonance forms differ in the
position of electrons and charge but not atoms. Every atom in an allylic
carbocation, free radical, or carbanion possesses a p-orbital and the pi-
electrons and charges or unpaired electrons are delocalized throughout these
orbitals.
5.6 Natural and Synthetic Rubber
Natural rubber is produced from a milky-white colloidal latex found in the
rubber tree. It is a polymeric terpene with isoprene being the recurring
polymeric unit. Polyisoprene rubber can also be produced synthetically by the
addition polymerization of isoprene by 1,4-addition. Other synthetic rubbers
include SBR (styrene-butadiene rubber), polybutadiene, and neoprene.
Rubber is strengthened, hardened, and made more elastic by a process called
vulcanization in which sulfur bridges form links within the polymeric chains.
These links become strained when the rubber is stretched and when released
the rubber assumes its original conformation.
CONNECTIONS 5.3 Terpenes
5.7 Oxidation of Alkenes
A. Hydroxylation with Potassium Permanganate
Treatment of alkenes with potassium permanganate produces 1,2-
diols in a cis configuration.
Chapter 5 Reactions of Alkenes and Alkynes
110
B. Ozonolysis
Ozonolysis cleaves the carbon-carbon double bond of an alkene to
form aldehydes and ketones.
5.8 Acidity of Terminal Alkynes
Terminal alkynes have weakly acidic hydrogens that can be abstracted by
strong bases such as sodium amide.
CONNECTIONS 5.4 The Treatment of Atherosclerosis
SOLUTIONS TO PROBLEMS
5.1 Addition and Elimination Reactions
H2C CH2
Br H
+ KOH H2C CH2
+ KBr + H2O
H2C CH2 + HBr H2C CH2
Br H
(a) Elimination
Addition
H2C CH2
OH H
H2SO4H2C CH2
+ H2O
H2C CH2 + H2O H2C CH2
OH H
(b) Elimination
AdditionH2SO4
Reactions of Alkenes and Alkynes Chapter 5
111
5.2 Addition Reactions of Alkenes
CH3CH CHCH3
CH3CH CHCH3 CH3CH CHCH3
Cl Cl
CH3CH CHCH3 CH3CH CHCH3
Br HCH3CH CHCH3 CH3CH CHCH3
HO H
CH3CH2CH2CH3Pt
+ H2a)
+ Cl2b)
+ HBrc)
H2SO4+ H2Od)
5.3 Addition Reactions
+ HCl
Cl H
(a)
CH2CH3CH2CH(b) + HBr CH2CH3CH2CH
HBr
CH2CH3CH2CH
BrH
or
5.4 Electrophilic Addition Mechanism
CH2CH3CH2CH
CH2CH3CH2CH
HBr
CH2CH3CH2CH
BrH
CH2CH3CH2CH
H
CH2CH3CH2CH
H
H+
H+
Br -
Br -
Chapter 5 Reactions of Alkenes and Alkynes
112
5.5 Halogenation of Alkenes
CH2CH3CH(a) + Cl2 CH2CH3CH
ClCl
H3C CH3
+ Br2
H3C Br
Br CH3(b)
5.6 Halogenation: Electrophilic Addition
CH2CH3CH(a) CH2CH3CH
ClCl
Cl +CH2CH3CH
Cl -
Cl
H3C CH3
(b) Br
5.7 Hydration of Alkenes
CHCH3CH3CH(a) + H2OH2SO4
CHCH3CH3CH
HOH
CHCH3CH3CH
CHCH3CH3CH
HOH
H+
CHCH3CH3CH
H
H2O
H
CHCH3CH3CH
HOH
- H+
(b)
Reactions of Alkenes and Alkynes Chapter 5
113
5.8 Carbocations
Arranged most to least stable:
CH3CCH3
CH3
CH3CHCH2CH3 CH3CHCH2
CH3
and CH3CH2CH2CH2> >
3O 2O 1O1O
most stable
5.9 Orientation of Addition
(a)
CH3CH2C CH2
CH3
CH3CH2C CH2
CH3
H
CH3CH2C CH2
CH3
HCl
CH3CH2C CH2
CH3
H
CH3CH2C CH2
CH3
ClH
H+
H+
+Cl -
predominantproduct
more stable3˚ carbocation
Cl -
+
less stable1˚ carbocation
(b)
CH3
CH3
H
CH3
OH
H
CH3 CH3
H
OH
H
+
+
H+
H2O - H+predominantproduct
more stable3˚ carbocation
less stable2˚ carbocation
- H+H2O
H+
Chapter 5 Reactions of Alkenes and Alkynes
114
5.10 Addition Reactions of Alkynes
CH3C CCH3 CH3C CCH3
Br Br
+ 1Br2(a)
CH3C CCH3 CH3C CCH3
Br Br
Br Br
+ 2Br2(b)
CH3C CCH3 CH3C CCH3
Cl Cl
(c) + 1Cl2
CH3C CCH3 CH3C CCH3
H H
H HNi
(d) + 2H2
5.11 Hydrogenation of Alkynes
CH3CH2C CCH3
C CCH3CH3CH2
HH
cisaddition
CH3CH2CH2CH2CH3Pt
Pt
2H2
1H2
5.12 Hydrogenation of Alkenes
+ H2
Pt
+ H2
Pt
H3C CH3 H3C CH3
HH
Reactions of Alkenes and Alkynes Chapter 5
115
5.13 Electrophilic Addition to Alkynes
CH3CH2CH2C CH CH3CH2CH2C CH
H
HBr
Br
+ 2HBr
CH3CH2CH2C CH CH3CH2CH2C CH
H
CH3CH2CH2C CH
HBr
CH3CH2CH2C CH
H
HBr
CH3CH2CH2C CH
H
HBr
Br
Reaction Mechanism
HBr adds totriple bond and then tothe resultingdouble bond.In each casethe more stablecarbocation is formed.
:Br:....
_
_
....
:Br: + H+
+
H+
5.14 Hydration of Alkynes
CHCH3C + H2OH2SO4
HgSO4CH3C CH2
OH CH3CCH3
O
enol ketone
5.15 Cationic Polymerization of Propene
CH2 CH
CH3
CH2 CH
CH3
CH2 CH+
CH3
.. ..H+ H..
CH2 CH CH2 CH+
CH3 CH3
CH2 CH
CH3
..H etc. etc.A -..
nH A
Chapter 5 Reactions of Alkenes and Alkynes
116
5.16 Free Radical Polymerization of 1,1-Dichloroethene
CH2 C
Cl
Cl
CH2 C
Cl
Cl
CH2 C
Cl
Cl
RO. .. ..RO..
ROOR RO +. .RO
CH2 C
Cl
Cl
CH2 C
Cl
Cl
CH2 C
Cl
Cl
.RO.. .. etc. etc.n
ORRO
5.17 1,2 and 1,4 Addition
CH2 CH CH CH2
CH2 CH CH
H
CH2
CH2 CH CH CH2
HBr
Br
+ 1HBr1,2 addition
1,4 addition
(a)
+ Br2
Br
Br
Br Br
1,2 addition
+
(b)
1,4 addition
Reactions of Alkenes and Alkynes Chapter 5
117
5.18 Electrophilic Addition Mechanism: 1,2 and 1,4 Addition
CH2 CH CH CH2
H
CH2 CH CH CH2
H
CH2 CH CH CH2
CH2 CH CH CH2
HBrCH2 CH CH CH2
HBr
Reaction Mechanism
++
H+
Resonance Forms
Br-
+
1,2 addition 1,4 addition
STEP 1: Electrophile, H+
is attracted to pi-cloud and usestwo pi-electrons to bond. Morestable allylic carbocation results.
STEP 2: The allylic carbocationis resonance stabilized. Resonanceforms show the two places it can beneutralized by bromide ion.
(a)
Br
Br
Br Br+
Reaction Mechanism
Br+
Resonance Forms
Br-
1,2 addition 1,4 addition
STEP 1: Electrophile, Br+
is attracted to pi-cloud and usestwo pi-electrons to bond. Morestable allylic carbocation results.
STEP 2: The allylic carbocationis resonance stabilized. Resonanceforms show the two places it can beneutralized by bromide ion.
(b)
Br Br
Chapter 5 Reactions of Alkenes and Alkynes
118
5.19 Resonance Forms, Hybrids, and Bonding Pictures
5.27 Reaction Mechanisms - Electrophilic Addition to Alkenes:
Section 5.1B
CH3CH CH2 CH3CH CH2
Br
CH3CH CH2
BrBr+
(a)Br Br
The carbocation in this case is actually a bromonium ion.
CH3C CHCH3
CH3
CH3C CHCH3
CH3
H
CH3C CHCH3
CH3
HCl+
H+(b)
Cl
Reactions of Alkenes and Alkynes Chapter 5
121
CH3 CH3 CH3Br
HH
+(c) H+ Br
CH3C CH2
CH3
CH3C CH2
CH3
H
CH3C CH2
CH3
HOH
CH3C CH2
CH3
HOH
- H+
+H
+
H+(d) H2O
5.28 Reaction Mechanisms - Electrophilic Addition to Alkynes
Section 5.2C
CH3C CCH3 CH3C CCH3
H
CH3C CCH3
HCl
CH3C CCH3
HCl
H
CH3C CCH3
HCl
HCl+ H+
+H+ Cl
Cl
5.29 Bromination: Section 5.1B2
Bromination involves cis addition due to an intermediate bromonium ion.Br
Br
+ Br2
5.30 Hydrogenation: Section 5.2B
CH3CHC CCH3
CH3
C CCH3CH CH3
HH
CH3
(a) Pd+ 1H2
CH3 CH3
CH3
H H
CH3+ 1H2Pd(b)
Chapter 5 Reactions of Alkenes and Alkynes
122
5.31 Reaction of Alkenes with Potassium Permanganate
H3C
KMnO4/H2O+
H3C H
OH OH
5.32 Hydration of Alkynes: Section 5.2D
CH
O
CH3CH2CCH3H2SO4HgSO4
+ H2O(a) CH3CH2C
CH3C CCH3
O
+ H2OH2SO4HgSO4
CH3CH2CCH3(b)
5.33 Electrophilic Addition to Conjugated Dienes: Section 5.4
CH2 C CH CH2
CH3
H+
- H+
H2O
resonance forms
+
(a)
1,2 addition 1,4 addition
H2C C
H
CH3
CH CH2H2C C
H
CH3
CH CH2
H2C C
H
CH3
CH CH2
OH
H2C C
H
CH3
CH CH2
OH
Reactions of Alkenes and Alkynes Chapter 5
123
Cl Cl
Cl ClCl
Cl
The electrophileattacks one of the double bonds toform an allyliccarbocation thatis described by two resonanceforms. Neutrali-zation forms two products. 1,2 addition 1,4 addition
resonance forms
+
+
+
Cl+(b)
Cl -
5.34 Resonance Forms and Resonance Hybrids: Section 5.5
CH3CH CH CH2 CH CH2CH3CH CH CH2CH3CH. ..
Resonance forms Resonance hybrid
(a)
CH2 C
O
CH2 C
O
CH2 C
O_
.. :.._
::..
_..
:..
(b)
H H H
CH3CH3 CH3 CH3(c)
+
+
++
O
(d)
O O O
Chapter 5 Reactions of Alkenes and Alkynes
124
5.35 Resonance Forms and Resonance Hybrids: Section 5.5
OO
O
OO
O
OO
O
OO
O
O
O
O
C
..-
C
2-
C
....
......
.. .. ....
....
..
....
C.. .. C
....
......
..
..- -
- -
-
5.36 Addition Polymers: Section 5.3
CH2 CF2 CH2 CH
Br
b)n
a)n
5.37 Oxidation of Alkenes: Section 5.7A
OHOH
CH3CHCH2KMnO4(a) CH3CH=CH2
OHOH
KMnO4(b)
5.38 Ozonolysis: Section 5.7B
Each place there is a carbon-carbon double bond it cleaves and each carbon
becomes a carbon-oxygen double bond.
Reactions of Alkenes and Alkynes Chapter 5
125
O
HCCH2CH2C CH
O O
O
O
CH3
CH3
C
CH2CH
O
CH3
O
CH3C CCH2CHCH2CH
O O O
CCH3
O
O
HCHc)
b)
2 HCHCH3CCH3a)
5.39 Ozonolysis: Section 5.7B
Since all of the examples are hydrocarbons, each place you see a carbon-
oxygen double bond, you are looking at a carbon that originally was involved in
Pay attention to orientation of addition as explained in Section 5.1C.
5.43 Reaction Mechanism: Section 5.1
OH
CH3
O
CH3 CH3CH2
OH
CH3 CH3
OH
CH3 CH3
:- H++..:..
:..
:+H+
5.44 Hydrogenation: Section 5.2B
Cis addition occurs.
CH3CH2C CCH3 C CCH3CH3CH2
H H
CH3CH2CHC CCH3
CH3
C CCH3CH3CH2CH
H H
CH3
(b) + 1H2
Pt
Pt(a) + 1H2
5.45 Reactions of Alkynes: Section 5.2
CH3CH2CH2C CH
Br
Br
+ 2HBr(a) CH3CH2CH2CCH3
CH3 (a) CH3CH2CH2CH=CH2 (b)
Reactions of Alkenes and Alkynes Chapter 5
127
Cl
Cl
CH3CH2C CCH3 + 2HCl CH3CH2CCH2CH3(b)
CH3CH2C CCH3CH3CH2C CCH3
Cl
Cl
Cl
Cl
(c) + 2Cl2
5.46 Units of Unsaturation: Sections 3.6, 5.1A.2, 5.2A
1-Buten-3-yne has one triple bond and one double bond. This represents three
units of unsaturation. One mole of the compound will add three moles of
bromine, one mole to the double bond and two to the triple bond.
5.47 Units of Unsaturation: Sections 3.6, 5.1A.4, 5.2A-B
Since the compound is non-cyclic all the units of unsaturation must be in the
form of carbon-carbon double bonds or triple bonds. Four mole-equivalents of
hydrogen are consumed so there must be four units of unsaturation: four double
bonds, two triple bonds, or one triple and two double bonds.
hydrogenation product starting material C8H10 + 4H2 C8H18
5.48 1,4 Addition: Section 5.4
5.49 Allylic Carbocations: Section 5.4-5.5
The three resonance forms show where this resonance stabilized carbocation
can be neutralized.
Chapter 5 Reactions of Alkenes and Alkynes
128
Br
Br
Br
resonance forms products
Br -
CH3CH=CH-CH=CH-CHCH2CH3
CH3CH=CH-CH-CH=CHCH2CH3
CH3CH-CH=CH-CH=CHCH2CH3+
+
CH3CH-CH=CH-CH=CHCH2CH3
CH3CH=CH-CH-CH=CHCH2CH3
CH3CH=CH-CH=CH-CHCH2CH3+
ACTIVITIES WITH MOLECULAR MODELS
1. Make molecular models of ethene and ethyne. Now convert these to theproducts formed when bromine (Br2) adds to the double bonds and triplebonds to form single bonds. How many bromines are needed to convert adouble bond to a single bond and a triple bond to a single bond? Howmany bromines are in your products and to which carbons did they add?
+ Br2
2Br2
Reactions of Alkenes and Alkynes Chapter 5
129
2. Make molecular models of 1-butene and 2-butene (cis or trans). Makemodels of the one product formed from the addition of HBr to 2-butene and thetwo products formed from 1-butene. Why is there a difference in the number ofaddition products. Which product predominates in the addition to 1-butene?
HBr
HBr
majorproduct
3. Make a model of 2-butyne and the product of cis addition of hydrogen.
H2
Pt
Chapter 5 Reactions of Alkenes and Alkynes
130
4. Make a model of cyclopentene and the product of trans addition of bromine.
