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Alkenes and AlkynesChapter #3
Alkene Introduction• Hydrocarbon with carbon-carbon double bonds• Sometimes called olefins, “oil-forming gas”• General formula CnH2n n≥2• Examples
n=2 C2H4
Common NamesUsually used for small molecules.Examples:
Vinyl carbons are the carbons sharing a double bond in blue
Vinyl hydrogens are the hydrogens bonded to vinyl carbons in red
CH2=CH2 CH2=CH-CH3 CH2=C-CH3
ethylene propylene isobutylene
CH3
IUPAC Nomenclature
• Parent is longest chain containing the double or triple bond.• -ane changes to –ene (or -diene, -triene) for double bonds, or –yne (or –diyne, -triyne).• Number the chain so that the double bond, or triple bond has the lowest possible number.• In a ring, the double bond is assumed to be between carbon 1 and carbon 2.
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C1-butene
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C
1-butene
2-methyl-2-butene
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C
1-butene
2-methyl-2-butene
3-methylcyclopentene
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C
1-butene
2-methyl-2-butene
3-methylcyclopentene
2-sec-butyl-1,3-cyclohexadiene
Name These Alkenes
CH2 CH CH2 CH3
CH3 C
CH3
CH CH3
CH3
CHCH2CH3H3C
1-butene
2-methyl-2-butene
3-methylcyclopentene
2-sec-butyl-1,3-cyclohexadiene
3-n-propyl-1-heptene
Alkene Substituents= CH2
methylene
- CH = CH2
vinyl
- CH2 - CH = CH2
allyl
- CH2 - CH = CH2
allyl
Name = ?
Alkene Substituents= CH2
methylene
- CH = CH2
vinyl
- CH2 - CH = CH2
allyl
- CH2 - CH = CH2
allyl
Name = Methylenecyclohexane Name =
Alkene Substituents= CH2
methylene
- CH = CH2
vinyl
- CH2 - CH = CH2
allyl
Name = Methylenecyclohexane Name = vinylcyclohexane
Alkyne Common Names
• Acetylene is the common name for the two carbon alkyne.
• To give common names to alkynes having more than two carbons, give alkyl names to the carbon groups attached to the vinyl carbons followed by acetylene.
Alkyne Examples
Alkyne Examples
Isopropyl methyl acetylene
Alkyne Examples
Isopropyl methyl acetylene sec-butyl Cyclopropyl acetylene
Cis-trans Isomerism• Similar groups on same side of double bond, alkene is cis.
• Similar groups on opposite sides of double bond, alkene is trans.
• Cycloalkenes are assumed to be cis.
• Trans cycloalkenes are not stable unless the ring has at least 8 carbons.
Name these:
C CCH3
H
H
CH3CH2
Name these:
C CCH3
H
H
CH3CH2
trans-2-pentene
Name these:
C CCH3
H
H
CH3CH2
trans-2-pentene
C CBr
H
Br
H
Name these:
C CCH3
H
H
CH3CH2
trans-2-pentene
C CBr
H
Br
H
cis-1,2-dibromoethene
Which of the following show cis/trans isomers?
a. 1-penteneb. 2-pentenec. 1-chloro-1-pentened. 2-chloro-1-pentenee. 2-chloro-2-pentene
Solution to the Question
H
H
H
Hydrogens cannon be both cis and trans. Reversing the hydrogens on the first carbon produces the same compound.
H
H
H
1-Pentene
H
2-Pentene does show cis/trans isomerism
H
H
2-Pentene
H
H
1-Chloro-1-pentene does show cis/trans isomerism
H
1-chloro-1-pentene
Cl
H
H
Cl cis trans
Solution to the QuestionCl
2-Chloro-1-pentene does not show cis/trans isomerism, since flipping the hydrogen atoms in C-1 produces the same compound
Cl
2-chloro-1-pentene
H
H
H
H
Cl
2-Chloro-2-pentene does show cis/trans isomerism, since since the colored atoms on the vinyl carbons are on the same side in cis and opposite sides on the trans isomer.
Cl
2-chloro-2-pentene
H
Htrans cis
Which of the following show cis/trans isomers?
a. 1-pentene-Nob. 2-pentene- Yesc. 1-chloro-1-pentene- Yesd. 2-chloro-1-pentene- Noe. 2-chloro-2-pentene- yes
E-Z Nomenclature• Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond. Highest priority is #1 and is the element with the largest atomic number.• If high priority groups are on the same side, the name is Z (for zusammen).• If high priority groups are on opposite sides, the name is E (for entgegen).
Example, E-Z
C C
H3C
H
Cl
CH2C C
H
H
CH CH3
Cl1
2
1
2
2
1
1
2
2Z 5E
Example, E-Z
C C
H3C
H
Cl
CH2C C
H
H
CH CH3
Cl1
2
1
2
2
1
1
2
2Z 5E
3,7-dichloro-(2Z, 5E)-2,5-octadiene
Physical Properties• Low boiling points, increasing with mass.
• Branched alkenes have lower boiling points.
• Less dense than water.
•Nonpolar (Hydrophobic)
Alkene Synthesis
• Dehydrohalogenation (-HX)• Dehydration of alcohols (-H2O)
OHH + H2O
minor major
Examples:Cl
NaOHminor major
+ + + NaCl + HOH
Zaitsev’s rule: The major product contains the most substituted double bond
Elimination Reactions:
Alkene ReactionsI. Addition Reactions
C=C
a. Hydration
C-C+ H-O-H
C=C
C=C
H O-H
b. Hydrogenation
C-C+ H-H
HH
c. Halogenation
+ X-X
Catalyst
H+
Catalyst = Ni, Pt, Pd
C-C
X X
Alcohol
Alkane
X = Cl, Br, IDihalide
Follows Markovnikov’s Rule
RegiospecificityMarkovnikov’s Rule: The proton (H+) of an acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.”
C=C
Examples:
CH3H
H
HH
C=CH CH3
H
+ H-O-HH+
+ H-Cl
H
C-CH
H Cl
H
H
C-CH
H O-H
H
CH3
CH3Major Products
Alkene Reactions (2)I. Addition Reactions (cont.)
d. Hydrohalogenation
C=C C-C+ H-X
C=C
H X
e. Glycol Formation
+ H-O-O-H C-C
H-O O-H
Alkyl halide
Glycol
Follows Markovnikov’s Rule
Alkene ReactionsStep 1: Pi electrons attack the electrophile.
Step 2: Nucleophile attacks the carbocation
Terpenes• Composed of 5-carbon isopentyl groups.• Isolated from plants’ essential oils.• C:H ratio of 5:8, or close to that.• Pleasant taste or fragrant aroma.• Examples:
Myrcene (From bay or myrcia plants)α-Pinene (From pine trees)Β-Selinene (From celery)Menthol (From peppermint oil)Camphor (From evergreen trees)R-Carvone (From spearmint)
Classification
• Terpenes are classified by the number of carbons they contain, in groups of 10.• A monoterpene has 10 C’s, 2 isoprenes. • A diterpene has 20 C’s, 4 isoprenes.• A sesquiterpene has 15 C’s, 3 isoprenes.
Terpenes
2-methyl-1,3-butadieneIsoprene
headtail
head
tail
head
Geraniol (roses)Head to tail link of two isoprenesCalled diterpene
OH
head
tail
head
tail
Menthol (pepermint)Head to tail link of two isoprenes another diterpene
Structure of Terpenes
Two or more isoprene units, 2-methyl-1,3-butadiene with some modification of the double bonds.
myrcene, frombay leaves
oCamphor (monoterpene)
ALKENE REVIEW
Describe the geometry around the carbon–carbon double bond.
a. Tetrahedralb.Trigonal pyramidalc. Trigonal planard.Bente.Linear
Answer
a. Tetrahedralb.Trigonal pyramidalc. Trigonal planard.Bente.Linear
Give the formula for an alkene.
a. CnH2n-4
b.CnH2n-2
c. CnH2n
d.CnH2n+2
e.CnH2n+4
Answer
a. CnH2n-4
b.CnH2n-2
c. CnH2n
d.CnH2n+2
e.CnH2n+4
Name CH3CH=CHCH=CH2.
a. 2,4-butadieneb.1,3-butadienec. 2,4-pentadiened.1,3-pentadienee.1,4-pentadiene
Answer
a. 2,4-butadieneb.1,3-butadienec. 2,4-pentadiened.1,3-pentadienee.1,4-pentadiene
Calculate the unsaturation number for C6H10BrCl.
a. 0b.1c. 2d.3
Answer
a. 0b.1c. 2d.3
U = 0.5 [2(6) + 2 – (12)] = 1
Name .
a. Trans-2-penteneb. Cis-2-pentenec. Trans-3-methyl-2-pentened. Cis-3-methyl-2-pentene
CC
H
H3C CH3
CH2CH3
Name
a. Trans-2-penteneb. Cis-2-pentenec. Trans-3-methyl-2-pentened. Cis-3-methyl-2-pentene
CC
H
H3C CH3
CH2CH3
Name
a. E-2-penteneb. Z-2-pentenec. E-3-methyl-2-pentened. Z-3-methyl-2-pentenee. Z-2-methyl-2-pentene
CC
H
H3C CH3
CH2CH3
Name
a. E-2-penteneb. Z-2-pentenec. E-3-methyl-2-pentened. Z-3-methyl-2-pentenee. Z-2-methyl-2-pentene
CC
H
H3C CH3
CH2CH3
a. ClCH2CH2Cl
b.ClCH=CHClc. CH2=CH2
d.CH2=CHCl
CC
H
H H
H
Cl2 NaOH
Answer
a. ClCH2CH2Cl
b.ClCH=CHClc. CH2=CH2
d.CH2=CHCl
Chlorine is added across the double bond, then HCl is lost.
a. (CH3)2CHOH
b.CH3CH2CH2OH
c. HOCH2CH2CH2OH
d.CH3CH(OH)CH2OH
CC
H
H CH3
H
H2O
catalyst
Answer
a. (CH3)2CHOH
b.CH3CH2CH2OH
c. HOCH2CH2CH2OH
d.CH3CH(OH)CH2OH
Water adds by Markovnikov’s orientation across the double bond.
Identify the product formed from the polymerization of tetrafluoroethylene.
a. Polypropyleneb.Poly(vinyl chloride), (PVC)c. Polyethylened.Poly(tetrafluoroethylene), Teflon
Answer
a. Polypropyleneb.Poly(vinyl chloride), (PVC)c. Polyethylened.Poly(tetrafluoroethylene), Teflon
Teflon is formed from the polymerization of tetrafluoroethylene.
a. CH3CCCH3
b.CH2=CHCH=CH2
c. CH3CH=CHCH3
d.CH3CH2CH2CH3
CC
H
H3C CH3
H
H2
Pd
Answer
a. CH3CCCH3
b.CH2=CHCH=CH2
c. CH3CH=CHCH3
d.CH3CH2CH2CH3
Hydrogen adds across the double bond to form an alkane.
a. (CH3)2CHOSO3H
b.CH3CH=CH2
c. (CH3)2C=O
d.CH3CH2COOH
HCH3C
OH
CH3
H2SO4
heat
7.15 Answer
a. (CH3)2CHOSO3H
b.CH3CH=CH2
c. (CH3)2C=O
d.CH3CH2COOH
Acid dehydrates alcohols to form alkenes.
Give the products from the catalytic cracking of alkanes.
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
Answer
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
Give the products from the dehydrogenation of alkanes.
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
Give the products from the dehydrogenation of alkanes.
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
End Chapter #3