ch #12 alkenes and alkynes. alkene introduction hydrocarbon with carbon-carbon double bonds...
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Ch #12 Alkenes and Alkynes
Alkene Introduction• Hydrocarbon with carbon-carbon double bonds• Sometimes called olefins, “oil-forming gas”• General formula CnH2n n≥2• Examples
n=2 C2H4
Common Names
Usually used for small molecules.Examples:
CH2 CH2
ethylene
CH2 CH CH3
propylene
CH2 C CH3
CH3
isobutylene
Vinyl carbons are the carbons sharing a double bond
Vinyl hydrogens are the hydrogens bonded to vinyl carbons
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
E-Z Nomenclature• Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond.• 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:
Anise oilBay leaves
Terpenes
Terpenes
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 TerpenesTwo or more isoprene units, 2-methyl-1,3-butadiene with some modification of the double bonds.
myrcene, frombay leaves
=>
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.
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. 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
Answer
a. CH3COOH
b.CH3CHO
c. CH3CH2OH
d.HOCH2CH2OH
e.CH3CH(OH)2
Ethylene oxide is formed first, followed by a ring opening to form ethylene glycol.
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.
a. [CH2CH(CH3)]n
b.[CH2CH2]n
c. [CH2=CH(CH3)]n
d.[CH2=CH2]n
CC
H
H CH3
H
polymerize
Answer
a. [CH2CH(CH3)]n
b.[CH2CH2]n
c. [CH2=CH(CH3)]n
d.[CH2=CH2]n
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.
7.15
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.
7.16 Dehydration of alcohols occurs by what mechanism?
a. SN1
b.SN2
c. E1d.E2
7.16 Answer
a. SN1
b.SN2
c. E1d.E2
The dehydration of alcohols occurs by an E1 mechanism.
7.17 Give the products from the catalytic cracking of alkanes.
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
7.17 Answer
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
7.18 Give the products from the dehydrogenation of alkanes.
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
7.18 Answer
a. Alkanesb.Alkenesc. Alkynesd.Alkanes + alkenese.Alkanes + alkynes
7.19
a. (CH3)3CO-, (CH3)3COH
b.CH3CH2O-, CH3CH2OH
c. NaI, acetoned.H2, Pd
CH3CHCH2CH3
Cl
CHCH2CH3H2C
?
7.19 Answer
a. (CH3)3CO-, (CH3)3COH
b.CH3CH2O-, CH3CH2OH
c. NaI, acetoned.H2, Pd
The Hofmann product (least substituted) is favored with a bulky base.
7.20
a. Pt, 500o Cb.H2, Pt
c. H2SO4, 150o C
d.NaI, acetonee.NaOH
CH3CH2CH2CH3
CHCH3CH3CH
CHCH2CH3H2C
CHCHH2C CH2
?
+
+
7.20 Answer
a. Pt, 500o Cb.H2, Pt
c. H2SO4, 150o C
d.NaI, acetonee.NaOH
Dehydrogenation occurs with a metal catalyst and heat.
End Chapter #3