Transcript Document

Alkenes, Reactions
R-H
R-X
R-OH
R-O-R
Alkenes
NR
NR



NR

 some
NR
NR
Metals
NR


NR
NR
Oxidation
NR
NR

NR

Reduction
NR

NR
NR

Halogens
NR
NR
NR
NR

Acids
Bases
Alkenes, reactions.
Addition
ionic
free radical
Reduction
Oxidation
Substitution
Reactions, alkenes:
1. Addition of hydrogen (reduction).
2. Addition of halogens.
3. Addition of hydrogen halides.
4. Addition of sulfuric acid.
5. Addition of water (hydration).
6. Addition of aqueous halogens (halohydrin
formation).
7. Oxymercuration-demercuration.
8. Hydroboration-oxidation.
9. Addition of free radicals.
10. Addition of carbenes.
11. Epoxidation.
12. Hydroxylation.
13. Allylic halogenation
14. Ozonolysis.
15. Vigorous oxidation.
1. Addition of hydrogen (reduction).
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— C = C — + H2
+
Ni, Pt, or Pd
|
|
 —C—C—
|
|
H
H
a) Requires catalyst.
b) #1 synthesis of alkanes
CH3CH=CHCH3
2-butene
+
H2, Ni 
CH3CH2CH2CH3
n-butane
Alkanes
Nomenclature
Syntheses
1. addition of hydrogen to an alkene
2. reduction of an alkyl halide
a) hydrolysis of a Grignard reagent
b) with an active metal and acid
3. Corey-House Synthesis
Reactions
1. halogenation
2. combustion (oxidation)
3. pyrolysis (cracking)
heat of hydrogenation:
CH3CH=CH2 + H2, Pt  CH3CH2CH3 + ~ 30 Kcal/mole
ethylene
32.8
propylene
30.1
cis-2-butene
28.6
trans-2-butene
27.6
isobutylene
28.4
fats & oils: triglycerides
O
CH2—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3
|
O
CH—O—CCH2CH2CH2CH2CH2CH2CH2CH2CH3
|
O
CH2—O—CCH2CH2CH2CH2CH2CH2CH3
“saturated” fat
O
CH2—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3
|
O
CH—O—CCH2CH2CH2CH2CH=CH2CH2CH2CH3
|
O
CH2—O—CCH2CH2CH=CHCH2CH2CH3
Ω - 3 “unsaturated” oil
Saturated triglycerides are solids at room temperature and
are called “fats”.
butter fat, lard, vegetable shortening, beef tallow, etc.
Unsaturated triglycerides have lower mp’s than saturated
triglycerides. Those that are liquids at room temperature
are called “oils”. (All double bonds are cis-.)
corn oil, peanut oil, Canola oil, cottonseed oil, etc.
polyunsaturated oils + H2, Ni  saturated fats
liquid at RT
solid at RT
oleomargarine
butter substitute
(dyed yellow)
Trans-fatty acids formed in the synthesis of margarine have
been implicated in the formation of “bad” cholesterol,
hardening of the arteries and heart disease. 
2. Addition of halogens.
| |
—C=C—
+
X2
|
|
 —C—C—
|
|
X
X
a) X2 = Br2 or Cl2
b) test for unsaturation with Br2
CH3CH2CH=CH2
1-butene
+
Br2/CCl4 
CH3CH2CHCH2
Br Br
1,2-dibromobutane
3. Addition of hydrogen halides.
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|
|
— C = C — + HX  — C — C —
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|
H X
a) HX = HI, HBr, HCl
b) Markovnikov orientation
CH3CH=CH2
CH3
CH2C=CH2
+
+
HI
HBr


CH3CHCH3
I
CH3
CH3CCH3
Br
Markovnikov’s Rule:
In the addition of an acid to an alkene the
hydrogen will go to the vinyl carbon that already
has the greater number of hydrogens.
CH3CH2CH=CH2
CH3
CH3CH=CCH3
CH3CH=CHCH3
+
+ HBr
+

HCl
HI


CH3CH2CHCH3
Cl
CH3
CH3CH2CCH3
Br
CH3CH2CHCH3
I
An exception to Markovikov’s Rule:
CH3CH=CH2
CH3
CH3C=CH2
+
+
HBr, peroxides  CH3CH2CH2Br
HBr, peroxides 
“anti-Markovnikov” orientation
note: this is only for HBr.
CH3
CH3CHCH2Br
Markovnikov doesn’t always correctly predict the product!
CH3
CH2=CHCHCH3
+
HI

CH3
CH3CH2CCH3
I
Rearrangement!

Ionic electrophilic addition mechanism
1)
2)
C C
C C
Y
+
YZ
+ Z
RDS
C C
Y
C C
Z Y
+ Z
mechanism for addition of HX
1)
2)
C C
C C
H
+
HX
+ X
RDS
C C
H
C C
X H
+ X
why Markovinkov?
CH3CH=CH2 + HBr
 CH3CHCH2
| 
H
or?
+ Br-

CH3CHCH2
 |
H
CH3CHCH3
|
Br
1o carbocation

2o carbocation
more stable
In ionic electrophilic addition to an
alkene, the electrophile always adds to
the carbon-carbon double bond so as to
form the more stable carbocation.
4. Addition of sulfuric acid.
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—C=C—
+
H2SO4
|
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 —C—C—
|
|
H
OSO3H
alkyl hydrogen sulfate
Markovnikov orientation.
CH3CH=CH2
+
H2SO4

CH3CHCH3
O
O-S-O
OH
5. Addition of water.
|
|
—C=C— +
H2O, H+
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 —C—C—
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|
H
OH
a) requires acid
b) Markovnikov orientation
c) low yield 
CH3CH2CH=CH2
+
H2O, H+
 CH3CH2CHCH3
OH
Mechanism for addition of water
RDS
1)
H
+
2)
C C
H
3)
C C
H OH2
C C
C C
H
+ H2O
C C
H OH2
C C
H OH
+ H
| |
— C = C — + H2O
H+

|
|
—C—C—
|
|
OH H
Mechanism for addition of water to an alkene to form an
alcohol is the exact reverse of the mechanism (E1) for the
dehydration of an alcohol to form an alkene.
Mechanism for dehydration of an alcohol = E1
1)
C C
H OH
+ H
C C
H OH2
RDS
2)
C C
H OH2
C C
H
+ H2O
3)
C C
H
C C
+ H
mechanism for addition of X2
1)
2)
C C
C C
X
+
X--X
+ X
RDS
C C
X
C C
X X
+ X
How do we know that the mechanism isn’t this way?
RDS
C C
X X
One step, concerted, no carbocation
C C
X X
CH3CH=CH2 + Br2 + H2O + NaCl
CH3CH=CH2 + Br--Br
Br
CH3CHCH2
Br Br

CHCHCH2
Br
H2O
+ CH3CHCH2
OH Br
Cl
+ CH3CHCH2
Cl Br
Some evidence suggests that the intermediate is not a
normal carbocation but a “halonium” ion:
|
|
—C—C—
Br 
The addition of X2 to an alkene is an anti-addition.
6. Addition of halogens + water (halohydrin formation):
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|
|
— C = C — + X2, H2O  — C — C — + HX
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OH X
a) X2 = Br2, Cl2
b) Br2 = electrophile
CH3CH=CH2
+
Br2(aq.) 
CH3CHCH2 + HBr
OH Br
mechanism for addition of X2 + H2O
1)
C C
2)
H2O +
3)
+
X--X
C C
X
RDS
C C
X
C C
H2O X
-H
C C
H2O X
C C
HO X
+ X
7. Oxymercuration-demercuration.
| |
|
|
— C = C — + H2O, Hg(OAc)2  — C — C — + acetic
|
|
acid
OH HgOAc
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— C — C — + NaBH4 
|
|
OH HgOAc
|
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—C—C—
|
|
OH H
alcohol
oxymercuration-demercuration:
a) #1 synthesis of alcohols.
b) Markovnikov orientation.
c) 100% yields. 
d) no rearrangements 
CH3CH2CH=CH2 + H2O, Hg(OAc)2; then NaBH4 
CH3CH2CHCH3
OH
With alcohol instead of water:
alkoxymercuration-demercuration:
| |
|
|
— C =C — + ROH, Hg(TFA)2  — C — C —
|
|
OR HgTFA
|
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— C — C — + NaBH4 
|
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OR HgTFA
|
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—C—C—
|
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OR H
ether
alkoxymercuration-demercuration:
a) #2 synthesis of ethers.
b) Markovnikov orientation.
c) 100% yields. 
d) no rearrangements 
CH3CH=CH2 + CH3CHCH3, Hg(TFA)2; then NaBH4 
OH
CH3 CH3
CH3CH-O-CHCH3
diisopropyl ether
Avoids the elimination with 2o/3o RX in Williamson Synthesis.
Ethers
nomenclature
syntheses
1. Williamson Synthesis
2. alkoxymercuration-demercuration
reactions
1. acid cleavage
8. Hydroboration-oxidation.
| |
|
|
— C = C — + (BH3)2  — C — C —
|
|
diborane
H
B—
|
|
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— C — C — + H2O2, NaOH 
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|
H
B—
|
|
|
—C—C—
|
|
H
OH
alcohol
hydroboration-oxidation:
a) #2 synthesis of alcohols.
b) Anti-Markovnikov orientation. 
c) 100% yields. 
d) no rearrangements 
CH3CH2CH=CH2 + (BH3)2; then H2O2, NaOH 
CH3CH2CH2CH2-OH
CH3
CH3C=CH2
+ H2O, Hg(OAc)2; then NaBH4 
Markovnikov
CH3
CH3C=CH2
CH3
CH3CCH3
OH
+ (BH3)2; then H2O2, NaOH 
anti-Markovnikov
CH3
CH3CHCH2
OH
Alcohols:
nomenclature
syntheses
1. oxymercuration-demercuration
2. hydroboration-oxidation
3.
4. hydrolysis of a 1o / CH3 alcohol
5.
6.
8.
9. Addition of free radicals.
| |
|
|
— C = C — + HBr, peroxides  — C — C —
|
|
H
X
a) anti-Markovnikov orientation.
b) free radical addition
c) HI, HCl + Peroxides
CH3CH=CH2 + HBr, peroxides  CH3CH2CH2-Br
Mechanism for free radical addition of HBr:
Initiating steps:
1) peroxide  2 radical•
2) radical• + HBr  radical:H + Br• (Br• electrophile)
Propagating steps:
3) Br• + CH3CH=CH2  CH3CHCH2-Br (2o free radical)
•
4) CH3CHCH2-Br + HBr  CH3CH2CH2-Br + Br•
•
3), 4), 3), 4)…
Terminating steps:
5) Br• + Br•  Br2
Etc.
In a free radical addition to an alkene, the electrophilic free
radical adds to the vinyl carbon with the greater number of
hydrogens to form the more stable free radical.
In the case of HBr/peroxides, the electrophile is the
bromine free radical (Br•).
CH3CH=CH2 + HBr, peroxides  CH3CH2CH2-Br
10. Addition of carbenes.
| |
|
|
— C = C — + CH2CO or CH2N2 , hν  — C — C —

•CH2•
| |
—C=C—
 
•CH2•
CH2
“carbene” adds across the
double bond
| |
— C = C — + CHCl3, t-BuOK 
 -HCl
|
|
— C— C —
CCl2
•CCl2• dichlorocarbene
| |
—C=C—
 
•CCl2•
hv
CH3CH=CH2 + CH2N2
CHCl3, t-BuOK
H
H3C C CH2
C
H2
H
H3C C CH2
C
Cl2
11.
Epoxidation.
| |
C6H5CO3H
— C = C — + (peroxybenzoic acid) 
|
|
— C— C —
O
epoxide
Free radical addition of oxygen diradical.
| |
—C=C—
 
•O•
H3C C C CH3
H H
2-butene
+
C6H5CO3H
peroxybenzoic acid
H H
H3C C C CH3
O
12. Hydroxylation. (mild oxidation)
| |
— C = C — + KMnO4
|
|
 —C—C—
|
|
OH OH
syn
OH
| |
|
|
— C = C — + HCO3H  — C — C — anti
peroxyformic acid
|
|
OH
glycol
CH3CH=CHCH3 + KMnO4  CH3CH-CHCH3
OH OH
2,3-butanediol
test for unsaturation purple KMnO4  brown MnO2
CH2=CH2
+
KMnO4
 CH2CH2
OH OH
ethylene glycol
“anti-freeze”
13. Allylic halogenation.
| |
|
| |
|
— C = C — C — + X2, heat  — C = C — C — + HX
|
|
H  allyl
X
CH2=CHCH3 + Br2, 350oC  CH2=CHCH2Br + HBr
a) X2 = Cl2 or Br2
b) or N-bromosuccinimide (NBS)
CH2=CHCH3 + Br2  CH2CHCH3
Br Br
addition
CH2=CHCH3 + Br2, heat  CH2=CHCH2-Br + HBr
allylic substitution
14. Ozonolysis.
| |
|
|
— C = C — + O3; then Zn, H2O  — C = O + O = C —
used for identification of alkenes
CH3
CH3CH2CH=CCH3 + O3; then Zn, H2O 
CH3CH2CH=O
+
CH3
O=CCH3
15. Vigorous oxidation.
=CH2 + KMnO4, heat  CO2
=CHR + KMnO4, heat  RCOOH
=CR2 + KMnO4, heat  O=CR2
carboxylic acid
ketone
CH3CH2CH2CH=CH2 + KMnO4, heat 
CH3CH2CH2COOH + CO2
CH3
CH3
CH3C=CHCH3 + KMnO4, heat  CH3C=O + HOOCCH3
KMnO4  CH3CHCHCH3
OHOH
mild oxidation  glycol
CH3CH=CHCH3
+
CH3CH=CHCH3
+
vigorous oxidation
hot KMnO4  2 CH3COOH
Reactions, alkenes:
1. Addition of hydrogen
2. Addition of halogens
3. Addition of hydrogen halides
4. Addition of sulfuric acid
5. Addition of water/acid
6. Addition of halogens & water (halohydrin formation)
7. Oxymercuration-demercuration
8. Hydroboration-oxidation
9. Addition of free radicals
10. Addition of carbenes
11. Epoxidation
12. Hydroxylation
13. Allylic halogenation
14. Ozonolysis
15. Vigorous oxidation
CH3
CH3C=CH2
isobutylene
“
“
“
+ H2, Pt 
CH3
CH3CHCH3
CH3
+ Br2/CCl4  CH3C-CH2
Br Br
+ HBr
+ H2SO4


CH3
CH3CCH3
Br
CH3
CH3CCH3
O
SO3H
CH3
CH3C=CH2
isobutylene
“
+ H2O, H+ 
CH3
CH3CCH3
OH
CH3
+ Br2(aq.)  CH3C-CH2Br
OH
CH3
CH3
CH3C=CH2 + H2O,Hg(OAc)2; then NaBH4  CH3CCH3
OH
“
+ (BH3)2; then H2O2, OH- 
CH3
CH3CHCH2
OH
CH3
CH3C=CH2
isobutylene
“
“
+ HBr, peroxides 
CH3
CH3CHCH2
Br
+ CH2CO, hv 
CH3
CH3C–CH2
CH2

CH3
CH3C–CH2
O
+ PBA
CH3
CH3C=CH2
isobutylene
“
“
“
+ KMnO4 
CH3
CH3C–CH2
OH OH
CH3
+ Br2, heat  CH2C=CH2 + HBr
Br
CH3
+ O3; then Zn/H2O  CH3C=O + O=CH2
+ KMnO4, heat 
CH3
CH3C=O + CO2