Transcript Unit 4 Chemical Kinetics and Chemical Equilibrium

Synthesis of Alkenes

Major approaches to the synthesis of alkenes:
 Elimination of Alkyl Halides
 Dehydrohalogenation
 E2 mechanism
 E1 mechanism
 Dehalogenation of Vicinal Dibromides

Dehydration of Alcohols
(CH3CH2)3N
Synthesis of Alkenes

Dehydrohalogenation can occur via either an
E2 or E1 mechanism.
+
 Loss of H and X ions from adjacent
carbons, forming a new pi bond
CH3
CH3CCH2CH3
Br
NaOH
D
H3C
H3C
H
C C
CH3
Synthesis of Alkenes

The most synthetically useful
dehydrohalogenation reactions occur under
E2 reaction conditions.
o
o
 3 or bulky 2 alkyl halide
 strong bases
 strong bulky bases are best when using
2o alkyl halides
 less likely to undergo substitution
reactions
(CH3)2CH
Synthesis of Alkenes
N H
(CH
)
CH
3
2
or Et N
(CH3CH2)3N

3
Common strong bulky bases
(CH3)2CH
(CH3)2CH N H
(CH3CH2)3N or Et3N
triethylamine
(CH3)2CH
CH3
N or
H t-Bu
(CH3CH
)2CH
CCH
3
3
diisopropylamine
O
(CH3)2CH
CH3
N or H
CH3CH
)2CH
t-BuO
CCH
3
3
-
-
O
t-butoxide ion
(CH3CH2)3N or E
H3C
H
C CCH3
CHCH
H3CH3CCH NCCH
3 3 or t-B
3
3
-
O
2,6-dimethylpyridine
Synthesis of Alkenes

Mechanism of E2 Dehydrohalogenation
 concerted reaction

anti-coplanar transition state
Synthesis of Alkenes

E2 elimination reactions can take place in
cyclohexanes only when proton and leaving
group can get into a trans-diaxial arrangement
 corresponds to anti-coplanar
Synthesis of Alkenes

Strong, less hindered bases (MeO-, EtO-, etc)
generally give the most substituted alkene
(Saytzeff’s rule)
Synthesis of Alkenes

Strong, bulky bases give a mixture of
Saytzeff’s product (more substituted) and the
Hoffmann product (least highly substituted
alkene)
 bulky bases often abstract a proton from a
less hindered carbon
Synthesis of Alkenes
Example: Predict the elimination product(s) of
the following reactions.
CH
CHCH
CH
3
2
CH CHCH CH 3
3
BrBr
2
3
KOH
KOH
C H OH, D
5 D
C2H52OH,
H
CH3
Br
H
DH
NaOCH3
CH3OH, D
Synthesis of Alkenes
Example: Predict the two possible elimination
product(s) for the following reaction. Which
one will be the major product?
Br
CH3
Et3N
D
Synthesis of Alkenes

Dehalogenation of Vicinal Dibromides
 two possible reagents
 NaI (E2 mechanism)
 Zn/HOAc (redox reaction)
Synthesis of Alkenes

Dehalogenation using I- takes place via a
concerted, stereospecific E2 mechanism
Anti-coplanar conformation required
Trans-diaxial conformation required for
cycloalkanes
Synthesis of Alkenes
Example: Predict the major elimination
product formed in the following reactions.
Br
H
H3C
C
H
C
NaI
CH3
Br
Br
Br
acetone
NaI
acetone
Synthesis of Alkenes
of Alcohols
C Dehydration
C


removal of water
CH3
CH3 C OH
CH3

H2SO4
D
CH3
CH2 C
CH3
+ H2O
equilibrium process
 drive reaction to completion by removing
alkene as formed (LeChatelier’s Principle)
Synthesis of Alkenes

Typical reaction conditions
 alcohol substrate
 Order of reactivity:
o
o
o
 3 > 2 > 1 alcohol

acid catalyst
 conc. H2SO4
 conc. H3PO4

heat
Synthesis of Alkenes
Mechanism of Dehydration (E1)
 Step 1: Protonation of the hydroxyl group
(fast)

Step 2: Ionization (RDS)
+
Synthesis of Alkenes

Step 3: Proton abstraction (fast)

Rearrangements to form more stable
carbonium ions are common in dehydration
reactions.

Saytzeff’s product preferred.
Synthesis of Alkenes
Example: Propose a mechanism for the
following reaction.
CH3
CH3CCH2OH
CH3
H2SO4
H3C
o
150 C
C=CHCH3
H3C
What type of reaction occurs?

Dehydration reaction with rearrangement
 E1 reaction with rearrangment
3
Synthesis
CH
3

CH3CCH2
+ H2SO4
CH3CCH2 O H
of Alkenes
CH3
+
O H
H
Step 1: Protonation of OH group
CH3
CH3
CH3
+
O H + HSO4-
H3CH
C 3CCH2
CH3CCH2OH
C=CHCH
o
3
H
CH
CH3
3
150
H3C
CH
3 2: Ionization with Methyl Shift
 Step
+ 2SO
H2SO
CH3CCH2 O H H
4 4
CHCH
3
3
+
CH3CCH2OH
CH3CCH2 O H
CH3
CH3
H
H2SO4
~CH3
o
150
H3C CH3
3
CH3C=CHCH
C CH2CH
+ H2O
3
H3C +
Synthesis of Alkenes

Step 3: Abstraction of proton
CH3 H
CH3C C CH3 + H2O
+
H
H C C C
H3C
+
C=CHCH3 + H3O
H3C
Synthesis of Alkenes
Example: Predict the major product formed in
the following reaction.
CH3
OH
H2SO4
D
Reactions of Alkenes

The most common reactions of alkenes are
addition reactions:
 the addition of a reagent to the pi bond with
subsequent formation of new sigma bonds
 number of elements of unsaturation
decreases
Reactions of Alkenes


The electrons in the p bond of C=C are
delocalized above and below the sigma bond
 more loosely held
In the presence of a strong electrophile, the
double bond acts as a nucleophile, donating
the p electrons to the electrophile and forming
a new s bond.
Reactions of Alkenes
Most reactions of alkenes are electrophilic
addition reactions.
Step 1: Attack of electrophile on pi bond
forming a carbonium ion:

Step 2: Nucleophile attacks carbonium ion
giving product.
Reactions of Alkenes

Addition of H-X to Alkenes
C
C
+ H
X
C
C
H
X
Reactions of Alkenes


In the previous example, the proton added to
the secondary carbon, forming the most
stable carbonium ion.
Markovnikov’s Rule:
 Asymmetric reagents such as H-X add to a
C=C so that the proton adds to the carbon
(in the double bond) that already has the
greater number of hydrogen atoms.
 “The rich get richer”
Reactions of Alkenes

Markovnikov’s Rule (extended):
 In an electrophilic addition to an alkene, the
electrophile adds in such a way as to give
the most stable intermediate.
Reactions of Alkenes
Example: Predict the product formed in each of
the following reactions.
CH3
HBr
HI
Reactions of HI
Alkenes

Anti-Markovnikov Addition of HBr
 In the presence of peroxides, HBr adds to
C=C via a free radical mechanism giving the
“Anti-Markovnikov” product.
CH3
H3C H
HBr
Br
CH3CH2O-OCH2CH3
Works only with HBr (not HCl or HI) due to
relative bond strengths.
O
O
Reactions COOC
of Alkenes

Some common peroxides:
O O HI
COOC
CH CO-OCCH
Benzoyl peroxide
3
3
HI
HI
O
O
CH3CO-OCCH
CH3CO-OCCH
3
3
Acetyl peroxide
O
O
OO
H3
O
O
Di-t-butyl peroxide
H3C H
Br
HBr
CH3
H
H
C
H
H
C
3
CH3CH2O-OCH2CH33
Diethyl
peroxide
Br
Br
HBr
Reactions of Alkenes
Example: Predict the product of the following
reaction.
HI
HBr
CH3CO-OCCH3
O
CH3
O
H3C H
HBr
Br