Download The Stereochemistry of E2 Reactions

The Stereochemistry of E2 Reactions
.
.
.
.
Dehydrohalogenation: Zaitsev’s Rule
Hammond-Leffler Postulate: the transition state for a
step that is downhill in energy should show a strong
resemblance to the reactant of that step.
.
.
.
.
.
.
.
.
.
.
.
.
.
The Stereochemistry of E2 Reactions
.
.
.
.
.
.
The Stereochemistry of E2 Reactions
.
.
.
.
.
.
.
.
Acid-Catalyzed Dehydration of Alcohols: E1 Reaction
B
CH3CH2O
H
E2
Elimination
RR
H
?
OH
δ
R
Nu
R
Nu
Substitution
RR
H
R
E2
Xδ
H
δ
R
R
Xδ
SN 2
B
H
R
R
R
R
X
E1
R
R
R
X
S N1
Acid-Catalyzed Dehydration of Alcohols: E1 Reaction
The temperature and concentration of acid required to dehydrate an
alcohol depend on the structure of the alcohol substrate:
Primary alcohol
Tertiary alcohol
Secondary alcohol
General reactivity order
Acid-Catalyzed Dehydration of Alcohols: E1 Reaction
Some primary and secondary alcohols also undergo rearrangements
of their carbon skeletons during dehydration.
CH3
.
H3C C
rearranged to
.
CH3
C
H
CH2
not formed
Acid-Catalyzed Dehydration of Alcohols: Mechanism
Rate Limiting Step
Dehydration of Alcohols: E1 Mechanism
Rate Limiting Step
Carbocation Stability
.
.
.
.
Dehydration of Primary Alcohols: E2 Mechanism
.
.
.
.
.
.
.
.
.
Dehydration of Primary Alcohols: E2 Mechanism
.
.
.
.
.
.
.
.
.
.
.
.
.
Rearrangement during Dehydration
A (80%)
.
B (20%)
.
.
.
.
.
.
Path B
B
H
CH3
H2C C
C
CH3
CH3 H
B
Path A
The formation of the more stable alkene is the general rule (Zaitsev's rule)
in the acid-catalyzed dehydration reactions of alcohols.
Rearrangement: Change in Ring Structure
Ring enlargement
Ring contraction
HO
H3O+
+
+
heat
B
H
+
+
+
Synthesis of Alkynes by Elimination Reactions
?
R
R
R
H
H
R
R
Br
Br Br
R
E2
R
H
R
E2
Br
H
R
R
H
Br
The Acidity of Terminal Alkynes
Relative Acidity
Relative Basicity