Elimination reactions
Chapter 6
1
Elimination reactions
 A problem that often occurs in substitution reactions is
elimination
2
E1-elimination
n = k [substrate]
 The mechanism is similar to that of the SN1 reaction: the
first step is formation of the carbocation via heterolytic
cleavage
3
Substitution vs elimination
 Elimination and substitution reactions are competing processes:
in this case, the tertiary carbocation gives
• primarily elimination in the presence of a strong base (EtO–)
• mainly substitution if there is only a good nucleophile present
 This can be considered as a ‘rule of thumb’
4
pKa’s revisited
Acid
HI
H3O +
HF
CH3CO2H
H2 S
NH4+
MeOH
HCCH
H2C=CH2
pKa
–10
–1.7
3.2
4.2
7.0
9.4
15.2
25
45
Base
CH3CO2¯
CN ¯
NH3
(CH3CH2)3N
CH3O¯
HO¯
CH3CH2O¯
(CH3)3CO¯
NH2¯
pK*
4.2
9.1
9.4
10.7
15.2
15.7
15.9
19
35
*The pK values are those of the conjugated acids
 If the pK of the conjugated base is higher than the pKa of the
acid deprotonation is possible (pK > 12-15: strong base)
5
The Saytzeff elimination
 Saytzeff elimination: in the elimination process, formation
of the most substituted double bond is favored
6
Explanation
 In the transition state for elimination, the increased stability
of the most substituted double bond is already felt so that
there is a lower energy barrier for elimination of Ha
7
Problems
 6.20: Predict the products of the E1 and SN1 reactions of the
following molecules in water:
8
The E2 elimination
n = k [substrate] [base]
 As in the SN2 reaction, the rate is dependent on the
concentration of both reaction partners
9
E2 vs SN2
 There is a strong similarity between the E2 and SN2
reactions: strong nucleophiles favor substitution, while
strong bases favor elimination
10
Steric bulk favors elimination
 We already saw that SN2 substitution on a tertiary carbon is
not possible, therefore E2 elimination will prevail (beside E1
elimination)
11
Other examples
 Note that at a primary carbon atom, only SN2 and no SN1
substitution is possible
12
Stereochemistry of the E2 rxn
Possible orientations of the proton that eliminates and the
leaving group
13
E2 and the anti-conformation
 The E2 elimination takes place in a single process in the anticonformation: proton abstraction, double bond formation and
cleavage of the C–L bond occur simultaneously
14
Resemblance to SN2
 The mechanism is ‘more or less’ analogous to the SN2
substitution
 The process is also called anti-elimination or antiperiplanar
elimination
15
How to visualize this aspect ?
 What are the products
of E2 elimination of
both diastereomers ?
16
The answer…..
17
Regiochemistry
 Generally, formation of the most substituted double bond is
favored
 Note the occurrence of E- and Z-isomers
18
Saytzeff vs Hofmann elimination
19
Influence of the base
20
Effect of the leaving group
 Especially quarternary ammonium leaving groups favor the
Hofmann product
21
The E1cB elimination
 The E1cB reaction resembles the SN2 reaction, with the
difference that there is an anion formed prior to the loss of
the leaving group
22
The three eliminations
 E1cB: the proton is removed first, an anion is formed
 E1: the leaving group departs first, a cation is formed
 E2: all processes occur at the same time
23
Example of an E1cB reaction
 This reaction is possible if there is a group present that can
stabilize the negative charge
24
Stability of anions
 The more substituted the carbanion, the less stable it is; this
is a result of the inductively electron donating alkyl groups
25
The synthetic outlook
26
Formation of CC-bonds
 Formation of CC-bonds is a synthetically important reaction
27
Summary
28
Summary (2)
29
Problems
 Make problems: 6.30, 6.31, 6.32, 6.38, 6.53, 6.54, 6.56, 6.59
30