Revision: All about Elimination Reactions | Chemistry Optional Notes for UPSC PDF Download

 Page 1


Eliminations	

An elimination is when the leaving group and another atom (typically a hydrogen) 	

leave the molecule and no new atoms are added	

- Two species have therefore been eliminated	

An elimination results in the formation of a new p bond	

Instead of substitution reactions, another reaction that can occur 	

when a leaving group is present is an elimination reaction 	

A convenient method to form alkenes	

(actually this is the reverse of an hydrogen halide addition to an alkene)	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
CH
3
HBr
Page 2


Eliminations	

An elimination is when the leaving group and another atom (typically a hydrogen) 	

leave the molecule and no new atoms are added	

- Two species have therefore been eliminated	

An elimination results in the formation of a new p bond	

Instead of substitution reactions, another reaction that can occur 	

when a leaving group is present is an elimination reaction 	

A convenient method to form alkenes	

(actually this is the reverse of an hydrogen halide addition to an alkene)	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
CH
3
HBr
There are three versions of an elimination reaction:  E1, E2 and E1cB	

(the E1cB mechanism is very rare and only occurs under very select conditions)	

E1:  Elimination, Unimolecular	

This mechanism is similar to the S
N
1 mechanism	

The leaving group departs in the rate determining step to generate a carbocation	

A base then abstracts a hydrogen from a carbon ADJACENT to the carbocation 	

to form a new p bond	

Elimination Reactions	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
B
H
3
CH
2
C
CH
2
CH
3
CH
3
Page 3


Eliminations	

An elimination is when the leaving group and another atom (typically a hydrogen) 	

leave the molecule and no new atoms are added	

- Two species have therefore been eliminated	

An elimination results in the formation of a new p bond	

Instead of substitution reactions, another reaction that can occur 	

when a leaving group is present is an elimination reaction 	

A convenient method to form alkenes	

(actually this is the reverse of an hydrogen halide addition to an alkene)	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
CH
3
HBr
There are three versions of an elimination reaction:  E1, E2 and E1cB	

(the E1cB mechanism is very rare and only occurs under very select conditions)	

E1:  Elimination, Unimolecular	

This mechanism is similar to the S
N
1 mechanism	

The leaving group departs in the rate determining step to generate a carbocation	

A base then abstracts a hydrogen from a carbon ADJACENT to the carbocation 	

to form a new p bond	

Elimination Reactions	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
B
H
3
CH
2
C
CH
2
CH
3
CH
3
Potential 
energy	

Reaction Coordinate	

S
N
1	

Potential 
energy	

Reaction Coordinate	

E1	

S
N
1 and E1 Reactions Have Identical Energy Diagrams for Rate Determining Step	

Cl
CH
3
H
3
C
H
3
C
Cl
CH
3
H
3
C
H
3
C
CH
3
CH
3
H
3
C
CH
3
CH
3
H
3
C
OCH
3
CH
3
H
3
C
H
3
C
H
2
C
CH
3
CH
3
B
CH
3
OH
Rate = k [substrate]	

 Rate = k [substrate]	


Page 4


Eliminations	

An elimination is when the leaving group and another atom (typically a hydrogen) 	

leave the molecule and no new atoms are added	

- Two species have therefore been eliminated	

An elimination results in the formation of a new p bond	

Instead of substitution reactions, another reaction that can occur 	

when a leaving group is present is an elimination reaction 	

A convenient method to form alkenes	

(actually this is the reverse of an hydrogen halide addition to an alkene)	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
CH
3
HBr
There are three versions of an elimination reaction:  E1, E2 and E1cB	

(the E1cB mechanism is very rare and only occurs under very select conditions)	

E1:  Elimination, Unimolecular	

This mechanism is similar to the S
N
1 mechanism	

The leaving group departs in the rate determining step to generate a carbocation	

A base then abstracts a hydrogen from a carbon ADJACENT to the carbocation 	

to form a new p bond	

Elimination Reactions	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
B
H
3
CH
2
C
CH
2
CH
3
CH
3
Potential 
energy	

Reaction Coordinate	

S
N
1	

Potential 
energy	

Reaction Coordinate	

E1	

S
N
1 and E1 Reactions Have Identical Energy Diagrams for Rate Determining Step	

Cl
CH
3
H
3
C
H
3
C
Cl
CH
3
H
3
C
H
3
C
CH
3
CH
3
H
3
C
CH
3
CH
3
H
3
C
OCH
3
CH
3
H
3
C
H
3
C
H
2
C
CH
3
CH
3
B
CH
3
OH
Rate = k [substrate]	

 Rate = k [substrate]	

Regioselectivity in E1 Reactions	

With the t-Butyl Chloride starting material shown, only one possible E1 product is possible 
as all three methyl groups are symmetrically equivalent	

Cl
CH
3
H
3
C
H
3
C
H
2
C
CH
3
CH
3
weak base
With an unsymmetrical tertiary chloride, however, different products can be obtained	

Cl
CH
2
CH
3
H
3
C
H
3
C
weak base
H
2
C
CH
3
CH
2
CH
3
H
3
C
CH
3
CH
3
In an E1 reaction, the more substituted alkene is favored	


Page 5


Eliminations	

An elimination is when the leaving group and another atom (typically a hydrogen) 	

leave the molecule and no new atoms are added	

- Two species have therefore been eliminated	

An elimination results in the formation of a new p bond	

Instead of substitution reactions, another reaction that can occur 	

when a leaving group is present is an elimination reaction 	

A convenient method to form alkenes	

(actually this is the reverse of an hydrogen halide addition to an alkene)	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
CH
3
HBr
There are three versions of an elimination reaction:  E1, E2 and E1cB	

(the E1cB mechanism is very rare and only occurs under very select conditions)	

E1:  Elimination, Unimolecular	

This mechanism is similar to the S
N
1 mechanism	

The leaving group departs in the rate determining step to generate a carbocation	

A base then abstracts a hydrogen from a carbon ADJACENT to the carbocation 	

to form a new p bond	

Elimination Reactions	

Br
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
CH
2
CH
3
H
3
CH
2
C
H
3
CH
2
C
B
H
3
CH
2
C
CH
2
CH
3
CH
3
Potential 
energy	

Reaction Coordinate	

S
N
1	

Potential 
energy	

Reaction Coordinate	

E1	

S
N
1 and E1 Reactions Have Identical Energy Diagrams for Rate Determining Step	

Cl
CH
3
H
3
C
H
3
C
Cl
CH
3
H
3
C
H
3
C
CH
3
CH
3
H
3
C
CH
3
CH
3
H
3
C
OCH
3
CH
3
H
3
C
H
3
C
H
2
C
CH
3
CH
3
B
CH
3
OH
Rate = k [substrate]	

 Rate = k [substrate]	

Regioselectivity in E1 Reactions	

With the t-Butyl Chloride starting material shown, only one possible E1 product is possible 
as all three methyl groups are symmetrically equivalent	

Cl
CH
3
H
3
C
H
3
C
H
2
C
CH
3
CH
3
weak base
With an unsymmetrical tertiary chloride, however, different products can be obtained	

Cl
CH
2
CH
3
H
3
C
H
3
C
weak base
H
2
C
CH
3
CH
2
CH
3
H
3
C
CH
3
CH
3
In an E1 reaction, the more substituted alkene is favored	

Saytzeff Elimination	

This preference for the more substituted alkene is referred to as the “Saytzeff” rule	

The preference for the more substituted alkene is due to the lower energy transition state	

H
3
C
H
3
C
CH
3
First obtain 3° cation	

H
3
C
H
2
C
CH
3
H
H
3
C
H
3
C
CH
3
H
The cation could have the base abstract 
either of the adjacent hydrogens	

B B
H
3
C
H
2
C
CH
3
H
B
!+
!+
H
3
C
H
3
C
CH
3
H
B
!+
!+
As seen with alkenes, the more alkyl substituents the more stable, 
thus the more substituted alkene transition state is favored	

(Sometimes translated as Zaitsev, Zaitzev, Saytzev)