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Substitution versus Elimination

Substitution and elimination reactions often compete with each other because it's a question of nucleophilic or basic properties.

subsitution vs elimination

Substitution and elimination reactions are strongly influenced by many experimental factors. Some of more important factors are outlined in the following table.

In the table, the significance of the effect is stated first, and then the "system" that will favour the reaction is stated.

This should help you deal with the questions....
1. When does an anion function as a Nu and when does it function as a B ?, and therefore,
2. When to I get substitution and when do I get elimination ?
 

Reaction
Solvent
Nu or Base
Leaving Group
Substrate
Example conditions
SN1
Very Strong
Polar solvents 
Weak
Good Nu and weak base 
Strong
Good LG 
Strong
3o or resonance stabilised 
alkyl halide / AgNO3 / aq. EtOH

alcohol / HX
SN2
Strong
Polar aprotic solvents 
Strong
Good Nu and weak base 
Strong
Good LG 
Strong
methyl or 1o

alkyl halide / NaI / acetone

alcohol / SOCl2 or PX3

E1
Very Strong
Polar solvents
Weak
Weak base 
Strong
Good LG 
Strong
3o or resonance stabilised 

alkyl halide / H2O

alcohol / H2SO4 /  heat

E2
Strong
Polar aprotic solvents 
Strong
Poor Nu and strong base 
Strong
Good LG 
Strong
3o
alkyl halide / KOH / heat

© Dr. Ian Hunt, Department of Chemistry, University of Calgary