Question

Compare elimination versus substitution of alkyl halides

Answer 1

Substitution versus Elimination

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



Substitution and elimination reactions are strongly influenced by many experimental factors.

1. Increasing the temperature tends to increase elimination due to disorder / entropy effects (recall DG - DH -TDS)

2. Increasing steric effects (look at both "R" and the "Nu/B") tends to increase elimination.

3. The basicity / nucleophilicity of the attacking species i.e. switching from ROH to RO- will increase the amount of elimination

Some of more important mechansitic 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 ?

ReactionSubstrateNu or BaseLeaving GroupSolventExamplesSN1Strong 
3o or resonance stabilised Weak 
Good Nu and weak base Strong 
Good LG Very Strong 
Polar solvents alkyl halide / AgNO3 / aq. EtOH

alcohol / HX (note X is a good Nu)SN2Strong 
Methyl or 1oStrong 
Good Nu and weak base Strong 
Good LG Strong 
Polar aprotic solvents 

alkyl halide / NaI / acetone

alcohol / SOCl2 or PCl3

E1Strong 
3o or resonance stabilised Weak 
Weak base Strong 
Good LG Very Strong 
Polar solvents

alcohol / H2SO4 /  heat

tertiary alkyl halide / weak base (ROH or H2O)

E2Strong 
3oStrong 
Poor Nu and strong base Strong 
Good LG Strong 
Polar aprotic solvents 

alkyl halide / KOH / heat,

primary alcohols / alcohol / H2SO4 /  heat