Substitution and Elimination

Chapter 8: Nucleophilic Substitution

Substitution versus Elimination

Substitution and elimination reactions are strongly influenced by many factors. Some of more important factors are outlined in the following table.
The significance of the effect is stated first, 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 do I get substitution and when do I get elimination ?

Reaction	Solvent	Nu or Base	Leaving Group	Substrate	Typical Conditions
SN1	Very Strong Polar solvents	Weak Good Nu and weak base	Strong Good LG	Strong 3^o or resonance stabilized	AgNO₃ / aq. EtOH
SN2	Strong Polar aprotic solvents	Strong Good Nu and weak base	Strong Good LG	Strong Methyl or 1^o	NaI / Acetone
E1	Very Strong Polar solvents	Weak Weak base	Strong Good LG	Strong 3^o or resonance stabilized	H₂SO₄, heat
E2	Strong Polar aprotic solvents	Strong Poor Nu and strong base	Strong Good LG	Strong 3^o	KOH, heat,