Solvent Effects on SN2 Reactions

Aprotic Polar Solvent:
Solvates cations extremely well
therefore: increases nucleophilicity of anions

Examples: DMSO, DMF, HMPA, Acetone

Protic Polar Solvent: must have an oxygen or nitrogen with H attached
Solvates anions well because it is good at hydrogen bonding
therefore: decreases nucleophilicity of anions

Examples: Methanol, Ethanol, Water

1. Nucleophilicity is solvent dependent when moving up and down the periodic
table.
For example:

F

Cl

Br

I

Nucleophilicity

In Polar
Aprotic
Solvents

In Protic
Solvents

2. Nucleophilicity decreases when moving from left to right across the periodic
table.
e.g. Carbon anions are better nucleophiles than Oxygen anions
3. The more concentrated the charge the better the nucleophile

Leaving Group Ability
Anything that spreads out charge stabilizes the anion and therefore is a better

leaving group.
The bigger the atom, the more polarizable, therefore more stable ----> better
leaving group