Page 141 - Pharm.Org.Chem-I 02-06-05-101

Factors affecting the SN1 reaction

1. Alkyl group reactivity
• SN1 requires formation of a carbocation intermediate
• The order of stability of carbocations is:

Tertiary > secondary > primary
• SN1 is mainly observed mainly with 3o > 2o alkyl halides [which

can rearrange to a 3o carbocation].
2. Effect of nucleophiles
The weaker the nucleophile, the faster and more effective the
SN1 reaction will be.
SN1 occurs with water or alcohol.

CN - > I- > OR- > OH- > Br- > Cl- > ROH > H2O
Nucleophilicity increases

3. Effect of solvent
Polar protic solvents are preferred because:
➢ They support the ionization of the alkyl halide.
➢ They stabilize the carbocation formed by solvation.
protic = solvent contains polar hydrogen.
e.g. H2O, alcohol.
Often, the solvent in an SN1 reaction is the nucleophile.

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