SN1 Reaction Mechanism: Step-by-Step

1The key idea

SN1 begins when the leaving group departs heterolytically, producing a carbocation. The carbocation then reacts with a nucleophile. For neutral oxygen nucleophiles, a proton transfer often completes the substitution.

2Where SN1 is plausible

Tertiary and some benzylic or allylic substrates can form relatively stabilized carbocations. Polar protic solvents facilitate ionization; weak nucleophiles such as water or alcohol often participate in solvolysis.

3Draw the ionization first

A curved arrow starts at the carbon–leaving group bond and points to the leaving group. The carbocation left behind is planar at the electron-deficient carbon. A nucleophile attacks in the next elementary step.

4Finish the substitution

If water attacks, the initial product is a protonated alcohol (oxonium ion); a molecule of solvent then removes a proton. The overall pathway may compete with E1 elimination, and the measured product ratio depends on conditions.

5Stereochemistry and rate

Attack on a planar carbocation from either face can reduce stereochemical purity and may give a mixture of configurations; ion-pair effects can prevent perfectly equal product ratios. Rate is commonly first order in the substrate under SN1 conditions.

Common mistakes

• Showing the nucleophile attacking before the leaving group departs in an SN1 mechanism.
• Omitting the deprotonation after neutral water or alcohol attacks.
• Claiming that all SN1 solvolyses produce exactly 50:50 enantiomers.
• Ignoring competing E1 alkene formation.

Quick concept check

Why can SN1 at a stereogenic carbon give both configurations?