SN1 is a two-step nucleophilic substitution mechanism involving a carbocation intermediate, whereas SN2 is a one-step mechanism in which bond formation and bond breaking occur simultaneously. SN1 is unimolecular in its slow step; SN2 is bimolecular.
The Mechanisms
In SN1, the carbon to leaving-group bond breaks by heterolytic fission, producing a carbocation and a leaving-group ion. This is the slow, rate-determining step. The nucleophile then donates an electron pair to the carbocation in a faster second step.
In SN2, the nucleophile attacks the carbon from the side opposite the leaving group. A single transition state forms as the carbontonucleophile bond forms and the carbonleaving-group bond breaks. There is no carbocation intermediate.
| Feature | SN1 | SN2 |
|---|---|---|
| Number of steps | Two | One |
| Rate equation | rate = k[halogenoalkane] | rate = k[halogenoalkane][nucleophile] |
| Favoured substrate | Tertiary | Primary |
| Nucleophile | Can be weak or neutral | Usually strong and negatively charged |
| Solvent | Polar protic | Polar aprotic |
| Stereochemical outcome | Mixture of configurations, often partial racemization | Inversion of configuration |
Tertiary halogenoalkanes favour SN1 because alkyl groups stabilize the carbocation. Primary carbocations are very unstable, so primary halogenoalkanes usually react by SN2. Steric hindrance around a tertiary carbon prevents effective backside attack, making SN2 unfavourable.
A common misconception is that “SN1” means one reaction step and “SN2” means two. The numbers instead describe the molecularity of the rate-determining elementary step: one reacting species for SN1 and two for SN2.
Exam Technique
For an IB Chemistry HL compare question, state both similarities and differences: both involve nucleophilic attack and loss of a leaving group, but their steps, rate equations and intermediates differ. In mechanism diagrams, curly arrows must begin at a lone pair or bond and end where the electron pair moves.
