SN1 Reaction
SN1 Reaction: The tertiary alkyl halide ionizes on its own (the rate-determining step) to give a stabilized tertiary carbocation and a leaving bromide ion…
What happens
The tertiary alkyl halide ionizes on its own (the rate-determining step) to give a stabilized tertiary carbocation and a leaving bromide ion; the nucleophile then attacks the flat, planar carbocation from either face.
Reaction summary
(CH₃)₃C–Br + H₂O → (CH₃)₃C–OH + HBr
Reagents and conditions
- Starting material: tert-Butyl bromide
- Reagents: Water (weak nucleophile)
- Products: tert-Butanol
Mechanism
- Ionization (rate-determining): The C–Br bond breaks heterolytically without any help from the nucleophile, forming a planar tertiary carbocation stabilized by hyperconjugation and three alkyl groups.
- Carbocation intermediate: The carbocation is sp² and planar, so the nucleophile can attack from either face, this is why SN1 at a stereocenter gives racemization.
- Nucleophilic attack + deprotonation: Water attacks the cation from either face; loss of a proton (to another water molecule) gives the neutral alcohol.
Selectivity
- Stereoselectivity: Racemization at a stereocenter, since the flat carbocation intermediate is attacked from both faces with roughly equal probability.
Common mistakes
- Assuming SN1 is fast for primary substrates, SN1 requires a stable carbocation, so it strongly favors tertiary (and resonance/benzylic/allylic-stabilized) substrates, not primary ones.
- Forgetting SN1 is first-order overall, only the substrate's concentration appears in the rate law, not the nucleophile's.
Memory trick
"SN1 = 1 step decides the rate, 1 species in the rate law": ionization is rate-determining and unimolecular.