Question

Why do tertiary alkyl halides undergo S\textsubscript{N1 reaction at a faster rate?

Hint:

Use the valence bond theory to determine geometry from hybridisation: - \(sp^3\): tetrahedral, - \(dsp^2\): square planar, - \(d^2sp^3\)/\(sp^3d^2\): octahedral. For low/high spin, check whether the ligand is strong or weak field.

Answer 1

Step 1: The S_N1 mechanism proceeds via formation of a carbocation intermediate.
Step 2: Tertiary alkyl halides have a carbon atom bonded to three alkyl groups. These groups stabilize the positive charge on the carbocation through:
- Hyperconjugation (delocalization of electrons from C–H bonds of adjacent alkyl groups), and - Inductive effect (electron-donating nature of alkyl groups).
Step 3: This stability lowers the activation energy of the rate-determining step (carbocation formation), hence speeding up the reaction.