Q5(b). Allylic SN1: kinetic and thermodynamic acetates

Introduction. Under SN1 conditions the tertiary allylic chloride ionises to an allylic cation with two resonance forms; acetate can attack either end, so two isomeric acetates form (allylic rearrangement).

Scheme: The 1,1-dimethylallyl cation is attacked at the tertiary (C-3) or primary (C-1) end.

1. Ionisation (rate-determining): Cl⁻ leaves to give the 1,1-dimethylallyl cation, (CH₃)₂C+-CH=CH₂ <→ (CH₃)₂C=CH-CH₂⁺.
2. Attack at the tertiary carbon (C-3) gives 2-methylbut-3-en-2-yl acetate, CH₂=CH-C(CH₃)₂-OAc. The tertiary carbon carries the larger share of the positive charge, and the acetate is close to it in the ion pair just after Cl leaves, so this product forms faster: the kinetic product.
3. Attack at the primary carbon (C-1) gives 3-methylbut-2-en-1-yl acetate (prenyl acetate), (CH₃)₂C=CH-CH₂-OAc. Its C=C is trisubstituted (more hyperconjugation) and the acetate is on an unhindered primary carbon, so it is more stable: the thermodynamic product.
4. With longer reaction time or heat, the tertiary allylic acetate re-ionises (it is a good SN1 substrate) and the mixture drifts to the primary acetate.

Conclusion. Products: 2-methylbut-3-en-2-yl acetate (kinetic) and 3-methylbut-2-en-1-yl acetate (thermodynamic), both from the same resonance-stabilised allylic cation.

Application. Prenyl (3-methylbut-2-enyl) acetate and related allylic esters are made this way for perfumes; the same kinetic/thermodynamic choice governs 1,2- versus 1,4-addition of HBr to butadiene.