QuestionOrganic / regioselective HBr addition
The major products [X] and [Y] formed in the following transformations are:
- (a) [X] Br on C bearing OCH₃; [Y] α-bromo ester
- (b) [X] Br on C bearing OCH₃; [Y] β-bromo ester
- (c) [X] Br on C2; [Y] α-bromo ester
- (d) [X] Br on C2; [Y] β-bromo ester
Answer: (b)
[X]: enol ether — H⁺ adds to C2, + at C1 is stabilised by O (oxocarbenium) → Br on the carbon bearing OCH₃. [Y]: α,β-unsaturated ester — conjugate (1,4) addition; Br⁻ ends on the β-carbon → (b).