Question

Explain why 3-bromocyclohexene is more reactive than 4-bromocyclohexene in hydrolysis with aqueous NaOH.

Answer 1

1. The bromine atom in 3-bromocyclohexene is positioned allylic to the double bond (at the 3-position), whereas in 4-bromocyclohexene it is not allylic.
2. Allylic bromides (like 3-bromocyclohexene) can form resonance-stabilized allylic carbocation intermediates when the bromide leaves during hydrolysis.
3. Resonance stabilization of the carbocation intermediate lowers the activation energy for nucleophilic substitution by OH⁻ ions.
4. In 4-bromocyclohexene, no allylic resonance stabilization occurs because the bromine is farther from the double bond, resulting in a less stable carbocation intermediate.
5. Consequently, the hydrolysis reaction proceeds faster with 3-bromocyclohexene due to the more stabilized intermediate, making it more reactive toward aqueous NaOH.

Thus, the position of the bromine relative to the alkene influences the reaction pathway and rate, with allylic bromides reacting more readily.