Question

Why does bromination take place at allylic position when cyclohexane is treated with N-bromosuccinamide? Explain with mechanism.

Answer 1

Answer on Question #54466 – Chemistry – Organic Chemistry

Question:

Why does bromination take place at allylic position when cyclohexene is treated with N-bromosuccinamide? Explain with mechanism.

Answer:

This bromination is a radical reaction which occurs in diluted solution of non-polar aprotic solvent. At the first stage NBS and HBr produce bromine:

Under heating Bromine forms bromine radical which attacks the cyclohexene molecule. Dissociation energies for allylic, alkylic and vinylic C-H bonds are 88 kcal/mol, 98 kcal/mol, 106 kcal/mol, respectively. Since the energy of C – H bond is the lowest at allylic position, the substitution occurs there:

Moreover, the formed cyclohexele radical is stable, because of the delocalization of electron via $\pi$-bond.

This makes allylic substitution more favorable than at other positions.

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