A chemist isolates a compound containing both an allylic methyl group and an unsubstituted vinylic double bond. They expose it to trace amounts of chlorine gas under UV light. What is the fundamental

Question

A chemist isolates a compound containing both an allylic methyl group and an unsubstituted vinylic double bond. They expose it to trace amounts of chlorine gas under UV light. What is the fundamental kinetic reason that dictates why the allylic position reacts exclusively, leaving the vinylic position completely untouched?

Prashant Kotian · Accepted Answer

Correct answer: B. In free-radical substitution, regioselectivity is kinetically controlled by the activation energy required for hydrogen abstraction. The activation energy is directly linked to the stability of the forming radical. The allylic radical is highly stabilized by resonance delocalization across the adjacent pi bond. In stark contrast, the vinylic radical cannot resonate with the pi bond and resides in a higher-energy, strongly held orbital. Thus, the activation energy for allylic abstraction is vastly lower, and the reaction proceeds overwhelmingly at that site.

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