An organic chemist compares the reaction rates of three different aromatic hydrocarbons—toluene, eth — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

The rate-determining step in NBS bromination is the abstraction of the benzylic hydrogen atom by a bromine radical. The activation energy for this step directly correlates with the stability of the resulting intermediate radical (Bell-Evans-Polanyi principle). Toluene forms a $1^\circ$ benzylic radical; ethylbenzene forms a $2^\circ$ benzylic radical; cumene forms a $3^\circ$ benzylic radical. Hyperconjugation from adjacent alkyl groups further stabilizes the radical. Therefore, the $3^\circ$ benzylic radical formed from cumene is the most stable, resulting in the lowest activation energy and the fastest reaction rate.