In the context of free-radical halogenation, why are the hydrogen atoms attached directly to the aro — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Vinylic and aryl $C-H$ bonds are extremely strong (BDE $\approx 110$ kcal/mol compared to $\approx 98$ kcal/mol for primary alkanes). This increased strength is due to the $sp^2$ hybridization of the carbon, which has greater s-character and holds the electrons closer to the nucleus. Furthermore, if a hydrogen were successfully abstracted, the resulting unpaired electron would reside in an $sp^2$ orbital that is situated in the plane of the molecule, perfectly orthogonal ($90^\circ$) to the adjacent pi-cloud. Therefore, it cannot participate in resonance delocalization, making the radical incredibly high in energy and practically impossible to form under mild conditions.