When chlorobenzene, isotopically labeled with exactly at the carbon attached to the chlorine atom, i — Aromatic Hydrocarbons Chemistry Question
Question
When chlorobenzene, isotopically labeled with $^{14}C$ exactly at the carbon attached to the chlorine atom, is treated with sodium amide ($NaNH_2$) in liquid ammonia, a mixture of labeled anilines is obtained. What is the approximate product distribution, and what mechanism does this observation prove?
💡 Solution & Explanation
The reaction of unactivated aryl halides with strong bases like $NaNH_2$ proceeds via an elimination-addition mechanism. The strong base removes an ortho proton, expelling the halide and forming a highly reactive benzyne intermediate. The symmetrical triple bond in benzyne can be attacked by the nucleophile ($NH_2^-$) at either carbon with roughly equal probability, leading to direct substitution (on the $^{14}C$) and cine-substitution (on the adjacent unlabelled carbon).