The kinetic isotope effect () for the electrophilic chlorination of standard benzene versus hexadeut — Haloalkanes and Haloarenes Chemistry Question
Question
The kinetic isotope effect ($k_H/k_D$) for the electrophilic chlorination of standard benzene versus hexadeuterobenzene ($C_6D_6$) using $Cl_2/FeCl_3$ is experimentally measured to be approximately $1.0$. What does this critical piece of kinetic data reveal about the reaction mechanism?
💡 Solution & Explanation
A primary kinetic isotope effect ($k_H/k_D \gg 1$, typically around $7$) is observed only when a $C-H$ bond is broken in the rate-determining step (RDS). Because the $C-D$ bond is stronger, breaking it would drastically slow the reaction. A value of $1.0$ indicates no isotope effect, meaning the $C-H$ bond is *not* broken during the RDS. Therefore, the slow step is the initial attack of the electrophile on the pi-system to form the arenium ion intermediate. The subsequent fast step is the abstraction of the proton (or deuteron) to restore aromaticity.