Among the following halide ions, which one is considered the best leaving group in nucleophilic subs — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Good leaving groups are the conjugate bases of strong acids; they are weak bases that stably accommodate a negative charge. Because $HI$ is the strongest acid among hydrogen halides, its conjugate base, $I^-$, is the weakest base and therefore the best leaving group. The leaving group ability order is $I^- > Br^- > Cl^- > F^-$.