Among halide leaving groups in substitution reactions, which is the best leaving group?

• A. I- (best leaving group).
• B. Br-
• C. F-
• D. Cl-

Answer: (A)

When a leaving group departs, its ability to do so depends on how stable the leaving group is as an anion. A good leaving group is the conjugate base of a strong acid and is not highly basic. Among halides, the iodide ion is the most stable and most polarizable anion. HI is a very strong acid, so I− is a very weak base. That weak basicity means the negative charge is better stabilized once the group leaves, allowing it to depart readily in substitution reactions. The carbon–iodine bond is also the weakest among the four, so breaking it requires less energy, further helping the leaving process. By comparison, fluoride forms a hard, highly basic F−, which is poorly stabilized, making it a poor leaving group; chloride and bromide are intermediate, with bromide better than chloride but not as good as iodide.