Organic Chemistry MCAT Practice Exam

In nucleophilic substitution reactions, a leaving group is a species that can depart from the parent molecule, allowing the nucleophile to take its place. The ideal leaving group is one that is stable once it has left, often because it is able to exist in a favorable ionic or molecular form.

Chloride ion is an excellent leaving group due to its ability to stabilize the negative charge after it departs. When a chlorine atom sheds its electron and leaves as Cl-, it becomes a stable ion, which greatly favors the process of nucleophilic substitution. The bond formed between the nucleophile and the substrate thus can proceed more easily when the leaving group can stabilize itself in the solution.

In contrast, water, while it can act as a leaving group in some specific contexts, is not typically thought of as a good leaving group in many nucleophilic substitution reactions, especially when compared to halides like chloride. Aldehydes and aromatic compounds are not appropriate leaving groups in nucleophilic substitutions because they do not tend to form stable species after leaving; aldehydes typically participate in different types of reactions and aromatic compounds are quite stable due to their resonance.

Consequently, chloride ion stands out as a reliable and standard leaving group in these types of