leaving group

When the nucleophile muscles in to take the carbon, something has to leave — and not just float away, but take the old bonding electrons with it. The piece that departs is the leaving group. Think of a game of musical chairs where the player who leaves must carry off the chair: the leaving group walks away carrying the electron pair, becoming an anion (or sometimes a neutral molecule).

What makes a good leaving group? It must be stable once it carries that negative charge — in other words, it must be a weak base. The best leaving groups are the conjugate bases of strong acids: iodide, bromide, chloride (from HI, HBr, HCl), and the workhorse sulfonate esters like tosylate and mesylate. A weak base is content holding its electrons, so it lets go of the carbon readily. Strong bases such as hydroxide (OH-), alkoxide (RO-), and amide (NH2-) are terrible leaving groups — they cling to those electrons and refuse to leave.

This is why you can't, in practice, kick the OH off an alcohol by simple substitution: hydroxide is too strong a base to leave. Chemists get around it by first converting OH into a good leaving group — protonating it to OH2(+) (so water, a weak base, leaves) or turning it into a tosylate. "Leaving-group ability" tracks acid strength: the more stable the departing anion, the better the leaving group, and the faster substitution (and elimination) go.