What is "leaving group"

n. (context chemistry English) An atom or group that is detached from the rest of a molecule during a reaction

In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis. Common anionic leaving groups are halides such as Cl, Br, and I, and sulfonate esters such as tosylate (TsO). Fluoride (F) functions as a leaving group in the nerve-agent sarin gas. Common neutral molecule leaving groups are water and ammonia.