What is "nucleofuge"

n. (context chemistry English) A leaving group that takes away a bonding electron pair with it

A nucleofuge (derived from nucleo, nucleus, the positive part in an atom and fuge, fugetive, to run away, to escape) is a leaving group which retains the lone pair from its previous bond with another species. For example, in the S2 mechanism a nucleophile attacks an organic compound containing the nucleofuge (the bromo group) which simultaneously breaks the bond with the nucleofuge.

After a reaction nucleofuges may contain either a negative or a neutral charge; this is governed by the nature of the specific reaction.

The word 'nucleofuge' is commonly found in older literature, but its use is less common in current literature in which the word leaving group dominates.