What is "nucleophilic substitution"

n. (context chemistry English) in aliphatic organic chemistry, a form of substitution reaction in which the leaving functional group is replaced with a nucleophile

In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group; the positive or partially positive atom is referred to as an electrophile. The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate.

The most general form for the reaction may be given as the following:

Nuc: + R-LG → R-Nuc + LG:

The electron pair (:) from the nucleophile(Nuc) attacks the substrate (R-LG) forming a new bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged.

An example of nucleophilic substitution is the hydrolysis of an alkyl bromide, R-Br, under basic conditions, where the attacking nucleophile is the OH and the leaving group is Br.

R-Br + OH → R-OH + Br

Nucleophilic substitution reactions are commonplace in organic chemistry, and they can be broadly categorised as taking place at a saturated aliphatic carbon or at (less often) an aromatic or other unsaturated carbon centre.