Nucleophilic substitution

In chemistry, a nucleophilic substitution (S_N) is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The molecule that contains the electrophile and the leaving functional group is called the substrate.^[1][2]

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

${\displaystyle {\text{Nuc}}\mathbf {:} +{\ce {R-LG -> R-Nuc}}+{\text{LG}}\mathbf {:} }$

The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R−LG) and bonds with it. Simultaneously, 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 hydroxyl (OH⁻) and the leaving group is bromide (Br⁻).

${\displaystyle {\ce {OH- + R-Br -> R-OH + Br-}}}$

Nucleophilic substitution reactions are common in organic chemistry. Nucleophiles often attack a saturated aliphatic carbon. Less often, they may attack an aromatic or unsaturated carbon.^[3]