Nucleophilic substitution reactions are part of organic chemistry and are also a fundamental class of substation reactions. The electron rich nucleophile selectively forms a bond with the positive or partially positive charge of an atom attached to a group. It can not only form a bond with this selected group, but it can also attack the group as well. A positive or partially charged atom is also known as an electrophile. Electrophiles are literally translated to mean electron-lover and they are reagents that are attracted to the electrons that participate in a chemical reaction. They accomplish this feat by accepting an electron pair in order to bond to a nucleophile. Because of these characteristics, electrophiles are known as a Lewis acid. The majorities of electrophiles are positively charged and have an atom that carries a partial positive charge, or one that is missing an octet of electrons.

In its most common or basic form the nucleophilic substitution reaction is given as; Nuc: + R-LG → R-Nuc + LG: The electron pair from the nucleophile attacks the substrate (R-L-G) creating a new bond. After this step takes place, the leaving group (LG) leaves with an electron pair. The primary product that is produced in this instance is R-Nuc. In this nucleophilic substitution reaction, the nuceophile is electrically neutral or negatively charge. On the other hand, the substrate is generally neutral or positively charged. Another example of a nucleophilic substitution reaction is the hydrolysis of an alkyl bromide in an alkaline environment and where the attacking nucleophile is the OH and the leaving group is Br. (R-Br + OH− → R-OH + Br−)

Nucleophilic substitution reactions are common in organic chemistry and they can be widely classed as taking place at a saturated aliphatic carbon or at a saturated aromatic or other unsaturated carbon center. Aliphatic compounds, as described in organic chemistry, are made up of carbon and hydrogen which are divided into two classes; aromatic compounds that contain benzene rings or like rings of atoms, and those that do not contain aromatic rings. They can be cyclic such as cyclohexane or acyclic as in hexane. Aromatic compounds are the structures of some rings of atoms that unexpectantly stable. Aromaticity is a chemical property where a conjugated ring of unsaturated bonds, empty orbitals and lone pairs display a stabilization stronger than would normally be expected. It is also classes as a manifestation of cyclic delocalization and of resonance.

Edward D. Hughes and Sir Christopher Ingold, in 1935, studied nucleopihic substitution reactions of alkyl halides and other related compounds. The introduced the concept that there are two main mechanisms at work, with both of them having to compete with one another. The two main mechanisms are the SN1 reaction and the SN2 reaction. With the ‘S’ standing for chemical substitution and the ‘N’ representing the nucleophile and the number shows the kinetic order of the reaction. The kinetic order is a certain order in chemical kinetics with regard to certain reactants. It is the power by which its concentration term in the rate equation is raised.