Quaternary compound

In chemistry, a quaternary compound is a cation consisting of a central positively charged nitrogen atom with four substituents, especially organic (alkyl and aryl) groups, discounting hydrogen atoms.

The best known quaternary compounds are quaternary ammonium salts, having a nitrogen atom at the centre. For example, in the following reaction, the nitrogen atom is said to be quaternized as it has gone from 3 to 4 substituents:

Other examples include substituted phosphonium salts (R₄P⁺), substituted arsonium salts (R₄As⁺) like arsenobetaine, as well as some arsenic containing superconductors. Substituted stibonium (R₄Sb⁺) and bismuthonium salts (R₄Bi⁺) have also been described.