Quaternary compound

In chemistry, a quaternary compound is a compound consisting of exactly four chemical elements.

In another use of the term in organic chemistry, a quaternary compound is or has a cation consisting of a central positively charged atom with four substituents, especially organic (alkyl and aryl) groups, discounting hydrogen atoms.

[1] The best-known quaternary compounds are quaternary ammonium salts, having a nitrogen atom at the center.

[2] 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.

[3] Substituted stibonium (R4Sb+)[4] and bismuthonium salts (R4Bi+) have also been described.