Quaternary compound

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

Quaternary ammonium cation. The Rs may be the same or different groups. (The Rs may also be connected, making a cyclic ion.)

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 centre.[2] For example, in the following reaction, the nitrogen atom is said to be quaternized as it has gone from 3 to 4 substituents:

${\displaystyle \mathrm {R_{3}N+RCl\longrightarrow R_{4}N^{+}\ Cl^{-}} }$

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.[5]