A quaternary ammonium salt is an ionic compound that has the following general structural formula.
n = positive whole number
R1, R2, R3, and R4 could be hydrogen atoms, alkyl groups, aryl groups, or any combination thereof. A– could be any anion present in salt.
The word “salt” in quaternary ammonium salt is a misnomer because a quaternary ammonium salt can not be prepared by an acid-base reaction.
see also quaternary ammonium ion, primary ammonium salt, secondary ammonium salt, tertiary ammonium salt