Question

What is quaternary ammonium salt?

Answer 1

We know that quaternary ammonium salts are cationic compounds. These quaternary ammonium salts are soluble in water.

Complete solution:
Quaternary ammonium salts are also called quats. Quaternary ammonium salts are cationic compounds. It contains four aryl or alkyl groups which are connected to the nitrogen and an ion such as bromide or chloride. We can prepare quaternary ammonium salt, benzalkonium by the reaction of alkyl dimethyl amine and benzyl chloride.

${\rm{C}}{{\rm{H}}_{\rm{3}}}{\left( {{\rm{C}}{{\rm{H}}_{\rm{2}}}} \right)_{\rm{n}}}{\rm{N}}{\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}} \right)_{\rm{2}}} + {\rm{ClC}}{{\rm{H}}_{\rm{2}}}{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}} \to \,{\left[ {{\rm{C}}{{\rm{H}}_{\rm{3}}}{{\left( {{\rm{C}}{{\rm{H}}_{\rm{2}}}} \right)}_{\rm{n}}}{\rm{N}}{{\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}}\,{{\rm{H}}_{\rm{5}}}} \right]^{\rm{ + }}}{\rm{C}}{{\rm{l}}^ - }$

In the above reaction, alkyl dimethylamine reacts with benzyl chloride to produce benzalkonium. Quaternary ammonium salts are not reactive toward strong acids, oxidants and even electrophiles. Towards nucleophiles, quaternary ammonium salts are stable.

Additional Information:
i) Quaternary ammonium salts are used as surfactants, disinfectants, as fabric softeners, as antistatic agents etc.
ii) Quaternary ammonium compounds also show antimicrobial activity. Certain quaternary ammonium is used as antimicrobials. Some examples are methylbenzethonium chloride, benzalkonium chloride, cetalkonium chloride, cetrimonium, sodium chloride, domiphen bromide, cetylpyridinium chloride, didecyldimethylammonium chloride, benzethonium chloride, tetraethylammonium bromide etc.
iii) Quaternary ammonium compounds are also good against amoeba, enveloped viruses and fungi. Quaternary ammonium compounds are known to be lethal to most of the organisms except Mycobacterium tuberculosis, endospores and the non-enveloped viruses.
iv) One important application of quaternary ammonium salts is in phase transfer catalysts. These phase transfer catalysts work in accelerating the reaction between the reagents which are dissolved in the immiscible solvents.

Note: Students may get confused between quaternary ammonium salt and ammonium salt. Ammonium salts are ionic compounds having the formula $\left( {\rm{R}} \right){\rm{4}}{{\rm{N}}^ + }{{\rm{A}}^ - }$ . The quaternary ammonium cations are charged permanently, and it does not depend on the pH of the used solution.