Quaternary Amines

The term quaternary amines are synonyms to quarts. These types of amines are charged permanently, are purely unconnected to the pH of the solution, and are salts of quaternary ammonium cations. There are a lot of uses of quaternary amines such as consumer applications like antimicrobial detergents, hair conditioners, fabric softeners, and inactivating enveloped viruses; the recent example of SARS-CoV-2 can also be considered. The preparation or synthesis of quaternary amines is called quaternization. The quaternization reaction further has several utilities like the synthesis of compounds with unequal alkyl groups. This reaction is also known as the Menshutkin reaction. Examples are Benzalkonium chloride, Cetalkonium chloride, domiphen bromide, etc.

Synthesis

These compounds are synthesised simply by adding an alkyl group to the tertiary amines which consist of a halocarbon. Halocarbon compounds are those compounds that have covalent bonds with halogen atoms, which leads to the formation of compounds like organofluorine, organobromine, organoiodine compounds; the most common halocarbon compound is chlorine halocarbons and is known as organochlorines. This reaction used to synthesise the quaternary amines is known as Menshutkin reaction, but this is an older name; contemporary chemistry names it quaternization, a self-explanatory term. This reaction is often also used to synthesise compounds containing unequal alkyl chain lengths. 

For instance, during the synthesis of cationic surfactants (cationic surfactants are quaternary ammonium salts and are thus permanently charged, formed by quaternization of tertiary amines). In this synthesis, the alkyl group on amine is longer than the others.

Synthesis reaction is:  

CH3(CH2)nN(CH3)2 + ClCH2C6H5  →   [CH3(CH2)nN(CH3)2CH2C6H5]  + Cl-

The chemical reaction of quaternary amines

Quaternary amines are unreactive for most of the electrophiles, oxidants, and acids, even the stronger ones. They are also unreactive and quite stable with nucleophiles. However, the ions of quaternary amines degrade upon reacting with some exceptionally strong bases. These amines undergo reactions like Sommelet-Hauser rearrangement and Stevens rearrangement. In some very harsh conditions and with very strong nucleophiles quaternary amines undergo a dealkylation reaction. Some other cations of ammonium salts can also show reactions like Hofmann elimination and Emde degradation. 

Applications of quaternary amines

Quaternary amines are known to show antimicrobial activities and are used as antimicrobials and disinfectants. These amines are good at destroying the cell membrane and viral envelope and thus are toxic and fatal to pathogens except endospores and non-enveloped viruses. These types of ammonium compounds are cationic detergents and disinfectants. These are very effective in combination with phenol. These are deactivated by anionic detergents and are also used as a phase transfer catalyst, which accelerates reactions between those reagents which are dissolved in immiscible solvents.

Conclusion

Amines are classified into sub-categories based on the number of carbon atoms that are linked to the Nitrogen and the charge present on the nitrogen atom. When the nitrogen atom of ammonia is attached to 4 alkyl groups it is termed quaternary amines also known as quats. Quaternary amines are charged permanently and are purely unconnected to the pH of the solution. These compounds are synthesised by the alkylation of the tertiary amines which consists of a halocarbon. Halocarbon compounds form covalent bonds with halogen atoms, which leads to the formation of compounds like organofluorine, organobromine, organoiodine compounds. 

The most common halocarbon compound is chlorine halocarbons and is known as organochlorines. The reaction which can synthesise the quaternary amines was initially known as Menshutkin reaction but now it is regarded as quaternization Quaternary amines are unreactive for most of the electrophiles, nucleophile, oxidants, and acids but it forms quite stable compounds with nucleophiles. On reaction with strong bases, the ions of quaternary amines degrade. Quaternary amines can undergo several reactions like Sommelet- Hauser rearrangement and Stevens rearrangement reactions. On reacting with very strong nucleophiles quaternary amines undergo a dealkylation reaction. 

Quaternary amines are widely used in the fabric and detergent industry. Quaternary amines are known to show antimicrobial activities and are used as antimicrobials and disinfectants as they can disrupt the cell membrane and viral envelope and prove to be toxic and fatal to a large no. of organisms except endospores of micro-organisms and non-enveloped viruses.