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Concept
Quaternary ammonium cation
Summary
In chemistry, quaternary ammonium cations, also known as quats, are positively-charged polyatomic ions of the structure , where R is an alkyl group, an aryl group or organyl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged, independent of the pH of their solution. Quaternary ammonium salts or quaternary ammonium compounds (called quaternary amines in oilfield parlance) are salts of quaternary ammonium cations. Polyquats are a variety of engineered polymer forms which provide multiple quat molecules within a larger molecule.
Quats are used in consumer applications including as antimicrobials (such as detergents and disinfectants), fabric softeners, and hair conditioners. As an antimicrobial, they are able to inactivate enveloped viruses (such as SARS-CoV-2). Quats tend to be gentler on surfaces than bleach-based disinfectants, and are generally fabric-safe.
Quaternary ammonium compounds are prepared by the alkylation of tertiary amine. Industrial production of commodity quat salts usually involves hydrogenation of fatty nitriles, which can generate primary or secondary amines. These amines are then treated with methyl chloride.
The quaternization of alkyl amines by alkyl halides is widely documented. In older literature this is often called a Menshutkin reaction, however modern chemists usually refer to it simply as quaternization. The reaction can be used to produce a compound with unequal alkyl chain lengths; for example when making cationic surfactants one of the alkyl groups on the amine is typically longer than the others. A typical synthesis is for benzalkonium chloride from a long-chain alkyldimethylamine and benzyl chloride:
CH3(CH2)\mathit{n}N(CH3)2{} + ClCH2C6H5 ->{} [CH3(CH2)\mathit{n}N(CH3)2CH2C6H5]+Cl-
Quaternary ammonium cations are unreactive toward even strong electrophiles, oxidants, and acids. They also are stable toward most nucleophiles.
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