Quaternary ammonium cation
Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR4+, R being an alkyl group or an aryl group. Unlike the ammonium ion (NH4+) 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 with an anion.
Quaternary ammonium compounds are prepared by alkylation of tertiary amines, in a process called quaternization. Typically one of the alkyl groups on the amine is larger than the others. A typical synthesis is for benzalkonium chloride from a long-chain alkyldimethylamine and benzyl chloride:
- CH3(CH2)nN(CH3)2 + ClCH2C6H5 → [CH3(CH2)nN(CH3)2CH2C6H5]+Cl-
While not very reactive, quaternary ammonium salts undergo Sommelet–Hauser rearrangement and Stevens rearrangement, as well as dealkylation under harsh conditions. Quaternary ammonium cations can also undergo the Hofmann Elimination and Emde degradation if there are hydrogens beta to the nitrogen.
Quaternary ammonium salts are used as disinfectants, surfactants, fabric softeners, and as antistatic agents (e.g. in shampoos). In liquid fabric softeners, the chloride salts are often used. In dryer anticling strips, the sulfate salts are often used. Spermicidal jellies also contain quaternary ammonium salts.
As antimicrobials 
Quaternary ammonium compounds have also been shown to have antimicrobial activity. Certain quaternary ammonium compounds, especially those containing long alkyl chains, are used as antimicrobials and disinfectants. Examples are benzalkonium chloride, benzethonium chloride, methylbenzethonium chloride, cetalkonium chloride, cetylpyridinium chloride, cetrimonium, cetrimide, dofanium chloride, tetraethylammonium bromide, didecyldimethylammonium chloride and domiphen bromide. Also good against fungi, amoeba, and enveloped viruses, quats act by disrupting the cell membrane. Quaternary ammonium compounds are lethal to a wide variety of organisms except endospores, Mycobacterium tuberculosis and non-enveloped viruses.
In contrast to phenolics, quaternary ammonium compounds are not very effective in the presence of organic compounds. Yet, they are very effective in combination with phenols. Quaternary ammonium compounds are deactivated by soaps, other anionic detergents, and cotton fibers. Also, they are not recommended for use in hard water. Effective levels are at 200 ppm. They are effective at temperatures up to 212 °F (100 °C).
Quaternary ammonium salts are popular chemicals used in foodservice industry as sanitizing agents.
As phase transfer catalysts 
In organic synthesis, quaternary ammonium salts are employed as phase transfer catalysts (PTC). Such catalysts accelerate reactions between reagents dissolved in immiscible solvents. The highly reactive reagent dichlorocarbene is generated via PTC by reaction of chloroform and sodium hydroxide.
Plant growth retardants 
Cycocel (chlormequat chloride) reduces plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Therefore, their effects are primarily on stem, petiole and flower stalk tissues. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with darker green color.
Health effects 
Quaternary ammonium compounds can display a range of health effects, amongst which are mild skin and respiratory irritation  up to severe caustic burns on skin and gastro-intestinal lining (depending on concentration), gastro-intestinal symptoms (e.g., nausea and vomiting), coma, convulsions, hypotension and death.
They are thought to be the chemical group responsible for anaphylactic reactions that occur with use of neuromuscular blocking drugs during general anaesthesia in surgery. Quaternium-15 is the single most often found cause of allergic contact dermatitis of the hands (16.5% in 959 cases)
Reproductive effects of trace amounts 
Quaternary ammonium-based disinfectants (Virex and Quatricide) were tentatively identified (by researcher Patricia Hunt - who previously uncovered the reproductive system effects of bisphenol A) and were also identified by researchers at other labs, as the most probable cause of jumps in birth defects and fertility problems in caged lab mice.
Charge issues with quaternary ammonium compounds during their quantification by ESI-MS/MS 
As quaternary ammonium compounds are classified as large volume chemicals (extensively used as antimicrobial agents, herbicides and detergents), they are one of the major environmental contaminants. Therefore, it is of interest to know about their impact on ecosystem. However, their quantification from environmental and biological samples is a major problem using conventional chromatography techniques as they are highly soluble in water. Interestingly, while analyzing them by liquid chromatography coupled tandem mass spectroscopy it has been found that they follow an exception rule. Under standard electrospray ionization (ESI) conditions mono- and di-quaternary ammonium compounds form molecular ions with the formula of m q / z q rather than ( m + z )/ z. Formation of m q / 2 is observed for di-quaternary ammonium compounds (like diquat) as precursor ion and m q / 1 as product ion due to the loss of one of the quaternary charge during CID. In di-quaternary ammonium compounds, this process can also result in the formation of fragment ions with higher mass as compared to their precursor ion. Hydrophilic interaction liquid chromatographic separation has been reported to demonstrate a successful separation of quaternary ammonium compounds for their quantification in ESI-MS/MS with higher precision. 
See also 
- Benzalkonium chloride, benzethonium chloride, methylbenzethonium chloride, cetalkonium chloride, cetylpyridinium chloride, cetrimonium, cetrimide, dofanium chloride, tetraethylammonium bromide, and domiphen bromide – antimicrobial ingredients found in various over-the-counter products
- Diquat -diquaternary ammonium compound recognized as a contact herbicide
- Cetyl trimethylammonium bromide (CTAB), stearalkonium chloride – cationic surfactants commonly used in toiletries
- Cocamidopropyl betaine (CAPB), a common Zwitterionic surfactant used ubiquitously in toiletries
- Denatonium, the most bitter compound known
- Dimethyldioctadecylammonium chloride
- Paraquat, an herbicide
- Polyquaternium, designations for quaternary ammonium-containing polymers used for personal care products
- Quaternary ammonium muscle relaxants
- Tetra-n-butylammonium bromide and Aliquat 336, common phase transfer catalysts
- Tetramethylammonium chloride
- Tetramethylammonium hydroxide
- Tetramethylammonium pentafluoroxenate, containing the unusual pentagonal pentafluoroxenate (XeF5-) ion
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- Quaternary ammonium (PIM G022)
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