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.
Quaternary ammonium compounds are prepared by the alkylation of tertiary amines with a halocarbon. 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)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.
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, amoebas, and enveloped viruses, quats are believed to 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 commonly used in the foodservice industry as sanitizing agents.
As phase transfer catalysts
In organic synthesis, quaternary ammonium salts are employed as phase transfer catalysts (PTCs). 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.
In the 1950's, distearyldimethylammonium chloride (DHTDMAC), was introduced as a fabric softener. This compound was discontinued because the cation biodegrades too slowly. Contemporary fabric softeners are based on salts of quaternary ammonium cations where the fatty acid is linked to the quaternary center via ester linkages; these are commonly referred to as betaines and are susceptible to degradation, e.g., by hydrolysis. Characteristically, the cations contain one or two long alkyl chains derived from fatty acids linked to an ethoxylated ammonium salt. Other cationic compounds can be derived from imidazolium, substituted amine salts, or quaternary alkoxy ammonium salts.
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.
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)
Possible reproductive effects in laboratory animals
Quaternary ammonium-based disinfectants (Virex and Quatricide) were tentatively identified as the most probable cause of jumps in birth defects and fertility problems in caged lab mice. See also Hunt and Hrubek (Reproductive Toxicology, 50:163-70, 2014).
The quantification of quaternary ammonium compounds in environmental and biological samples is problematic using conventional chromatography techniques because the compounds are highly soluble in water. While analyzing them by liquid chromatography coupled tandem mass spectrometry 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 rather than .[clarification needed] Formation of is observed for di-quaternary ammonium compounds (like diquat) as precursor ion and 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.
- 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
- Silicone quaternary amine
- Tetra-n-butylammonium bromide and Aliquat 336, common phase transfer catalysts
- Tetramethylammonium chloride
- Tetramethylammonium hydroxide
- Tetramethylammonium pentafluoroxenate, containing the unusual pentagonal pentafluoroxenate (XeF5−) ion
- Triazene cleavage
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