Inorganic chemistry,quaternary ammonium cations, also known asquats, are positively-chargedpolyatomic ions of the structure[NR₄]⁺, where R is analkyl group, anaryl group^[1] ororganyl group. Unlike theammonium ion (NH+4) and the primary, secondary, or tertiary ammoniumcations, thequaternary ammonium cations are permanently charged, independent of thepH of their solution.Quaternary ammonium salts orquaternary ammonium compounds (calledquaternary amines inoilfield parlance) aresalts of quaternary ammonium cations.Polyquats are a variety of engineeredpolymer forms which provide multiple quat molecules within a larger molecule.

Quats are used in consumer applications including asantimicrobials (such asdetergents anddisinfectants),fabric softeners, andhair conditioners. As an antimicrobial, they are able to inactivateenveloped viruses (such asSARS-CoV-2). Quats tend to be gentler on surfaces thanbleach-based disinfectants, and are generally fabric-safe.^[2]

Quaternary ammonium compounds are prepared by thealkylation oftertiary amine. Industrial production of commodity quat salts usually involves hydrogenation offatty nitriles, which can generate primary or secondary amines. These amines are then treated withmethyl chloride.^[4]

The quaternization of alkyl amines byalkyl halides is widely documented.^[5] In older literature this is often called aMenshutkin reaction, however modern chemists usually refer to it simply asquaternization.^[6] The reaction can be used to produce a compound with unequal alkyl chain lengths; for example when makingcationic surfactants one of the alkyl groups on the amine is typically longer than the others.^[7] A typical synthesis is forbenzalkonium chloride from a long-chain alkyldimethylamine andbenzyl chloride:

CH₃(CH₂)_nN(CH₃)₂ + ClCH₂C₆H₅ → [CH₃(CH₂)_nN(CH₃)₂CH₂C₆H₅]⁺Cl⁻

Quaternary ammonium cations are unreactive toward even strongelectrophiles,oxidants, andacids. They also are stable toward mostnucleophiles. The latter is indicated by the stability of thehydroxide salts such astetramethylammonium hydroxide andtetrabutylammonium hydroxide even at elevated temperatures. The halflife of Me₄NOH in 6M NaOH at 160 °C is >61 h.^[8]

Because of their resilience, many unusual anions have been isolated as the quaternary ammonium salts. Examples includetetramethylammonium pentafluoroxenate, containing the highly reactive pentafluoroxenate (XeF⁻
₅) ion.Permanganate can be solubilized inorganicsolvents, when deployed as itsNBu⁺
₄ salt.^[9][10]

With exceptionally strong bases, quat cations degrade. They undergoSommelet–Hauser rearrangement^[11] andStevens rearrangement,^[12] as well as dealkylation under harsh conditions or in presence of strong nucleophiles, like thiolates. Quaternary ammonium cations containing N−C−C−H units can also undergo theHofmann elimination andEmde degradation.

• Tetramethylammonium ion:(CH₃)₄N⁺, also denotedMe₄N⁺ (Me =methyl group)
• Tetraethylammonium ion:(C₂H₅)₄N⁺, also denotedEt₄N⁺ (Et =ethyl group)
• Tetrapropylammonium ion:(n-C₃H₇)₄N⁺, also denotedPr₄N⁺ (Pr =propyl group)
• Tetrabutylammonium ion:(n-C₄H₉)₄N⁺, also denotedBu₄N⁺ (Bu =butyl group)

Quaternary ammonium salts are used asdisinfectants,surfactants,fabric softeners, and asantistatic agents (e.g. inshampoos). Inliquid fabric softeners, thechloride salts are often used. Indryeranticling strips, thesulfate salts are often used. Older aluminiumelectrolytic capacitors andspermicidal jellies also contain quaternary ammonium salts. Quats are also used in contraception formulations, veterinary products, diagnostic testing, vaccine production, and nasal formulations.^[14]

Concerns have been raised about the level of understanding of safety profile of quat disinfectants on people. As of August 2020, half of disinfectants theUnited States Environmental Protection Agency suggested as effective againstCOVID-19 contained one of the quats, and often a quat as the sole ingredient.^[15]Salmonella andE. coli O157:H7 exposed to quats have developed cross resistance to antibiotics. A subject of concern is the potential effect of increased use of quats related toCOVID-19 pandemic on antibiotic resistance in a larger microbial community in nature and engineered environment.^[16]

Quaternary ammonium compounds have antimicrobial activity.^[18] Quaternary ammonium compounds, especially those containing long alkyl chains, are used asantimicrobials anddisinfectants. Examples arebenzalkonium chloride,benzethonium chloride, methylbenzethonium chloride,cetalkonium chloride,cetylpyridinium chloride,cetrimonium,cetrimide, dofanium chloride,tetraethylammonium bromide,didecyldimethylammonium chloride anddomiphen bromide. Also good againstfungi,amoebas, andenvelopedviruses (such asSARS-CoV-2),^[19][20] most quaternary ammonium compounds are believed to act by disrupting thecell membrane orviral envelope.^[21] (Some QACs, such asdequalinium and similar bis-QACs, show evidence of a different mode of action.)^[22]

Quaternary ammonium compounds are lethal to a wide variety of organisms exceptendospores andnon-enveloped viruses, both having no accessible membrane coat to attack. It is possible to solve the endospore problem by adding chemicals which force them to germinate.^[23][24] They have reduced efficacy againstgram-negative bacteria,mycobacteria, and bacteria inbiofilms due to them having additional layers that need to be penetrated or disrupted. Some bacteria such as MRSA have acquired resistance genes,qacA/B andqacC/D, that pump the cation out of the cell.^[22]

In organic chemistry, quaternary ammonium salts are employed asphase transfer catalysts (PTCs). Such catalysts accelerate reactions between reagents dissolved in immiscible solvents. The highly reactive reagentdichlorocarbene is generated via PTC by reaction ofchloroform and aqueoussodium hydroxide.^{[citation needed]}

In the 1950s,distearyldimethylammonium chloride (DHTDMAC), was introduced as afabric 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 asbetaine-esters or ester-quats and are susceptible to degradation, e.g., byhydrolysis.^[26] Characteristically, the cations contain one or two longalkyl chains derived from fatty acids linked to anethoxylated ammonium salt.^[27] Other cationic compounds can be derived fromimidazolium,guanidinium, substituted amine salts, or quaternaryalkoxy ammonium salts.^[28]

• Cationic surfactants used as fabric softeners
• 
  Distearyldimethylammonium chloride, an early generationfabric softener with lowbiodegradability that was phased out.
• 
  Another diesterquat, a contemporary fabric softener.
• 
  Diethyl ester dimethyl ammonium chloride used as a fabric softener.
• 
  Another diesterquat used as a fabric softener.

The antistatic qualities that make quaternary ammonium salts useful as fabric softeners also make them useful inhair conditioners andshampoos.^[29] The idea was pioneered byHenkel with a 1984 patent.^[30] Examples includecetrimonium chloride andbehentrimonium chloride.^[31]

Cycocel (chlormequat chloride) reduces plant height by inhibiting the production ofgibberellins, 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.^[32]

Several quaternary ammonium derivatives exist in nature.^[33] Prominent examples includeglycine betaine,choline,carnitine, butyrobetaine,homarine, andtrigonelline. Glycine betaine, anosmolyte, stabilizes osmotic pressure in cells.^[34]

Choline is a precursor for the neurotransmitteracetylcholine. Choline is also a constituent oflecithin, which is present in many plants and animal organs.^[35] It is found inphospholipids. For example,phosphatidylcholines, a major component ofbiological membranes, are a member of thelecithin group of fatty substances in animal and plant tissues.^[36]

Carnitine participates in thebeta-oxidation of fatty acids.^{[citation needed]}

Quaternary ammonium compounds can display a range of health effects, amongst which are mild skin and respiratory irritation^[37] up to severe caustic burns on skin and thegastrointestinal wall (depending on concentration), gastrointestinal symptoms (e.g., nausea and vomiting), coma, convulsions, hypotension and death.^[38]

They are thought to be the chemical group responsible foranaphylactic reactions that occur with use ofneuromuscular blocking drugs duringgeneral anaesthesia insurgery.^[39]Quaternium-15 is the single most often found cause of allergiccontact dermatitis of the hands (16.5% in 959 cases).^[40]

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. The quat ingredients in the disinfectants includealkyl dimethyl benzyl ammonium chloride (ADBAC) anddidecyl dimethyl ammonium chloride (DDAC).^[41][42] A similar link was tentatively identified in nurses.^[43] The studies contradict earlier toxicology data reviewed by the U.S. Environmental Protection Agency (U.S. EPA) and the EU Commission.^[44]

Thequantification of quaternary ammonium compounds can be challenging. Some methods include precipitation of solid salts withtetraphenylborate. Another method, an Epton titration, involves partitioning between water-chloroform in the presence of an anionic dye. Individual cations are detectable byESI-MS and NMR spectroscopy.^[4]

• Ammonium
• Benzyltrimethylammonium fluoride
• Iminium

• "Toxicities of quaternary ammonium".Internationally Peer Reviewed Chemical Safety Information (INCHEM). World Health Organization.

• Quaternary ammonium compounds
• Disinfectants
• Food safety

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