224 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Manufacture Most quaternary ammonium compounds are manufactured by reacting a tertiary amine with an ester. In some cases the amine or the ester is formed in situ. Examples: Alkyltrimethylammonium Salts A well-known member of this group of cationics is cetyltrimethylam- monium bromide. It may be prepared by one of the following methods: (a) By reacting cetyl bromide with trimethylamine: + -- C•0H•Br + (CH•)•N C•0H•N(CH•)•Br. Excess amine is employed, which must be totally removed when isolating the product, traces which cannot be detected by normal analytical methods being sufficient to produce the characteristic fishy odour of trimethylamine. The cetyl bromide is prepared by the action of anhydrous hydrogen bromide on cetyl alcohol. The crude cetyltrimethylammonium bromide before purification may contain impurities arising from impure reagents and from side-reactions resulting from inadequate control of the reaction conditions. These impuri- ties can be cetyl alcohol, hexadecene, hexadecyldimethylamine, tri- and tetra-methylamine bromides, among others. (b) By reacting cetyl bromide (or chloride) with dimethylamine to yield cetyldimethylarnine: C•6H•Br + (CH•)•NH--•C•0H•N(CH•)• + HBr which is isolated and quaternised in the further step: + -- C•6H•N(CH•)• + CH•Br--C•0H•N(CH•)•Br. by reacting with methyl bromide. This reaction is vigorously exothermic and produces a crude cetyl- trimethylammonium bromide which is more readily purified than that from the preceding synthesis. (c) Cetylamine is reacted with excess methyl bromide in the presence of caustic soda: C•0H•NH• + 3CH•Br -- C•0H0•N(CH•)•Br + 2HBr. The sodium bromide which is produced in this reaction is difficult to remove completely from the product. Alternatively, the primary amine may be methylated by reaction with formaldehyde and formic acid, followed by isolating the cetyldimethylamine and proceeding as in (b).

CATIONIC SURFACE-ACTIVE AGENTS 225 Alkyldimethylbenzylammonium salts This type of cationic is typified by stearyldimethylbenzylammonium chloride. For this compound, one synthesis only is employed in industry. Stearyldimethylbenzylammonium chloride.--Stearyldimethylamine is re- acted with benzyl chloride: C•sH3,N(CH•)• +•CH•C1 -+C•sH•7N(CC1ICH• I This reaction is exothermic but the following difficulties are encountered: unless an excess of benzyl chloride is used, the crude product contains free tertiary amine which is difficult to remove if an excess of benzyl chloride is employed, this is more difficult still to remove from the product. Differences between commercial samples of this type of material generally relate to the quantity of free amine, amine hydrochloride, benzyl alcohol and benzyl chloride which are present as impurities. Some attempts have been made to synthesise by reacting the long- chain alkyl halide with benzyldimethylamine: C•H37C1 + (CH•)•NCH• • S.D.B.A.C. but the reaction velocity is low and poor yields are obtained. Purification of the crude is more difficult 'than with the alternative reaction above, and the general economics of the process a•e unsatisfactory. Alkyl pyridinium halides Cetylpyridinium chloride represents this type of compound. One synthesis only is commercially employed, the reaction between cetyl chlcride and pyridine, itself a tertiary amine. N• • C•H•C1 + x • + C1- N I The cetyl chloride .ks prepared by hydrohalogenation of the alcohol under such conditions that no secondary halide is produced. •