GB628401A - Improvements in or relating to the nuclear chlorination of aromatic carboxylic acids - Google Patents

Abstract

Nuclear chlorination of aromatic carboxylic acids in which the carboxyl groups are nuclear and which have at least one nuclear hydrogen atom, is effected by treatment with an aqueous solution of hypochlorous acid at pH 2.0 to 6.0, preferably 3.5 to 5.0. The aromatic acid may be generated in situ by the hydrolysis of such compounds as the acid anhydride or chloride or salt of the acid. It is preferred to use hypochlorous acid in a slight excess and in the form of metal salts. Calcium and sodium are preferred hypochlorites, although potassium lithium, strontium, magnesium and barium hypochlorites are mentioned. Aromatic acids mentioned are benzoic, phthalic, iso- and terephthalic, salicylic, toluic, p-acetyl-benzoic, anthranilic, o-benzoylbenzoic, o-, m-, and p-bromo- or chloro-benzoic, and anisic acids, dichloro-, dihydroxy-, and nitro-substituted benzoic acids and bromo-, chloro-, e.g. di- and tri-chloro, nitro-, and hydroxy-substituted phthalic acids. Small quantities of alcohol or acetone may be added to effect solution of the aromatic acid. The pH is maintained within the required range by buffer salts or by the addition of small amounts of a strong, e.g. hydrochloric, acid during the reaction, although when a dicarboxylic acid is chlorinated to the mono derivative the reaction produces its own buffer effect. With a dicarboxylic acid the acid salt of the corresponding chlorinated acid may be obtained from which the free acid may be liberated. In examples, phthalic anhydride is treated with aqueous sodium hypochlorite containing a small proportion of sodium hydroxide to yield (1) the mono sodium salt of 4-mono-chloro-phthalic acid which is converted to the free acid by treatment with calcium chloride and sulphuric acid; and (2) 4,6-dichloro-phthalic acid, the reaction being maintained within the desired pH range by the addition of hydrochloric acid; and in (3) phthalic anhydride treated with aqueous calcium hypochlorite, yields the calcium salt of 4-mono-chloro-phthalic acid, which yields the free acid on treatment with sulphuric acid. In the Specification as open to inspection under Sect. 91 it is stated that the hypochlorite may be generated in situ, e.g. by adding chlorine to a solution of sodium hydroxide and in example (3) chlorine is passed through a solution of phthalic acid in sodium hydroxide, the mono-sodium salt of mono-chloro-phthalic acid thus obtained and the free acid liberated by treatment with hydrochloric acid. This subject-matter does not appear in the Specification as accepted.