GB651311A - Improvements in preparation of polychlorophenoxyacetic acids - Google Patents

Abstract

Polychlorophenoxyacetic acids are made by reacting an alkali metal salt of a polychlorophenol and an alkali metal salt of chloroacetic acid in a reactive medium comprising water and a substantially water-insoluble organic liquid which is a solvent for the polychlorophenol, acidifying the reaction mixture and separating the desired acid which is precipitated. By this means the yield is increased since the alkaline hydrolysis of the chloroacetic acid salt to glycollic acid is minimized as is the extent of polychlorophenol impurity in the final product. In an illustration the reaction mixture is refluxed to completion, acidified preferably with dilute sulphuric acid and cooled to room temperature. On acidification at preferably elevated temperatures, e.g. 70-90 DEG C. two layers are formed the desired acid, here 2,4-dichlorophenoxyacetic acid, is dissolved in the two phases. On cooling an emulsion is formed and the acid precipitates starting at 50-60 DEG C. Cooling to below 60 DEG C., particularly below 40 DEG C. is preferred. This acid is mainly in the organic phase although 10-20 per cent thereof remains dissolved in the organic liquid as does the excess phenol. The precipitated acid may be filtered off and the organic layer recycled or purified by, e.g. distillation before recycling. Preferably, vigorous agitation is applied during reaction and acidification. The reaction temperature is conveniently the boiling point of the mixture, e.g. 70-120 DEG C. and is preferably as high as is possible if necessary superatmospheric pressure being used. Preferably the mixture is taken to the reaction temperature as quickly as is possible. The acid is obtained in a powdery form. Specified solvents are hydrocarbons, e.g. paraffins, olefines, cycloparaffins, monocyclic aromatics, e.g. benzene, toluene and xylene and halogenated hydrocarbons, e.g. halogenated paraffins and olefins such as carbon tetrachloride, perchlorethylene and ethylene chloride, halogenated cycloparaffins, e.g. chlorocyclohexane and halogenated monocyclic hydrocarbons such as monochloro- and orthodichlorobenzene. The use of monochlorobenzene is preferred when synthesizing 2 : 4-dichlorophenoxyacetic acid. Usually organic liquids of B.Pt. 70-200 DEG C. are used. The ratio of water to organic liquid varies within wide limits being usually from 1 : 10 to about 1 : 1.5 by weight. Likewise the amounts of solvents to reactants varies but preferably keep the reaction mixture fluid throughout the reaction, e.g. indicated amounts of water are 10 to 100 per cent and of solvent 100 to 200 per cent by weight of reactants. Alkali metal salts of the reactants are obtained by the addition of alkali metal hydroxide to the reaction mixture, preferably being the last ingredient added. Preferably a substantial molar excess of phenol to acid is used, e.g. 1.2-1.6 mols. of 2,4-dichlorophenol per mol. of chloro-acetic acid. Alkali metal salts specified are those of lithium, potassium and sodium, the latter being preferred. The amount of alkali metal hydroxide used is preferably 100-110 per cent of that theoretically required to neutralize the reactants and more particularly is 100 per cent. In examples: (1) sodium chloroacetate and 2 : 4-dichlorophenolate, monochlorobenzene and water are refluxed for 3 hours, acidified with diluted sulphuric acid, filtered and washed at 15-20 DEG C., the 2 : 4-dichlorophenoxy-acetic acid thus obtained is then oven dried; (2) monochloroacetic acid, 2,4 - dichlorophenol, monochlorobenzene, water and sodium hydroxide are refluxed for 3 hours and the product collected as in (1); (3) recycled organic liquid containing 2,4-dichlorophenol is mixed with fresh 2,4-dichlorophenol, monochloroacetic acid, monochlorobenzene, water and sodium hydroxide, refluxed as rapidly as possible, acidified at 80 DEG C. with dilute sulphuric acid, cooled to room temperature, filtered to remove product, washed and the filtrate allowed to settle and the organic layer returned to a fresh batch, and in (4) a recycled organic liquid is mixed with fresh 2,4-dichlorophenol and chloroacetic acid, warmed to 30-40 DEG C. and anhydrous flake sodium hydroxide added to attain refluxing temperature, the process then continuing as in (3). The Specification as open to inspection under Sect. 91 relates to the preparation of all aryloxyaliphatic acids by similar processes. Thus it is stated that any aromatic nuclear substituted-hydroxy compound, e.g. phenols, cresols, xylenols, and naphthols and their halogeno, alkoxy, aryloxy, amino, arylamino, p nitro and carboxy substituted derivatives may be used, e.g. beta-naphthol and p1-amino-p-hydroxydiphenyl together with alpha-halogenated aliphatic acids, e.g. dichloro-acetic, alpha-chloropropionic and alpha-chlorobutyric acids, and in an example (5) recycled organic liquid is mixed with fresh beta-naphthol and chloroacetic acid, warmed to 30-40 DEG C., aqueous sodium hydroxide added and the process continued as in example (3) above to obtain beta-naphthoxyacetic acid. This subject-matter does not appear in the Specification as accepted.