The chlorination of 4‐aminobenzoic acid and its methyl ester

Abstract

AbstractChlorination of 4‐acetylaminobenzoic acid (III) afforded 4‐acetylamino‐3,5‐dichlorobenzoic acid (IV, m.p. 237‐238°), which could be saponified easily to 4‐amino‐3,5‐dichlorobenzoic acid (II, m.p. 290°). Direct chlorination of 4‐aminobenzoic acid (I) in a mixture of glacial acetic acid and hydrochloric acid afforded 3,4,4,6,6‐pentachloro‐1‐cyclohexen‐5‐one‐2‐carboxylic acid (V, m.p. 178‐180°), a compound already obtained by Zincke by the chlorination of 4‐hydroxybenzoic acid (VI). Compounds with a higher content of chlorine could be prepared by the chlorination of the methyl ester of I (VIII). The reaction carried out in a neutral medium led to the nitrogen containing compound IX with m.p. 151‐153° and gross formula C8H5NO2Cl8 to which the structure of a methyl 1,2,3,3,5,5,6‐heptachloro‐4‐chloriminocyclohexane‐1‐carboxylate was attributed. Reduction with stannous chloride converted IX into methyl 4‐amino‐3,5‐dichlorobenzoate (X), which can be used as a starting product for IX. Chlorination of VIII in a mixture of glacial acetic acid and hydrochloric acid yielded a compound XI with m.p. 129‐131° and gross formula C8H5O3Cl7 as the main reaction product to which the structure of a methyl 1,2,3,3,5,5,6‐heptachlorocyclohexan‐4‐one‐1‐carboxylate has been attributed. Acid hydrolysis converted IX into XI. From the same reaction mixture two other stereo‐isomers of XI also were isolated, be it in minor quantities (XII m.p. 140–142°, XIII m.p. 213‐215°). The stereoisomerism of the three compounds is caused by the different spatial arrangement of the chlorine atoms attached to carbon atoms 1, 2 and 6. Reduction converted the three isomers into methyl 3,5‐dichloro‐4‐hydroxybenzoate (XIV). Chlorination of the methyl esters of VI and XIV also afforded amounts of XI. A fourth chlorination product could be isolated from the reaction mixture of VIII, viz. a compound with m.p. 199‐200° and gross formula C8H6O4Cl6 (XVI). Reduction with stannous chloride converted it into a compound XVII with m.p. 121‐122° and gross formula C8H8O4Cl2. Both compounds contain an extra hydroxyl group. Lack of material prevented further establishment of the structure. The infrared spectra of a number of polychloro‐compounds are given.