Patent GB-396735-A (GB)
Title: The separation or purification of chlorinated derivatives of phthalic acids
IPC: C07C51/42
Filed: 1932-02-09
Abstract: The separation and purification of mixtures of chlorinated phthalic acids or anhydrides is effected by treating mixtures of such acids and/or anhydrides (which may also contain phthalic acid and/or anhydride) with sulphuric acid of from 100 to about 50 per cent strength so as to convert one or more of the organic acids into anhydride and/or vice versa, and removing one or more of the constituents of the resulting mixture of acids and anhydrides by the differential action of an organic solvent.  The action of the acid depends on its concentration and on the constitution of the bodies treated, the results being grouped as follows:--(A) Phthalic anhydride or its chlorinated derivatives which contain no chlorine in the 3- or 6- positions, i.e. -4-chlor- and 4,5-dichlorophthalic anhydride are converted to the acids by treatment with 98-100 per cent sulphuric acid; acid below 98 per cent strength does not affect either acids or anhydrides: (B) anhydrides containing one chloro atom in 3- or 6-position, i.e. 3-chloro, 3,4- and 3,5-dichlorophthalic anhydrides are converted to the acids by the action of about 95 per cent sulphuric acid, a strength of 80-55 per cent being particularly suitable; acid above 95 per cent strength converts the phthalic acids to the anhydride form: (C) anhydrides containing chlorine atoms in 3- and 6- positions, i.e. 3,6-dichloro- and tetrachlorophthalic anhydrides are converted to acids only by sulphuric acid weaker than 50 per cent strength; stronger acid results in the acid form being converted to anhydride.  After such a partial conversion of a mixture of chlorinated derivatives an organic solvent which is substantially immiscible with the sulphuric acid solution and exerts a preferential solubility for one or more of the constituents is added; thus toluene is specified as a general solvent for the anhydrides.  In examples: (1) a mixture of 3,6- and 4,5-dichlorphthalic anhydrides is treated with 98 per cent sulphuric acid at 100 DEG  C. and diluted; the mixture is then extracted with toluene which dissolves the 3,6-dichlorphthalic anhydride leaving 4,5-dichlorphthalic acid behind; (2) a mixture of 3,6- and 3,4-dichlorphthalic anhydrides is treated similarly as in (1), using a weaker sulphuric acid solution to give 3,6-dichlorphthalic anhydride and 3,4-dichlorphthalic acid; (3) phthalic anhydride is chlorinated in oleum to give a mixture of 3,4-, 4,5- and 3,6-dichlorphthalic anhydrides; the solution is diluted to give an acid concentration of 98-99 per cent sulphuric acid which is then poured on ice; by this means the 4,5-isomer is converted to the acid form, the other two being unchanged; (4) the mixture of dichlorphthalic anhydrides obtained by chlorinating phthalic anhydride in oleum in presence of iodine is diluted with water as in (3) to convert the 4,5- isomer to the acid form. Extraction of the precipitated mixture with toluene leaves this 4,5-isomer behind, and the toluene solution is then shaken with 56 per cent sulphuric acid whereby 3,4-dichlorphthalic acid is precipitated out; pure 3,6-dichlorphthalic anhydride may be obtained by crystallization from the toluene solution: (5) phthalic anhydride is chlorinated in aqueous potash and crystals of impure acid potassium-4-chlorphthalate filtered off.  The mother liquors are precipitated with calcium chloride, and the acids (phthalic, 3- and 4-chlorphthalic, dichlorphthalic) then obtained from this mixture of calcium salts.  The dry mixture of acids is treated with 99,5 per cent sulphuric acid, diluted and extracted with toluene whereby 3-chlorphthalic and some dichlorphthalic anhydrides pass into solution leaving phthalic and 4-chlorphthalic acids behind.  The toluene solution is concentrated and agitated with 56 per cent sulphuric acid whereby 3-chlorphthalic acid precipitates out. 3,6-dichlorphthalic anhydride can be crystallized out from the filtrate.