GB702953A - Improvements relating to 2,8-disulphonic acid of 3,7-diaminodibenzothiophenedioxide - Google Patents

Abstract

2, 8-Disulpho-3, 7-diaminodibenzothiophene dioxide is prepared by sulphonating benzidine or 3, 7-diaminodibenzothiophene-dioxide, and heating the reaction product at a temperature of 80-140 DEG C. in the presence of 8 to 101 per cent. sulphuric acid until the 2, 8-disulphonic acid isomer content in the product is substantially increased. Preferably, the sulphonation product is heated to 2-5 hours at 100-125 DEG C. with 85-100 per cent. sulphuric acid. The 2, 8-isomer may be separated either before or after the treatment with the sulphuric acid, by adding an excess of a trialkylamine. In a modification, the 2, 8-isomer may be prepared from other sulphonic acid derivatives of 3, 7-diaminodibenzothiophene dioxide by heating with sulphuric acid as above described. In examples, (1) benzidine sulphate is sulphonated by treatment first with 25 per cent. oleum at 65 DEG C. and then adding 85 per cent. sulphuric acid at 140 DEG C., the reaction mixture is diluted to bring the sulphuric acid to 85 per cent. concentration and then heated to 122 DEG C. for 2 hours, after which the 2, 8-disulpho-3, 7-diaminodibenzothiophene dioxide is separated as the triethylamine salt; (2) benzidine sulphate is sulphonated and the product heated to 122 DEG C. for 2 hours in 100 per cent. sulphuric acid; (3) 3, 7-diaminodibenzothiophene dioxide is sulphonated and the product heated to 125-130 DEG C. for 1 hour at a sulphuric acid concentration of 67-68 per cent.; (4) benzidine sulphate is sulphonated and the product heated for 4-5 hours at 115-120 DEG C. at a sulphuric acid concentration of 45-50 per cent.; (5) benzidine sulphate is sulphonated and the product heated at a sulphuric acid concentration of 95-6 per cent. for 2 hours at 122 DEG C.; (6) benzidine sulphate is sulphonated and the product heated for half an hour at 140 DEG C. or 5 hours at 110 DEG C. at a sulphuric acid concentration of 100 per cent.; (7) benzidine sulphate is sulphonated and the product heated to 140 DEG C. for one hour at a sulphuric acid concentration of 101 per cent.; and (8) benzidine is sulphonated, the 2, 8-disulphonic acid isomer isolated as the triethylamine salt, and the residue subjected to repeated heating after adding 8 per cent. sulphuric acid, followed by cooling after each heating step to separate further quantities of the 2, 8-isomer.