Preparation of 2,3,5,6-tetrabromo-4-hydroxybenzoic acid
A mixture consisting of 150 g of 3,5-dibromo-4-hydroxybenzoic acid and 750 ml of 70% oleum is stirred vigorously for 30 min, a reflux condenser, and a dropping funnel whose tip just dips into the oleum, the flask being placed in a water-bath at 28-29° C. Then five portions (6, 6, 6, 5, 5 ml) of bromine (dried by concentrated sulfuric acid) are added at 29° C, each portion in 10 min at 45-min intervals. The reaction mixture is set aside overnight and then poured slowly on ice with stirring. The precipitated crude 2,3,5,6-tetrabromo-4-hydroxybenzoic acid is reprecipitated from ammonia. The yield is 76 % and the m.p. is 222°C After recrystallization from dilute ethanol 2,3,5,6-tetrabromo-4-hydroxybenzoic acid melts at 228° C.
Arch. Pharm., 61, 657 (1956).