One hundred fifty grams (0.59 mole) of iodine is dissolved in 2.5 l. of 95 per cent ethyl alcohol in a 5-l. round-bottomed flask set on a tripod and provided with a mechanical stirrer and a reflux condenser. To this solution is added 300 g. (0.84 mole) of powdered dry crude p-chloromercuribenzoic acid (p. 159) (Note 1). The mixture is stirred and heated. The acid gradually goes into solution and the color of iodine disappears. The hot stirred mixture is treated with iodine until a yellow color persists for at least ten minutes. This is conveniently done by weighing out iodine in 10-g. portions, dissolving in a little alcohol, and adding through the condenser. The total amount of iodine required depends on the purity of the mercurated acid. It should not exceed 210 g. (0.83 mole)If any insoluble material is left after an excess of iodine has been added, it is removed by rapid filtration through a preheated suction filter.

On cooling, the filtrate yields about 175 g. of p-iodobenzoic acid. Concentration of the mother liquor yields crystals which are contaminated with mercuric iodide. The latter may be removed by grinding the crystals in a mortar with water and enough sodium iodide or potassium iodide to destroy the red color of the mercuric iodide (Note 2). The mixture is filtered and the crystals are washed with a little sodium iodide solution and then with water. The washed p-iodobenzoic acid melts at 266–267°. The total yield is 150–170 g. (72–81 per cent of the theoretical amount).