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).