Five hundred grams (1.5 moles) of crude p-tolylmercuric chloride (p. 519) and 720 g. (4.6 moles) of potassium permanganate are added to a solution of 1200 g. (30 moles) of sodium hydroxide in 18 l. of water contained in a large enameled pail (or similar vessel) placed on a 15-cm. Fletcher burner and provided with a mechanical stirrer. The mixture is stirred and heated as near boiling as possible for fifteen minutes. The highest temperature conveniently obtainable is 95°. The tolylmercuric chloride dissolves (Note 1), forming the sodium salt of p-hydroxymercuribenzoic acid. The solution is cooled somewhat and alcohol is added to react with the excess of potassium permanganate (250 cc. is generally required to completely decolorize the solution). The manganese dioxide thus precipitated is removed by filtering with suction (Note 2). The filtrate should be clear and colorless. (If it is green, not enough alcohol has been added.)

After the filtrate has become thoroughly cool (Note 3), it is acidified with hydrochloric acid, about 1700 cc. of technical concentrated acid being necessary. The precipitated p-chloromercuribenzoic acid is difficult to filter because of its flocculent character. It is best to allow it to stand at least overnight and then filter by suction after siphoning most of the clear supernatant liquid. The acid retains much water and should be dried at 110° (Note 4). The yield is 350–420 g. (61–74 per cent of the theoretical amount). The crude acid may be purified by reprecipitation with hydrochloric acid from a solution in sodium hydroxide (Note 5). The crude acid is sufficiently pure for the preparation of p-iodobenzoic acid (p. 325). It can be used for this purpose without drying.