In a 3-l. round-bottomed flask, fitted with a thermometer, a reflux condenser, and an inlet tube extending nearly to the bottom, is placed 141 g. (1 mole) of freshly distilled o-chlorobenzaldehyde (Note 1). To the upper end of the condenser is attached a tube leading to a flask containing water for the absorption of hydrogen chloride. The reaction vessel and absorption flask are weighed accurately, and a current of chlorine, dried by sulfuric acid, is passed into the aldehyde, the temperature of which is maintained at 140–160°. The rate of the current of gas is so regulated that little or no chlorine escapes. The reaction and absorption flasks are removed and weighed about every three hours. After about fifteen hours (Note 2) the absorption of chlorine at 160° practically ceases.

The total increase in weight (Note 3) amounts to 26–29 g. (75–84 per cent of the theoretical amount). The reaction product is distilled under reduced pressure, when the pure o-chlorobenzoyl chloride passes over at 93–95° /10 mm. or 137–139° /60 mm., leaving a small quantity of high-boiling residue which appears to consist mainly of an intermediate compound (Note 2). The yield is 122–126 g. (70–72 per cent of the theoretical amount) (Note 4).