A. Ethyl 3,5-dichloro-4-hydroxybenzoate. A tared 2-l. round-bottomed flask is equipped with an efficient reflux condenser connected through a calcium chloride drying tube to a gas-absorption trap, and is set up on a steam bath in a hood. In the flask are placed 250 g. (1.5 moles) of ethyl 4-hydroxybenzoate (Note 1) and 444 g. (266 ml., 3.3 moles) of sulfuryl chloride. The mixture is warmed on the steam bath (gently at first) until gas is no longer evolved (about 1 hour). Then 50 ml. of sulfuryl chloride is added to the reaction flask, and the warming is continued until gas is no longer evolved. This entire chlorination procedure requires about 1.5 hours. The excess sulfuryl chloride is then removed by attaching a water pump through an empty safety flask to the reaction flask and warming on the steam bath until no evidence of vapor can be detected above the white solid in the reaction flask. The residue in the flask should weigh about 350 g. (Note 2). Recrystallization from a mixture of 600 ml. of ethanol and 140 ml. of water gives 315–334 g. (83–88%) of ethyl 3,5-dichloro-4-hydroxybenzoate hydrate (Note 3) melting at 110–114° with decomposition.

B. 3,5-Dichloro-4-hydroxybenzoic acid. In a 2-l. round-bottomed flask equipped with a reflux condenser and set upon a steam bath are placed 315 g. (1.25 moles) of ethyl 3,5-dichloro-4-hydroxybenzoate hydrate and 600 ml. of Claisen's alkali (Note 4). Saponification is brought to completion by heating on the steam bath for 1 hour. A yellow homogeneous solution results which is diluted with 400 ml. of water and acidified to Congo red paper by pouring it into a rapidly stirred solution of 320 ml. of concentrated hydrochloric acid and 380 ml. of water in a 4-l. beaker. The thick white slurry is then cooled to 0–10°; the acid is collected on a 20-cm. Büchner funnel and washed with two 250-ml. portions of cold water (Note 5). After being freed from as much water as possible by suction, the filter cake is broken up and dissolved in a boiling mixture of 1 l. of ethanol and 350 ml. of water. Cooling to 0° gives the first crop of crystals, weighing about 200 g. Concentration of the mother liquors to 750 ml. and cooling to 0° yields an additional 40–50 g. of product of equal purity. After drying in an oven at 85–95° for 8 hours, the white crystalline acid weighs 240–250 g. (93–97%) and melts at 266–268°.

C. 2,6-Dichlorophenol. A mixture of 250 g. (1.2 moles) of dry 3,5-dichloro-4-hydroxybenzoic acid and 575 g. (600 ml., 4.8 moles) of redistilled dimethylaniline is placed in a 2-l. round-bottomed flask provided with a thermometer and a short air-cooled condenser and is heated slowly in an oil bath. Evolution of gas commences at 130° and is vigorous at 150°. The solution is heated at 190–200° for 2 hours or until the evolution of gas has ceased. After cooling, the solution is poured by portions into 600 ml. of concentrated hydrochloric acid in a 3-l. separatory funnel, with cooling from time to time by holding the funnel under a stream of cold water. When the solution is thoroughly cooled and acid to Congo red paper, the phenol is extracted with three 250-ml. portions and three 100-ml. portions of ether (Note 6). The combined ether extracts are washed with 15 ml. of 6 N hydrochloric acid, dried overnight over 20 g. of anhydrous sodium sulfate, and filtered into a 2-l. Claisen flask, and the ether is removed by distillation. After cooling, the residue begins to solidify, and 500 ml. of petroleum ether (40–60°) is added. After the phenol has been brought into solution by refluxing gently on the steam bath it is poured into a 1-l. beaker and cooled to 0°. A crop of white crystals (130–140 g.) is collected, having a melting point of 64.5–65.5°. By concentration of the mother liquor to 200 ml., a second crop (25–40 g.) melting at 64–65° can be collected. The total yield amounts to 157–180 g. (80–91%) (Note 7).