To a solution of 122 g. (1 mole) of pure salicylaldehyde (Note 1) in 1000 cc. of normal sodium hydroxide solution at room temperature, is added 1420 g. (1.2 moles) of 3 per cent hydrogen peroxide. The mixture darkens slightly in color and the temperature rises to 45–50°. The solution is allowed to stand for fifteen to twenty hours, whereupon a few drops of acetic acid are added in order to neutralize any excess alkali, and the solution evaporated to complete dryness on the water bath under reduced pressure.

The solid residue is finely crushed and warmed nearly to boiling with 500 cc. of toluene; the mixture is then poured into the folded filter paper of an extraction apparatus (p. 375) and extracted with boiling toluene for five hours. The toluene is allowed to cool and is decanted from the catechol, which crystallizes out. The insoluble material is again ground up and extracted in the apparatus with the decanted toluene. The combined product, weighing 70–76 g., consists of light brown plates melting at 104°, and is thus pure enough for many purposes. A further 6–12 g. of catechol can be obtained on distilling off the bulk of the toluene from the mother liquor. In order to obtain an entirely pure product, the crude catechol should be distilled under reduced pressure, when it passes over entirely at 119–121° /10 mm. (or 113–115° /8 mm.), and the distillate recrystallized from about five times its weight of toluene. In this way, colorless plates melting at 104–105° are obtained. The yield of the purified product is 76–80 g. (69–73 per cent of the theoretical amount) (Note 2).

In a 5-l. flask fitted with a stirrer, a thermometer reaching to the bottom, a fractionating column about 40 cm. long (Note 1), and a return inlet tube, are placed 912 g. (7.4 moles) of crystalline guaiacol and 1500 g. (1015 cc., 8.8 moles) of 48 per cent hydrobromic acid (p. 26). This apparatus is connected with a condenser, an automatic separator, and an absorption vessel, as shown in Fig. 10 (Note 2). The mixture is gently heated, with continual stirring, at such a rate that the vapor at the head of the column maintains a temperature of 85–95°. The mixture of water and guaiacol which condenses passes through the automatic separator (Note 3), the guaiacol being returned to the reaction vessel and the water collected in the tap funnel, from which it is removed from time to time. The gaseous methyl bromide is collected in an ice-cooled bottle containing a weighed quantity (about 1600 g.) of methyl alcohol (Note 4).

After six to seven hours, the temperature at the head of the column begins to rise above 95° and finally reaches 98°, at which point practically no more guaiacol passes over with the distillate. This requires about one hour more, after which the heating is stopped (Note 5). The increase in weight of the absorption bottle amounts to 400–500 g. (57–72 per cent of the theoretical amount of methyl bromide); the water collected weighs 550–560 g. (Note 6).

The residue in the flask, which has become pink in color, is extracted at 85–95° with three 1500-cc. portions of toluene (Note 7), and the united extracts are distilled under reduced pressure on the steam bath until no more moisture passes over with the vapor. The solution (2–2.5 l.) is then allowed to cool, whereupon 385–390 g. of catechol separates in colorless plates melting at 104–105°. The mother liquor is further concentrated under reduced pressure and the residue finally distilled, pure catechol passing over at 124–125° / 12 mm. The distillate, on recrystallization from a small quantity of toluene, yields 8–10 g. of pure material.

The aqueous residue from the three extractions with toluene yields, when the bulk of the water has been distilled off on the steam bath under reduced pressure, a further quantity of catechol by extraction with hot toluene, but the crystals so obtained are contaminated with a red impurity. It is therefore preferable to distil the entire residue, again collecting the fraction boiling at 124–125° /12 mm. and recrystallizing it from fresh toluene. In this way 295–305 g. of pure product is obtained. The forerun contains a little aqueous hydrobromic acid, which may be employed in a subsequent batch. The total yield of pure catechol is 690–705 g. (85–87 per cent of the theoretical amount) (Note 8).