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).