A.
Hexahydrogallic acid. A solution of
50 g. (0.266 mole) of recrystallized gallic acid monohydrate (Note
1) in
225 ml. of 95% ethanol (Note
2) is placed in a
1-l. high-pressure hydrogenation bomb (Note
3) with
8 g. of 5% rhodium-alumina catalyst (Note
4). The bomb is then closed,
hydrogen admitted at full tank pressure (2200 lb., (Note
5)), and the temperature raised to 90–100° (Note
6) while agitation is commenced. When the
hydrogen uptake is complete (8–12 hours), heating is discontinued and the bomb is allowed to cool. The residual
hydrogen is bled off, and the contents of the bomb are rinsed out with two 40-ml. portions of warm distilled water and then heated to boiling on the
steam bath for 5 minutes to dissolve any product which has crystallized on the catalyst. After removal of the catalyst by suction filtration (Note
7), the colorless filtrate (Note
8) is concentrated on the steam bath under reduced pressure (preferably using a
rotary evaporator). The viscous residue which may have begun to deposit crystals is diluted with
75–100 ml. of ethyl acetate and the product allowed to crystallize at 0° for several hours or overnight. The product is collected on a
9-cm. Büchner funnel and washed with
75 ml. of cold 3:1 ethyl acetate-absolute ethanol and finally with
100 ml. of 30–40° petroleum ether. When dry, it weighs
21–24 g. (
45–51%); an additional
2–4 g. can usually be obtained by concentration of the mother liquors and crystallization from
ethyl acetate. Recrystallization is achieved by dissolution of the combined products in the minimum amount of boiling water (Note
9), suction filtration if necessary to remove suspended matter (Note
10), addition of hot
ethanol to bring the volume of the solution to about 110 ml., and finally addition of about
35 ml. of acetone, sufficient to produce a faint cloudiness. The solution is allowed to cool slowly to room temperature and is then stored at 0° overnight. The fine, colorless crystals are collected on a
7-cm. Büchner funnel and washed with
80 ml. of cold 5:3 absolute ethanol-acetone, then with
100 ml. of 30–40° petroleum ether. The product when dry weighs
18–20 g. (
38–43%). The yield may be increased somewhat by concentration of the combined mother liquor and washings, and treatment as before with
ethanol and
acetone. The melting point is not a useful criterion of purity, since the
hexahydrogallic acid decomposes on heating (Note
11). The product is apparently substantially the all-
cis isomer.
2