Preparation of 2,2′-(hydrazine-1,2-diyl)dibenzoic acid
A solution is made of 33.4 grams (0.2 mole) of pure o-nitrobenzoic acid in 120 ml. 40% sodium hydroxide solution and 40 ml. water, by heating to 100°. Zinc dust is added to this solution, in small portions with vigorous stirring, at such a rate that the temperature of the solution stays at 100-105° without external heating, continuing the addition until the originally deep yellowish brown solution is completely decolorized. About 40-50 grams of zinc dust is required, depending on its quality, and the addition requires about 15 minutes. As soon as the solution becomes colorless, 600 ml. hot water is added, and the excess zinc dust is filtered off and washed with hot water. The filtrate is treated with 20 ml. 2 N acetic acid and a few drops of sodium bisulfite solution (to prevent oxidation), and then 140 ml. of concentrated hydrochloric acid is added dropwise with good stirring. At this point, the solution should be strongly acid to litmus, but not to Congo red, so that rearrangement does not occur. (The acetic acid added earlier facilitates attainment of the correct acidity.) A filtered test portion of the solution should give no precipitate on addition of acetic acid, and additional acid is added if necessary. The solution is then cooled and the 2,2′-(hydrazine-1,2-diyl)dibenzoic acid, which separates in a crystalline, easily filterable form, is filtered off with suction and washed thoroughly with cold water.
The fundamental processes of dye chemistry, by H. E. Fierz-David, 164, 1949