This is taken from J. Chem. Soc., 2, 1927, p. 2339 :
Preparation of 2,5-dihydroxybenzaldehyde (gentisinaldehyde)
Neubauer and Flatow (Z. Physiol. Chem., 1907, 52, 375) prepared 2,5-dihydroxybenzaldehyde by the oxidation of salicylaldehyde but the present method with m-hydroxybenzaldehyde as initial material gives an improved yield of a less crude product.
A solution of m-hydroxybenzaldehyde (61 gr) and NaOH (25 gr) in water (400 cc), while being vigorously stirred and kept at 30-35°C, is treated simultaneously from separate dropping funnels with aqueous solutions of potassium persulphate (150 gr in 1500 cc) and NaOH (200 cc, 40%), the additions being made during 90 min. at such rates that the mixture is always alkaline. After 36 hours of standing, the deep brown solution is rendered faintly acidic (Congo Red paper), and unchanged m-hydroxybenzaldehyde (31 gr) is extracted with ether. The solution is then strongly acidified (350 cc of conc HCl) and heated slowly to 70°C, the dark brown amorphous precipitate which forms gradually, being filtered off (this substance, which chars above 330°, is almost insoluble in all organic solvents, but readily soluble in aq. Na2CO3 ; it contains the aldehyde group as shown by the formation of the p-nitrophenylhydrazone, and invariably constitutes about 40% of the yield). The 2,5-dihydroxybenzaldehyde is extracted from the filtrate by ether, ether removed by evaporation, and the residue extracted by benzene. Yield : 13 gr (mp. 89-92°). This substance crystallises from benzene in bright yellow needles, but they still contain solvent. They rapidly effloresce at room temperature, leaving yellow needles, mp. 98-99°.
Gentisinaldehyde could not be obtained from 2-nitro-5-hydroxybenzaldehyde by reduction, diazotisation and decomposition of he azo-compounds. The latter however could be obtained in 30% yield.
