Question

Why is the reaction of vanillin semicarbazone from vanillin run at a pH of about 5? And how is the pH of about 5 achieved in the experiment? The experiment uses a heated solution of semicarbazide hydrochloride and sodium acetate trihydrate mixed with 95% ethanol and water added with a hot solution of vanillin in 95% ethanol. I believe the acetic acid is used as a buffer to achieve the pH, but maybe its the heat? or possibly ethanol?

Answer 1

Vanillin-semicarbazone was prepared by the reaction of vanillin (1.52 g) and semicarbazone (0.52 g) in 95% ethanol or methanol as a conventional solvent following the standard procedure. The mixture was refluxed at 50 ºC for 45 min. The product, precipitated as a yellow solid, was collected by filtration and washed out with solvent. The formation of vanillin-semicarbazone in ethanol as a standard solvent is catalyzed with sulfuric acid, since the water elimination after the nucleophilic attack of semicarbazide on vanillin and acid medium is necessary for the synthesis.

The excess of methanol was removed from the crude glycerol by distillation. After distillation, the acidity ofcrude glycerol was adjusted to pH 5 by addition of 85 % phosphoric acid.