Place 152 g (1 mol) of a good sample of commercial vanillin, m.p. 81-82°C, in a 1-L 3-necked flask equipped with a reflux condenser, a mechanical stirrer and two separatory funnels (one of which is supported over the top of the reflux condenser). Melt the vanillin by warming on a water bath and stir vigorously. Charge one funnel with a solution of 82 g of pure potassium hydroxide in 120 mL of water and the other funnel with 160 g (120 mL, 1.04 mol) of purified dimethyl sulphate at the same rate. Stop the external heating after a few minutes; the mixture continues to reflux gently from the heat of the reaction. The reaction mixture should be pale reddish-brown since this colour indicates thet it is alkaline; should the colour change to green, an acid reaction is indicated and this condition should be corrected by slightly increasing the rate of addition of the alkali. When half to three-quarters of the reagents have been added, the reaction mixture becomes turbid and separates into two layers. As soon as all reagents have been run in (about 20 minutes), pour the yellow reaction mixture into a large porcelain basin and allow to cool without disturbance, preferably overnight. Filter the hard crystalline mass of 3,4-dimethoxybenzaldehyde, grind it in a glas mortar with 300 mL of ice-cold water, filter at the pump and dry in a vacuum desiccator.
The yield of 3,4-dimethoxybenzaldehyde (Veratraldehyde), m.p. 43-44°C, is 160 g (96%). This product is sufficiently pure for most purposes; it can be purified without appreciable loss by distillation under reduced pressure, b.p. 158°C/8mmHg; m.p. 46°C. The aldehyde is easily oxidised in the air and should therefore be kept in tightly stoppered bottle.
Reference: Vogel's Textbook Of Practical Organic Chemistry, Fifth Edition, page 987
