5-Bromovanillin

3-Bromo-4-hydroxy-5-methoxybenzaldehyde, or 5-bromovanillin, is a relatively obscure trisubstituted benzaldehyde used an intermediate in the synthesis of syringaldehyde and other related compounds. At room temperature, it appears as a light yellow crystalline solid with an intoxicating smell rather like vanillin. It can be made with relative ease via low temperature bromination of vanillin.


Method:

Into a  500ml two necked round bottom flask, I poured a solution of 30.42g of vanillin in 200ml of methanol. A separatory funnel (stopcock closed!) containing 35.2g of dry Bromine was attached to one neck of the flask, and the other was stoppered off. The flask was placed in an ice bath (with salt added to decrease temperature in bath further). After leaving contents of flask to cool for a while, the addition of the bromine was begun, a couple of drops at a time , swirling the flask to mix contents each time. Care was taken to make sure the temperature did not rise much over this period. Addition of all the bromine took about 1 hour. After this, I removed the mixture from the ice bath, and stirred it periodically for 1 hour. Mixture was then cooled in another ice bath to around 0 C, whereupon 200ml of water were gradually added (In same way as the bromine, via separatory funnel). The mixture was then filtered to collect the precipitated product, which was washed on the filter with 400ml water, then 200ml 70% methanol solution (0 C). After drying, I obtained 45g of 5-bromovanillin as a fine light yellow crystalline powder, a 97% yield if pure.

C6H3(OCH3)(OH)COH + Br2 ==> C6H2Br(OCH3)(OH)COH + HBr