The reduction of vanillin to vanillyl alcohol can be carried out with sodium borohydride. Considering the extreme reactivity of most hydrides (e.g. NaH, LiAlH4) towards water, sodium borohydride is surprisingly stable in aqueous solution, reacting slowly with alcohols and water as long as the pH is high (basic) and the temperature is low. Insoluble in ether, NaBH4 is a mild and selective reducing agent in aqueous NaOH and ethanol solutions, reducing aldehydes and ketones rapidly at room temperature, and esters and alcohols very slowly. It is inert towards functional groups that are readily reduced by LiAlH4, including carboxylic acids, epoxides, lactones, nitro groups, nitriles, azides, amides, and acid chlorides. The reduction of vanillin will be performed under basic reaction conditions using aqueous NaOH, which serves to stabilize the borohydride, as well as increase the solubility of vanillin in water. When the reduction is complete, the product is still dissolved in the reaction mixture because it remains in phenoxide form. The work-up sequence isolates the vanillyl alcohol from the reaction mixture and, at the same time, destroys any excess borohydride.
Procedure: Reduction of Vanillin to Vanillyl Alcohol (adapted from an article by J. Fowler in J. Chem Ed, 69(2) pg A43, 1992)
Vanillin (0.76 g, 5 mmol) was dissolved in aqueous NaOH (1M, 5 mL) in a small beaker, and the contents were stirred until a homogeneous yellow solution formed. The contents of the beaker were cooled in an ice bath until they were cool to the touch. A reaction tube was used to obtain NaBH4 filled to the 0.4-mL gradation mark (ca. 0.15 g). [NOTE: Do NOT weigh the NaBH4 on a balance! Do NOT bring it out of the hood at all for any reason!] The sodium borohydride was carefully added to the vanillin solution with continuous mixing. Small aliquots of the reaction mixture were used to rinse all of the borohydride out of the reaction tube. The reaction mixture was then allowed to stand at room temperature for twenty minutes.
The reaction mixture was again placed in an ice bath in the fume hood, and aqueous HCl (2.5 M) was added dropwise while swirling. The solution became colorless and ceased frothing. When tested with pH paper, the mixture was strongly acidic. A small amount of white precipitate was observed. Continued cooling led to the formation of a large amount of white crystals. This product was collected via vacuum filtration and was washed with 3 x 1-mL portions of ice water. The mass of this crude product was recorded, and a small portion was saved for crude melting point determination.
The crude product was recrystallized from water. The mass and melting point of the dry, purified product were determined. (XX g, XX–XX °C)