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Answer
Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones. Thiamine is responsible for biosynthesis of acyloin-like compounds. In the first step of reaction, the cyanide anion (as sodium cyanide) reacts with the aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity reversal of the carbonyl group, which then adds to the second carbonyl group in a second nucleophilic addition. Proton transfer and elimination of the cyanide ion affords benzoin as the product. This is a reversible reaction.