In Experiment 12B we used an umpolung reagent in an umpolung synthesis to synthesize benzoin from benzaldehyde. Umpolung synthesis is a synthetic approach that allows us to switch the identity of a reacting species. For example, the normal chemical reactivity of a nucleophile can be switched to one of an electrophilic compound. Two common agents are cyanide and thiamine (Vitamin B). The mechanisms for both agents are the same. In this experiment, thiamine was used to turn benzaldehyde, an electrophile, into a nucleophile. This nucleophilic benzaldehyde then reacts with another molecule of benzaldehyde to make benzoin. This means that benzaldehyde is the source of the electrophile and nucleophile in this reaction. The two molecules of benzaldehyde
