A Case of the Benz. Part II

The so-called condensation of benzaldehyde to benzoin was originally performed with a cyanide catalyst. The origins of this reaction go back as far as an 1832 report by Wöhler and Liebig. The reaction can be performed in a biomimetic manner with thiamine as a catalyst under basic conditions as proposed by Ronald Breslow in a 1958 article in the Journal of the American Chemical Society.  In the undergraduate laboratory curriculum, the reaction mixture is prepared during one laboratory period and worked up during the subsequent session. Workup involves simply filtering the solid benzoin from the reaction mixture and washing it with cold water and/or methanol. We performed the synthesis in parallel with the previously described Diels-Alder reaction which also requires two laboratory sessions for completion. The amount of thiamine catalyst that is used seems considerable since it is a fluffy white substance. Experimental notes usually state that the thiamine catalyst is likely to be susceptible to degradation by extremes of pH and heat. Some reactions call for heating the reaction at 60 °C to initiate the reaction. The reaction features the rather dramatic formation of a white solid from a transparent yellowish reaction mixture. The reaction appears to be quite robust. Typically, recrystallization is not recommended for this step.benzaldehyde2benzoin01

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