ARIZONA STATE UNIVERSITY BENZILIC ACID SYNTHESIS PART I LABORATORY 1 Organic Chemistry 237
Benzilic Acid Synthesis Part I Laboratory
Arizona State University
PURPOSE OF THE EXPERIMENT
This experiment is Part I of a three-day synthesis to convert benzaldehyde to benzilic acid, with benzoin as the intermediate. Part I is the Thiamine-Catalyzed Benzoin Condensation. BACKGROUND REQUIRED
You should be familiar with vacuum filtration, recrystallization, melting point measurement, infrared spectroscopy, thin-layer chromatography, and reflux. BACKGROUND INFORMATION1
In 1958, Breslow discovered that in basic solution, thiazolium salts are effective catalysts for the benzoin condensation. What was the broader significance of Breslow’s work was the recognition that several biological cofactors contain a thiazole ring. Enzymes often require additional small molecules called coenzymes as co-catalysts. Much of the bond breaking and bond making in biochemical reactions involve coenzymes. Thiamine pyrophosphate (TPP) is a coenzyme derived from vitamin B1, also known as thiamine. Thiamine catalyzes the benzoin condensation in vitro in the absence of an enzyme. The mechanism of the thiamine-catalyzed benzoin condensation of benzaldehyde is shown in Figure 1 on the following page. A carbanion forms when a hydroxide ion deprotonates the thiamine thiazole ring. Carbanion A, being a good nucleophile, undergoes reversible addition to the benzaldehyde carbonyl, forming an alkylated thiazole derivative, B. The resonance effect of the thiazole ring increases the acidity of the α-hydrogen atom on the carbon atom adjacent to the benzene ring. Increased acidity facilitates proton transfer from the α-carbon to the oxygen, forming carbanion C. Carbanion C then adds to another molecule of benzaldehyde, forming oxyanion D. Oxyanion D is in equilibrium with oxyanion E, which, in turn, eliminates the catalyst and forms benzoin. 1 Converting Benzaldehyde to Benzilic...
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