Chemistry 30A3 Fall 2011 Labs Dr. Troendle 48

Chemistry 30A3 Fall 2011 Labs Dr. Troendle 48 - carbanion...

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Chemical Biology 3OA3, September 2009 Page 48 The Benzoin Condensation Two molecules of an aromatic aldehyde, when heated with a catalytic amount of sodium or potassium cyanide in aqueous ethanol, react to form a new carbon-carbon bond between the carbonyl carbons. The product is an α-hydroxy ketone (a class of compounds with the generic name benzoin). The mechanism for cyanide-catalyzed benzoin formation involves a rather long sequence of steps. (The mechanism can be found in most of the introductory organic chemistry texts.) It starts with reversible cyanide ion addition to the carbonyl group of one benzaldehyde to form the anion of the cyanohydrin, which in aqueous ethanol rapidly equilibrates with the neutral cyanohydrin. The acidity of the C-H bond adjacent to the cyano group is enhanced by resonance stabilization of the anion and under the basic conditions of the reaction (NaCN is basic) the isomeric
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Unformatted text preview: carbanion is formed. This adds to a second molecule of benzaldehyde; proton interchange and loss of cyanide ion lead to benzoin. There are two requirements for an effective catalyst of the benzoin condensation. First, the catalyst must give significant amounts of carbonyl adduct (steps 1 and 2), but not form such a strong bond that the catalyst is not easily lost in the last step. Second, the catalyst must stabilize the anion sufficiently to allow the C-H bond to be broken readily, but not so much that the anion becomes unreactive. For more than 100 years, the only species that had been found that satisfied these requirements was the cyanide ion. However, in 1958 Breslow discovered that the conjugate base of a thiazolium salt also was an effective catalyst; it added reversibly to aldehydes and stabilized the -anion by resonance....
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This note was uploaded on 01/08/2012 for the course CHM 30A3 taught by Professor Ricktroendle during the Fall '11 term at UNF.

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