Changing the Reactant Structure to Divert or Trap a Proposed Intermediate

2 Changing the Reactant Structure to Divert or Trap a Proposed Intermediate 

The nature of an intermediate can sometimes be deciphered by synthesizing a new reactant that is similar to the actual reactant under study, but for which the intermediate being proposed can react in a new and predictable manner. There are no standard ways to approach this kind of experiment, and you must design your own experiments on a case-by-case basis. We draw upon an enzymatic reaction as an example of the application of this technique. 

The enzyme mandelate racemase interconverts the enantiomers of mandelate ion (2-hydroxyphenylacetic acid). 

A carbanion alpha to the carboxylate was proposed as the intermediate formed. To test for the presence of this intermediate, an analog of mandelate ion, /, was synthesized and subjected to racemization by the enzyme. The product isolated was //, which can result from formation of 

Powers, V. M., Reynolds, L. J., Whitman, C. P., Kozarich,. W., and Kenyon, G. L. Evidence for the Generation of. Alpha.-carboxy-.aIpha.-hydroxy-f -xylylcne from p-(Bromomelhyl)mandelateby Mandelate Racemase. /. Aw. Chem. Sue., 110,323 (1988). 

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