An MCAT cliché are questions designed to see if you can distinguish between proposed mechanisms for hydrolysis. Problems often refer indirectly to the original research, in which, to determine the likely mechanism, researchers labeled the alkoxy portion of an ester with 18O. If hydrolysis occurred by an SN2 process, the carboxylate group would leave with the 18O, and the alcohol portion of the reaction products would be lack radioactivity. On the contrary, however, it was found that the alcohol portion did contain the radioactivity, so the acyl substitution mechanism was established as the most likely one. Further support is given by the fact that the reaction occurs with retention of configuration in the alcohol, rather than inversion of configuration, which would occur through an SN2 process. On my MCAT there was a passage looking for me to interpret similar experimental results in a biochemical context. The key to the passage was understanding the acyl transfer mechanism.

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