Compelling evidence for the existence of a tetrahedral intermediate in nucleophilic acyl substitution was obtained in a series of elegant experiments carried out by Myron Bender in 1951. The key experiment was the reaction of aqueous −OH with ethyl benzoate (C6H5COOCH2CH3) labeled at the carbonyl oxygen with 18O. Bender did not allow the hydrolysis to go to completion, and then examined the presence of a label in the recovered starting material. He found that some of the recovered ethyl benzoate no longer contained a label at the carbonyl oxygen. With reference to the accepted mechanism of nucleophilic acyl substitution, explain how this provides evidence for a tetrahedral intermediate.
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