Methyl benzoate is hydrolysed with NaOH dissolved in H2^18O-enriched water (so that the hydroxide is ^18OH-). Assume acyl-oxygen cleavage (BAC2). The ^18O label is found in

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Methyl benzoate is hydrolysed with NaOH dissolved in $H X_{2} X_{18} X_{2}^{218} O$ -enriched water (so that the hydroxide is $X_{18} X_{2}^{218} OH X^{-}$ ). Assume acyl-oxygen cleavage (BAC2). The $X_{18} X_{2}^{218} O$ label is found in

In BAC2 the nucleophile

X_{18} X_{2}^{218} OH X^{-}

attacks the carbonyl carbon and the tetrahedral intermediate expels

CH X_{3} O X^{-}

(the C-O bond to the alkyl group stays intact). The label thus ends up in the carboxylate and methanol carries only

X_{16} X_{2}^{216} O