Ethanoic acid reacts with ethanol enriched in oxygen-18, CH3CH2-^18OH, in the presence of a few drops of conc. H2SO4. After equilibrium is reached the oxygen-18 is found

only in the ester, ethyl ethanoate

in neither, since the label is lost as CH3CH2OH vapour

equally in the ester and in the water

only in the ester, ethyl ethanoate

JEE Main levelSingle correct optionDifficulty 2 of 3ap064

Ethanoic acid reacts with ethanol enriched in oxygen-18, $CH X_{3} CH X_{2} - X_{18} X_{2}^{218} OH$ , in the presence of a few drops of conc. $H X_{2} SO X_{4}$ . After equilibrium is reached the oxygen-18 is found

In Fischer esterification the acyl carbon is attacked by the oxygen of the alcohol, and the C-O bond of the acid (acyl-oxygen fission) breaks, so the

OH

lost as water comes from the carboxylic acid. The alcohol oxygen stays bonded to the ethyl group, so

CH X_{3} CO - X_{18} X_{2}^{218} OCH X_{2} CH X_{3}

carries the label and the water does not.