CH\(_3\) - CH\(_2\) - COOCH\(_2\) - CH\(_3\) From the condensed structure above, the reactants are

The compound CH₃-CH₂-COOCH₂-CH₃ contains the ester functional group (-COO-). To identify the reactants that formed this ester, split the structure at the ester linkage (between the carbonyl carbon and the oxygen bonded to the alkyl group).

The ester bond in -COO- comes from two parts:

• The acid part (to the left of -COO-): CH₃-CH₂-COO-. This corresponds to the carboxylate portion of propanoic acid (CH₃CH₂COOH), a three-carbon carboxylic acid.
• The alcohol part (to the right of -COO-): -CH₂-CH₃. This corresponds to the alkyl group from ethanol (CH₃CH₂OH), a two-carbon alcohol.

The ester is therefore ethyl propanoate, formed from propanoic acid and ethanol:

\[\text{CH}_3\text{CH}_2\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}\]

The correct reactants are propanoic acid and ethanol.

Exam tip: To identify the parent acid and alcohol of an ester, break the molecule at the single-bond oxygen in the -COO- group. The fragment bonded to the carbonyl (C=O) gives the acid; the fragment bonded through the oxygen gives the alcohol.