What type of molecule contains an ester link ─COO─?

The presence of an ester link ─COO─ is characteristic of esters, which are formed when an alcohol reacts with a carboxylic acid. The structure of an ester includes a carbonyl group (C=O) directly bonded to an alkoxy group (─O─R, where R is an alkyl group). This specific linkage is responsible for the unique properties of esters, including their common use in fragrances and flavorings.

In contrast, alcohols contain a hydroxyl group (─OH) and lack the carbonyl component necessary to form an ester. Carboxylic acids are defined by the presence of a carboxyl group (─COOH), and while they are involved in the formation of esters, they do not contain the ester link themselves. Aldehydes feature a carbonyl group at the end of a carbon chain (R─CHO) but also do not possess the ester functional group. Hence, the correct identification of an ester link ─COO─ directly leads to recognizing the molecule as an ester.