What is the structure of a carboxylic acid?

• A. R(C=O)R'
• B. R(C=O)H
• C. RCO2R'
• D. RCO2H

Answer: (D)

The structure of a carboxylic acid is represented by RCO2H. In this notation, R stands for a hydrocarbon chain or any substituent group attached to the carboxylic acid functional group. The carboxylic acid functional group consists of a carbon atom (C) double-bonded to an oxygen atom (O) and also single-bonded to a hydroxyl group (OH).

This characteristic grouping is known as a carboxyl group (–COOH), which exemplifies the defining feature of carboxylic acids. The presence of both a carbonyl group (C=O) and the hydroxyl group (–OH) is essential for classifying a compound as a carboxylic acid, as these components directly contribute to the functional properties of carboxylic acids, including their acidity.

In contrast, other options represent different structures. For example, the presence of two R groups linked by a carbonyl (as in the first option) indicates a ketone or possibly an ester if there is an -OR group instead of -OH. Therefore, the option that clearly fits the definition of a carboxylic acid, with the –COOH group specified, is the