Which of the following functional groups is present in alkanoic acid?

1. Analyze the question You are asked to identify the functional group that defines alkanoic acids (also called carboxylic acids).

2. Explain the concept Alkanoic acids are named from the parent alkane by replacing the "-e" with "-oic acid" (e.g. methane → methanoic acid, ethane → ethanoic acid). Their functional group is the carboxyl group, –COOH. It is made up of:

• a carbonyl group (C=O), and
• a hydroxyl group (–OH) attached to the same carbon.

General formula: CnH2n+1COOH (or R–COOH).

3. Check each option

• A. –COOH: carboxyl group. This is the functional group of alkanoic acids. ✔
• B. –OH: hydroxyl group. It is found in alkanols (alcohols), such as ethanol.
• C. –COOR: ester group. It is found in alkyl alkanoates (esters), such as ethyl ethanoate.
• D. –CHO: aldehyde group. It is found in alkanals, such as ethanal.

4. Answer The functional group in alkanoic acid is the carboxyl group, –COOH, which is option A.

5. Exam tip Match the name ending to the functional group:

• -ol → –OH
• -al → –CHO
• -oic acid → –COOH
• -oate → –COOR

Remember: "oic acid" means the acid has both the C=O and O–H in one group.