Which of the following is a characteristic functional group of esters?

• A. Carboxyl group ($-COOH$)
• B. Hydroxyl group ($-OH$)
• C. Carbonyl group ($-C=O$)
• D. Ester group ($-COO-$)

Answer: (D)

Verified step by step guidance

1. Understand the structure of an ester: Esters are organic compounds derived from carboxylic acids where the hydroxyl group (-OH) is replaced by an alkoxy group (-O-R).
2. Identify the functional group of an ester: The characteristic functional group of esters is the ester linkage, which is represented as -COO-. This consists of a carbonyl group (C=O) adjacent to an ether linkage (O-R).
3. Differentiate between the given options: The carboxyl group (-COOH) is characteristic of carboxylic acids, the hydroxyl group (-OH) is characteristic of alcohols, and the carbonyl group (-C=O) is found in aldehydes and ketones.
4. Recognize the ester group: The ester group (-COO-) is unique to esters and is not the same as the carboxyl, hydroxyl, or carbonyl groups individually.
5. Conclude the identification: Based on the structural understanding, the ester group (-COO-) is the correct characteristic functional group of esters.