When naming a carboxylic acid according to IUPAC rules, which carbon atom should be designated as carbon $1$ in the parent chain?

• A. The carbon atom of the carboxyl group ${\mathrm{COOH}}_{}$
• B. The carbon atom of the first substituent group
• C. The carbon atom in the longest continuous chain that is not part of the carboxyl group
• D. The carbon atom farthest from the carboxyl group

Answer: (A)

Verified step by step guidance

1. Understand that in IUPAC nomenclature for carboxylic acids, the parent chain is the longest carbon chain that includes the carboxyl group (–COOH).
2. Identify the carboxyl group (–COOH) in the molecule, which is the functional group characteristic of carboxylic acids.
3. According to IUPAC rules, the carbon atom of the carboxyl group is always assigned as carbon number 1 in the parent chain.
4. Number the carbon atoms in the chain starting from the carboxyl carbon (carbon 1) and proceed along the longest chain to give substituents the lowest possible numbers.
5. This numbering ensures that the carboxyl carbon is the reference point for naming and that the acid functional group is prioritized in the nomenclature.