What is the correct structure for 5-methyl-4-octanone?
The Correct Answer and Explanation is:
The correct answer is III.
To determine the correct structure for 5-methyl-4-octanone, we must break down the IUPAC name into its component parts and apply the rules of chemical nomenclature.
First, the parent name “octanone” tells us two things. The “octan” prefix indicates that the longest continuous carbon chain in the molecule contains eight carbon atoms. The “-one” suffix signifies that the principal functional group is a ketone, which is characterized by a carbonyl group (a carbon double-bonded to an oxygen atom) located within the carbon chain.
Next, the number “4” in “4-octanone” specifies the location of this carbonyl group. According to IUPAC rules, the carbon chain is numbered starting from the end that gives the carbonyl carbon the lowest possible number. Therefore, the carbonyl group is located on the fourth carbon of the eight-carbon chain.
Finally, the “5-methyl” part of the name indicates there is a substituent attached to the parent chain. Specifically, a methyl group (–CH3) is bonded to the fifth carbon of the chain.
Let’s examine structure III based on these rules. The longest continuous carbon chain has eight carbons, making it an octane derivative. To assign the lowest number to the carbonyl carbon, we must number the chain from left to right. This places the carbonyl group at the C4 position. Continuing this numbering, we find a methyl group attached to the C5 position. When we combine these parts, the systematic name for structure III is indeed 5-methyl-4-octanone, which perfectly matches the question.
The other structures are incorrect. Structure I is 6-methyl-2-heptanone. Structure II is 3-methyl-4-heptanone. Structure IV is 6-methyl-3-heptanone. Structure V is a cyclic ketone, specifically 2,2-dimethylcyclooctanone. None of these match the required name.
