What is the correct structure for 5-methyl-4-octanone
The Correct Answer and Explanation is:
The correct answer is IV.
To identify the correct structure for 5-methyl-4-octanone, we must break down its IUPAC name into its constituent parts, which provide a clear blueprint for the molecule’s assembly.
The parent name “octanone” tells us two key things. The “octan” part indicates that the longest continuous carbon chain in the molecule contains eight carbon atoms. The suffix “one” signifies that the molecule is a ketone, meaning it contains a carbonyl functional group (a carbon atom double-bonded to an oxygen atom, C=O) located somewhere within this carbon chain.
The number “4” in “4-octanone” specifies the position of this ketone group. According to IUPAC rules, we number the carbon chain starting from the end that gives the principal functional group, in this case the carbonyl group, the lowest possible number. So, the C=O group is on the fourth carbon of the eight-carbon chain.
The prefix “5-methyl” indicates that there is a substituent attached to the main chain. The “methyl” part means the substituent is a single-carbon alkyl group (–CH3). The number “5” specifies that this methyl group is bonded to the fifth carbon atom of the main chain.
Let’s examine structure IV using these rules. First, we find the longest carbon chain containing the carbonyl group, which is indeed eight carbons long. To name it, we number the chain to give the carbonyl group the lowest number. Numbering from the right end places the carbonyl group on carbon 4. Numbering from the left would place it on carbon 5, so we must number from the right. With this numbering system, we then locate the methyl group, which is attached to the fifth carbon. Therefore, structure IV is correctly named 5-methyl-4-octanone.
The other structures are incorrect:
- Structure I is 6-methyl-2-heptanone.
- Structure II is 2-methyl-3-hexanone.
- Structure III is 6-methyl-4-heptanone.
- Structure V is a cyclic ketone, 3,3-dimethylcyclooctanone.
Thus, only structure IV precisely matches the name 5-methyl-4-octanone.
