# Naming this oxo-carboxylic acid

On a textbook question, the answer key revealed this structure to be 3-methyl-2-oxopentanoic acid:

I found this strange, as the carbon from the left (#3) is methyl, but the oxo prefix is on the right for the $\ce{C=O}$. Why is this?

• Why does it not say 3-methyl-5-oxopentanoic acid or 4-methyl-2-oxopentanoic acid? Is the nomenclature correct? – l33tcodes Mar 22 '17 at 18:20
• Yes it is correct – ringo Mar 22 '17 at 18:20
• Why does it not say 3-methyl-5-oxopentanoic acid or 4-methyl-2-oxopentanoic acid? Please explain – l33tcodes Mar 22 '17 at 18:21
• Why would it be either of those? The methyl and oxo groups are right next to each other. – ringo Mar 22 '17 at 18:22
• @ringo how is it correct? – Yb609 Mar 22 '17 at 18:23

## 1 Answer

Starting from the group with the highest priority, the carboxylic acid group, we label the longest carbon chain:

This molecule might otherwise be named a ketone, but since the carboxylic acid group has a higher priority, it is treated as another substituent (oxo), located at the 2 position. The methyl group is attached to the 3 position, hence the name 3-methyl-2-oxopentanoic acid.