CHXI12:ORGANIC CHEMISTRY SOME BASIC PRINCIPLES AND TECHNIQUES
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The correct IUPAC nomenclature for the following compound is
1 5 – Methyl – 2 – oxohexan – 6 – oic acid
2 2 – Methyl – 5 – oxohexanoic acid
3 2 – Formyl – 5 – methylhexan – 6 – oic acid
4 5 – Formyl – 2 – methylhexanoic acid.
Explanation:

So, 2 – Methyl –5 – oxo hexanoic acid, is the correct IUPAC name.