-7
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On a textbook question, the answer key revealed this structure to be 3-methyl-2-oxopentanoic acid:

enter image description here

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?

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  • $\begingroup$ Why does it not say 3-methyl-5-oxopentanoic acid or 4-methyl-2-oxopentanoic acid? Is the nomenclature correct? $\endgroup$ – l33tcodes Mar 22 '17 at 18:20
  • $\begingroup$ Yes it is correct $\endgroup$ – ringo Mar 22 '17 at 18:20
  • $\begingroup$ Why does it not say 3-methyl-5-oxopentanoic acid or 4-methyl-2-oxopentanoic acid? Please explain $\endgroup$ – l33tcodes Mar 22 '17 at 18:21
  • $\begingroup$ Why would it be either of those? The methyl and oxo groups are right next to each other. $\endgroup$ – ringo Mar 22 '17 at 18:22
  • $\begingroup$ @ringo how is it correct? $\endgroup$ – Yb609 Mar 22 '17 at 18:23
10
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Starting from the group with the highest priority, the carboxylic acid group, we label the longest carbon chain:

enter image description here

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.

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