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I was asked the IUPAC Name of the following compound in JEE Mains (2nd September, Shift 1) 2020.

enter image description here

There were two names occuring to me:

  1. 5-formyl-2-methyl-hex-2-enoic acid
  2. (2,5)-dimethyl-6-oxo-hex-2-enoic acid

I finally chose (2), but the temporary answer key says (1). Can someone please tell which is the correct one and why?

Note: As I realised after receiving a comment, I've accidentally made the double bond have (E) configuration. This is an unintended error due to the website I used to construct the compound's structure. Please note that the original compound did not have any stereochemistry indicated

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    $\begingroup$ Neither one. There is no (E) designation for the double bond. Possible help: chemistry.stackexchange.com/questions/117065/… $\endgroup$ – user55119 Sep 3 '20 at 15:16
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    $\begingroup$ @user55119: Actually, the OP drawing had (E) config. The original question asked in JEE had no specific config. It should be mentioned in the post, I think. $\endgroup$ – Rahul Verma Sep 3 '20 at 15:24
  • $\begingroup$ I think the simple answer would be to first define the parent chain. As there are no rings, there is no need to leave the aldehyde group out of the parent chain. So the option with formyl should be incorrect. $\endgroup$ – B.Anshuman Sep 4 '20 at 3:28
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According to the new recommendations of IUPAC nomenclature rules, when choosing the senior parent structure, you have to first consider the senior parent structure, which has the maximum number of substituents corresponding to the principal characteristic group or senior parent hydride in accord with the seniority of classes (functional groups). Then, if there is a choice, choose the principal chain, which has the greater number of skeletal atoms. Next in order is the chain, which has the greater number of multiple bonds, and then has the greater number of double bonds (This is according to Blue Book in 2013, which was displayed in 5 steps you have mentioned from Wikipedia).

Accordingly, your most senior function is carboxylic group, which would have the highest priority:

6-Oxohexanoic acid

Then, there is a choice to choose the principal chain, which has the greater number of skeletal atoms. Both choice have six carbons, but the numbered chain indicated in the image has additional function, thus it is the principle chain. You also need to consider the stereochemistry if they are indicated. Here, there is a double bond in trans-orientation. That should be included in the name. Although, $\ce{C}$-5 is a chiral carbon, its stereochemistry is not indicated so you can leave it alone. Therefore the correct IUPAC name is: (2E)-2,5-dimethyl-6-oxohex-2-enoic acid.

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  • $\begingroup$ Just as a sidenote, the original question had no stereochemistry indicated. It's just that, I couldn't find any online tool to draw the compound without indicating strereochem $\endgroup$ – Fitz Watson Sep 4 '20 at 2:30
  • $\begingroup$ "but the numbered chain indicated in the image has additional function, thus it is the principle chain" Could you please cite an official source saying so, because to challenge the answer key, a verified source citation id compulsory. $\endgroup$ – Fitz Watson Sep 4 '20 at 2:33
  • $\begingroup$ It is in the blue book. For instance given answer, 5-formyl-2-methyl-hex-2-enoic acid, is also carbon 6 chain. When you have choice to choose, you go with the one with most functional groups. Methyl and halo groups are valued equally but oxo group has a priority. $\endgroup$ – Mathew Mahindaratne Sep 4 '20 at 5:35
  • $\begingroup$ Further, in case of formyl group attached to carbon-4, then you could use the name (2*E*)-4-formyl-2-methylhex-2-enoic acid (also, between methyl and hex does not have a hyphen). And, given answer is wrong by not giving stereochemistry of the double bond, anyway. $\endgroup$ – Mathew Mahindaratne Sep 4 '20 at 5:37
  • $\begingroup$ I couldn't find the point saying "when both chains have same number of carbons, choose the one with more functional groups" in blue book. Could you please locate it for me? $\endgroup$ – Fitz Watson Sep 4 '20 at 5:58
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"Formyl" and "oxo" are used to denote when the aldehyde group is a substituent/carbon skeleton component respectively. Seeing as the aldehyde group is clearly in the main chain, the "oxo" prefix must be used and therefore the second option must be the correct one. Regardless of this, seeing as the carboxylic acid group must have the highest priority, the aldehyde must be numbered 6, so we can rule out the first option.

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  • $\begingroup$ Actually what the second option does is that it takes the methyl group on C-5 into the parent chain, hence rendering the -CHO as a substitution. This leads to the formyl name on C-5. But, the question is how do we decide which is supposed to be the parent chain? $\endgroup$ – Fitz Watson Sep 3 '20 at 16:30

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