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I have heard two definitions of metamers.

  1. Compounds having the same molecular formula but different number of carbon atoms on either side of the functional group.
  2. Compounds having the same molecular formula but different alkyl groups on either side of the functional group.

Which one of these is correct?

Are pentan-2-one and 3-methylbutan-2-one metamers?

pentan-2-one and 3-methylbutan-2-one

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    $\begingroup$ How are they different, to begin with? $\endgroup$ – Ivan Neretin Jul 4 at 12:20
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    $\begingroup$ @IvanNeretin Different alkyl groups might mean a more branched alkyl group, but with same amount of carbon atoms. $\endgroup$ – user80708 Jul 4 at 12:22
  • $\begingroup$ @IvanNeretin I asked a question in the second part of the question. I thought the answer will change according to which definition is correct. $\endgroup$ – user80708 Jul 4 at 12:23
  • $\begingroup$ All right, so they are different, after all. Your example fits the second definition but not the first one. Well, hopefully someone will bring in the correct definition from IUPAC. $\endgroup$ – Ivan Neretin Jul 4 at 12:33
  • $\begingroup$ I am pretty sure that it is the second definition,but I can't find a source for that right now. Lemme come back with a reliable source :) $\endgroup$ – Yusuf Hasan Jul 4 at 12:37
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There is no IUPAC definition for metamers or metamerism in the gold book (not even one that makes it obsolete), and Wikipedia doesn't even have a real article about it, see Metamerism.

In chemistry, the chemical property of having the same proportion of atomic components in different arrangements (obsolete, replaced with isomer). In organic chemistry, compounds having the same molecular formula but different number of carbon atoms (alkyl groups) on either side of functional group ( i.e., -O-,-S-, -NH-, -C(=O)-) are called metamers and the phenomenon is called metamerism.

I cannot find any authoritative source for this, but there is a question discussing it on our platform: What is metamerism? Also related: Are methyl n-propyl ether and methyl iso-propyl ether metamers?

Any of the two definitions cited in the question match the general definition of isomers.

isomer (DOI: 10.1351/goldbook.I03289)
One of several species (or molecular entities) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.

More particularly, they match the definition of constitutional isomers.

constitutional isomerism (DOI: 10.1351/goldbook.C01285)
Isomerism between structures differing in constitution and described by different line formulae e.g. $\ce{CH3OCH3}$ and $\ce{CH3CH2OH}$.

Metamers/ metamerism is an archaic, ambiguous, and deprecated term in chemistry. Don't use it.

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