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How many ether metamers are possible for $\ce{C4H10O}?$

According to me, the answer should be 2 but it was given 3. I know 3 structures can be made:

$$\ce{CH3—O—CH2—CH2—CH3}\tag{1}$$

$$\ce{CH3—CH2—O—CH2—CH3}\tag{2}$$

$$\ce{CH3—O—CH(CH3)2}\tag{3}$$

Structure 2 and 3 are metamers, but 1st and 3rd structures are positional isomers(?), then how the answer can be 3?

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    $\begingroup$ Funny that my first answer was down voted because I right gave ethers as example of metaners apparently out of black. Lately I see more people comfortable with that. By the way I do not see much, if any, use of this particular class of isomerism which is covered by the other classifications. $\endgroup$ – Alchimista Aug 23 at 13:02
  • $\begingroup$ Metamer, really? Im glad to be able to say that none of my teachers or professors has ever bothered to introduce this new metacategory. $\endgroup$ – Karl Aug 24 at 19:58
  • $\begingroup$ Can anyone point me to a IUPAC definition of the term "metamer"? Google was unsuccessful for me, en:wp has nothing but a one -liner saying the term is obsolete, nothing on de:wp. $\endgroup$ – Karl Aug 25 at 6:46
  • $\begingroup$ Metamerism definition (3rd bullet) includes position isomers! $\endgroup$ – rv7 Aug 26 at 3:43
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"Metamerism" is an extremely old, obsolete, and ill-defined property. It is also not educationally or synthetically useful. An additional, and pointless, way to categorise chemical compounds.

See this answer to the question "What is metamerism": https://chemistry.stackexchange.com/a/104576/17952

If you have to answer this in a test, do your best with whatever definition your teacher has provided. And tell him that he should stop teaching such nonsense.

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Whilst the 1st and 3rd are positional isomers, I think you can also class them as metamers as well - n-propyl and i-propyl will have some significant differences. For instance n-propyl alcohol can be easily oxidized all the way to a carboxylic acid, whereas i-propyl alcohol can only easily be oxidized to the ketone. If the alkyl chain was a pentyl, for instance I think you can class the 2-pentyl and 3-pentyl radicals as positional isomers and a different metamer to 1-pentyl.

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