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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, 2019 at 13:02
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    $\begingroup$ 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, 2019 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 Google was unsuccessful for me, en:wp has nothing but a one -liner saying the term is obsolete, nothing on de:wp.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, 2019 at 6:46
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    $\begingroup$ Metamerism definition (3rd bullet) includes position isomers! $\endgroup$ Aug 26, 2019 at 3:43

3 Answers 3

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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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Structural isomerism is categorised into 5 main types; which are listed from highest priority to lowest as:

  1. Tautomerism

  2. Functional isomerism

  3. Metamerism- Metamerism is a type of structural isomerism where the carbon-side chain/ alkyl group attached to a polyvalent functional group differs( here ether is the polyvalent functional group).

  4. Chain isomerism

  5. Positional isomerism - a type of structural isomerism where the position of the functional group differs.

According to this, we check for metamerism before positional isomerism.

Here, since there are different alkyl groups on the right side of the oxygen atom in ether*( propyl group in structure (1), isopropyl group in structure (3))* we consider them to be metamers.

Only if the 2 compounds were not metamers, we would look for positional isomerism.

This youtube playlist is a useful way to understand structural isomerism https://www.youtube.com/playlist?list=PL_A4M5IAkMac-tIcsX-GmJB6Jfv5wjVTp (the 7th and 8th video are specifically on metamerism)

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