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My book doesn't classify them as positional isomers but the position of the methoxy group is different in both the molecules. So, I guess they should be positional isomers.

I understand that the two compounds are metamers. But can't a metamer be a positional isomer?

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  • $\begingroup$ As far as my knowledge, I also believe that they should be positional (structural) isomers. Did your textbook specifically state that they weren't positional isomers? $\endgroup$ – Nanoputian Aug 11 '15 at 21:19
  • $\begingroup$ Yes. There was a question to identify positional isomers from four pair of options. This wasn't in the list. $\endgroup$ – the_random_guy42 Aug 11 '15 at 22:24
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2-methoxy-2-methylpropane is a synonym for MTBE (Methyl tertiary-butyl ether). The name "propane" is a little bit missleading, because it comes from the standardized IUPAC nomenclature system, which names the ether-oxygen together with the smaller alkyl group an "alkoxy" group with the suffix "-oxy" (in our case: methoxy) and takes the larger alkyl part as its base identifier (here propane) when describing ethers. So MTBE is referred as an ether with all its chemical properties.

Now to the isomerism: Structural isomers have the same chemical formula but different constitutions (arrangement of atoms, bonds, groups). So they definitely must be constitutional isomers.

Constitutional isomerism is divided into 5 subcategories:

  • Chain isomerism
  • Positional isomerism
  • Functional isomerism
  • metamerism
  • tautomerism

As you already mentioned: they are metamers. Thats because different alkyl rests are attached to the same functional group (ether). This is the first reason why they are not likely to be positional isomers. And the other reasons might come from the "moving substituent" (alkoxy group), which has a carbon atom (the carbon structure has to be constant within positional isomers, therefore moving the -OCH3 could mean a difference) and is no functional group.

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