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?
"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.
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.
