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Question

Give isomeric relations between these compounds.

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My answer

  • A and B show metamerism and positional isomerism.
  • A and C show chain isomerism and metamerism.
  • B and C show chain isomerism and metamerism.

Given answer

  • A and B show metamerism and positional isomerism.
  • A and C show chain isomerism, metamerism and positional isomerism.
  • B and C show metamerism and positional isomerism.

My doubt

How does A & C and B & C show positional isomerism?

Does positional isomerism even makes sense when the carbon chain skeleton is not the same? Because in definition of positional isomerism, it states that structure of carbon chain remains same.

And why does B and C does not chain isomerism?

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In organic chemistry, compounds having the same molecular formula but different number of carbon atoms (alkyl groups, etc.) on either side of functional group (i.e., $\ce{-O-,-S-, -NH-, -C(=O) -,}$ etc.) are called metamers and the phenomenon is called metamerism (Wikipedia). Thus, you agree that all three compounds are metamers.

Positional isomers are constitutional isomers that have the same carbon skeleton and the same functional groups (and/or substituents). In Positional Isomerism, the prime carbon chain remains the same in all aspects while the position of functional groups on and/or other substituents change position on a parent structure.

In chain isomerism or skeletal isomerism, components of the (usually carbon) skeleton are distinctly re-ordered to create different structures (Wikipedia).

Accordingly, A and B show metamerism and positional isomerism (position of $\ce{C=O}$ group change on $\ce{C-C-C-C-C}$ carbon skeleton).

A and C show metamerism, chain isomerism ($\ce{C-C-C-C-C}$ carbon skeleton changes significantly to $\ce{C-C-C(-C)-C}$ skeleton), and positional isomerism (position of $\ce{CH3}$ group changes on $\ce{C-C(=O)-C-C}$ carbon skeleton from 1-to-3 position).

Similarly, B and C also show metamerism, chain isomerism ($\ce{C-C(=O)-C-C-C}$ carbon skeleton changes significantly to $\ce{C-C(=O)-C(-C)-C}$ skeleton), and positional isomerism (position of $\ce{CH3}$ group changes on $\ce{C-C(=O)-C-C}$ carbon skeleton from 4-to-3 position).

Therefore, I also agree with you that B and C should show chain isomerism. If not, then n-pentane and 2-methylbutane would not have chain isomerism! :-)

Late Addition: Example for positional isomerism with substitution:

positional isomerism

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  • $\begingroup$ Isn't positional isomerism related to position of the functional group? Why is CH3 considered as functional group? If that's true, then n-pentane and 2-methylbutane must be positional isomers, since position of CH3 is changed. $\endgroup$ – user80708 Jul 5 at 3:57
  • $\begingroup$ Not necessarily. As long as configuration of substitution didn't change except the position, thet are considered positional isomers. See my addition to the answer. $\endgroup$ – Mathew Mahindaratne Jul 5 at 16:39

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