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How the following two structures are chain isomers?

1: ethylbenzene; 2: 1,2‐xylene

According to what I have studied in nomenclature when a compound has both cyclic and straight chain, we consider that the chain as parent chain according to the following order:

  1. functional group;
  2. multiple bonds;
  3. maximum number of carbons.

Following the above order, I came to the conclusion that both structures have benzene as their parent chain. Since both have benzene, then how they come to be chain isomers?

Please explain to me in simple terms because I am just a beginner.

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  • $\begingroup$ Forget benzene. Constitutional (structural) isomers have the same molecular formula with different atom connectivity. Remove ch3 and an ortho H from ethylbenzene and switch them. Now you have ortho-xylene. $\endgroup$ – user55119 Sep 2 at 16:07
  • $\begingroup$ This reminds me of a recent question chemistry.stackexchange.com/questions/119523/… Same problem, "chain isomers" have very little in common but their identic sum formula. $\endgroup$ – Karl Sep 2 at 21:47
  • $\begingroup$ The substituents (chains of carbon other than parent) should also be same For example 2-methylpentane and 3-methylpentane are positional isomer- $\endgroup$ – aryan bansal Sep 3 at 16:55

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