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I understand that chain isomerism is when two molecules differ only by the classification of the carbonic chain, for example one being branched and the other straight, or one being open and the other closed. But what happens when both molecules have the same classification?

For example, 2,3-Dimethylhexane:

2,3-Dimethylhexane

And 2-Methylheptane:

2-Methylheptane

Both are open, branched, saturated and homogeny chains, so I don't think they have chain isomerism. Of course one have a hexane chain and the other a heptane, but that doesn't change the chain classification. I also don't see why it should be thought as a position isomerism, since the second branch wasn't exactly "moved", but incorporated in the chain.

Also, if I may, what would be the case of two unsaturated chains like Propyne and Propadiene? They're both open and unsaturated chains, so I don't know if they should be classified as a chain isomerism or a position one.

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  • $\begingroup$ Chain isomerism = different carbon skeleton, but same molecular formula. According to this definition, these two are obviously chain isomers. Propyne and Propadiene are functional isomers: the former is an alkyne and the latter an allene. $\endgroup$ – Aniruddha Deb Jan 14 at 3:59
  • $\begingroup$ My textbook says that Propyne and Propadiene are chain isomers, because one have an "ethynic chain" and the other a "diethenyc chain" (it's in portuguese so it may get lost in translation). I didn't think they're functional isomers because they're both hydrocarbons, and I thought that would be their functional group $\endgroup$ – Anonymous Jan 14 at 4:14
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    $\begingroup$ They are constitutional (structural) isomers. Same molecular formula; different atom connectivity. $\endgroup$ – user55119 Jan 14 at 4:41
  • $\begingroup$ Is your definition that does not work. Two constitutional isomers don't differ in their (ill-defined) classification as you think but in what @user55119 said above. $\endgroup$ – Alchimista Jan 14 at 9:15

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