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Can someone tell me how many structural isomers exist for 1,2-dimethyl cyclohexane? My teacher said there is one structural isomer but I believe there aren't any because there is no other way to define the structure of the above compound such that they have the same molecular formula but a different structural formula. Also, we need two or more compounds to define isomerism, so how can a single molecule be ever defined as an isomer if there isn't any other compound to compare it with?

Also can structural isomers be defined if the position of substituents is already defined as in the case of above compound where the positions of the two methyl groups are defined beforehand (i.e. 1, 2)?

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    $\begingroup$ How many ways can you attach two methyl groups to a cyclohexane ring such that none of the structures has the same atom connectivity? Let's see some effort. $\endgroup$
    – user55119
    Feb 8 at 18:21
  • $\begingroup$ Did you search it on a search engine? Did you try anything yourself? What and how much progress did you make? Without these inclusions, we cannot answer your question because this is not a question-solving website. I hope you understand our point. Nothing personal and, welcome to Chem.SE $\endgroup$
    – user79161
    Feb 8 at 18:22
  • $\begingroup$ I guess there are 3 ways to connect two different methyl groups.And yes i did search for it everywhere. Peace.They say that there can be structural isomers for dichlorohexane which makes sense as i have 3 arrangements to place the two methyl groups.But what in the case of the above compound? $\endgroup$ Feb 8 at 18:37
  • $\begingroup$ We have already specifed the positions of the substituents so structural isomers are out of question right? Had it only been C6H10Cl2 i could have made different structural isomers but i am confused as the above compound is already a structural isomer. $\endgroup$ Feb 8 at 18:43
  • $\begingroup$ The molecular formula for 1,2 dimethyl cyclohexane is $\ce{C8H16}$ which is the same as numerous compounds. It would seem that the teachers question was some (stupid?) play on words. Could you give the exact wording of the question? $\endgroup$
    – MaxW
    Feb 8 at 20:06
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My teacher said there is one structural isomer

I think (hope) you are talking about stereo isomers (i.e. same Lewis structure, different substance).

There are two ways of counting. Either you can ask "How many substances are stereo isomers of cis-1,2-dimethylcyclohexane?" Or you can ask "How many stereo isomers exist of 1,2-dimethylcyclohexane?". The correct answers are off by one.

This is like asking "how many siblings do you have" vs. "how many siblings are there in your family (including you)".

How many structural isomers does 1,2-dimethyl-cyclohexane have?

The molecular formula for this substance is $\ce{C8H16}$. There are lots of substances with that formula but different Lewis structure.

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  • $\begingroup$ Yeah that makes sense.sorry about the wrong molecular formula lol.Could you also tell me why methane has 1 structural isomer.It shouldnt have any right? Becoz structural isomerism involves two or more molecules.But here we have just one.So how can we compare when there isnt a second molecule to compare with. $\endgroup$ Feb 8 at 20:58
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    $\begingroup$ How many siblings in a single-child family? $\endgroup$ Feb 8 at 21:06
  • $\begingroup$ Zero? Coz there is just one child.Had there been two or more we could have concluded that they were siblings of one another right? $\endgroup$ Feb 9 at 7:10
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If you are specifying it as 1,2 dimethyl cyclohexane you cant make any structures but with a general formula of dimethyl cyclohexane, you can make a few structures.

once you have named the compound completely, you can't really make any structural isomers of it since the name ensures that you can make only one compound out of what name you have been given, which is the entire point of IUPAC nomenclature. If this were however dimethyl cyclohexane, you could make 1,1 dimethyl cyclohexane, 1,2 dimethyl cyclohexane, and 1,3 cyclohexane.

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  • $\begingroup$ This was exactly what I was thinking. Thanks a lot. Also does methane have any isomers coz there is just one possible structure. Although wiki shows that it does have one. Tysm again :) $\endgroup$ Feb 10 at 16:53
  • $\begingroup$ again, methane is IUPAC named so no, it can't have any isomers, of any type whatsoever $\endgroup$
    – RBM
    Feb 10 at 17:07

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