Why is 1,3-dimethyl-cyclohexane a meso compound?

[![enter image description here](https://i.stack.imgur.com/LVwUL.jpg)](https://i.stack.imgur.com/LVwUL.jpg)


So i was trying to find the plane of symmetry in 1,3-dimethyl-cyclohexane in chair conformation, but then i realised the plane is present only at a particular orientation [see figure]. So why do we consider this to be a meso compound?




1,3-dimethylcyclohexane is not exactly a compound, but **three** different compounds, depending on the configuration of the chiral centers. Two of them (RR and SS) are enantiomers of each other, and the third one (RS) is meso.


Why meso?


Well, because it has no enantiomeric forms which couldn't be transformed to each other by single-bond rotations. That's what is important, and the symmetry plane is just one possible *evidence* of this situation. It is sufficient, but not necessary.


We can put it differently: you seem to know full well that all conformations are in fact one compound, since you can't separate them. Also, at least one of the conformations is surely a meso compound, since it has a mirror plane. Then what about any other conformation, possibly having no mirror plane? Is it a different compound? No.




---


On a side note, mirror plane is not necessary at all, and [Wikipedia](https://en.wikipedia.org/wiki/Meso_compound) gives us an example:



> 
> A meso isomer need not have a mirror plane. It may have an inversion or a rotoreflexion symmetry such as S4. For example, there are two meso isomers of 1,4-difluoro-2,5-dichlorocyclohexane but neither has a mirror plane, and there are two meso isomers of 1,2,3,4-tetrafluorospiropentane (see figure). In fact, a meso compound may have no symmetry in some conformations...
> 
> 
> 


[![Image from Wikipedia](https://i.stack.imgur.com/iNzkf.png)](https://i.stack.imgur.com/iNzkf.png)


 6