[SVXX]

I was reading Solomons a while ago when I was faced with a peculiar situation. It was the chapter on stereoisomerism and optical activity, and the section on cyclic systems.
They have described 1,2-dimethylcyclopentane and 1,3-dimethylcyclohexane to have a plane of symmetry as follows -:



I guess I have a misconception about what a plane of symmetry actually is and how to draw it on such molecules. Could someone point it out to me?

[a student]

in the latter molecule which you have drawn what is the position of that two methyl groups are they both in axial or equatorial or one in axial and the other in equatorial? may be this is why the mentioned plane doesn't exist 

[Jorriss]

Look at it in 3-d?

[SVXX]

@a student : It says the plane doesn't exist in both cis and trans forms. I am not concerned by which isomers we can isolate and which ones we cannot(due to ring flipping, where 4 isomers are present, we can isolate only 3). I'm asking why the plane of symmetry condition is not valid in this molecule. Can't I draw a plane like I have in the picture?
@Jorriss : Easy to say that...can you give me a diagram?

[Jorriss]

@Jorriss : Easy to say that...can you give me a diagram?

Google 'Cyclohexane chair conformation'

[SVXX]

@Jorriss : Easy to say that...can you give me a diagram?

Google 'Cyclohexane chair conformation'

That's very smart of you Jorriss, but surely you could have a little foresight and think that it had occurred to me earlier?
Your search phrase is so vague that Google would give me thousands of pages, none of which would have 1,2-dimethylcyclohexane or how to draw a plane of symmetry.
My problem is with why I cannot draw a plane of symmetry as I have drawn in 1,2-dimethylcyclohexane. My query is regarding how a plane of symmetry is drawn. I'm not asking about axial and equitorial configurations(which I have addressed as "cis" and "trans") and not about chair and boat conformations.
Jesus..

[helenn]

But what the others are trying to say is that 1,2-dimethylcyclohexane will exist as a 3D structure and you need to take that into account when thinking about symmetry in the molecule. It looks symmetrical how you have drawn it all in one plane, but the actual molecule is not planar. The diagrams on this page might help.
http://www.cem.msu.edu/~reusch/VirtualText/sterism2.htm

[SVXX]

If that's the case why does 1,3-dimethylcyclohexane (cis) have a plane of symmetry? Is it because the methyl groups are situated in such a way that non-planarity of the chair conformer doesn't cause a problem? You know, like if its distorted in the direction of one methyl group, it is distorted equally in the direction of the other.
Thanks for the page btw. I get what you mean, cyclohexane is not planar and is slightly distorted(existing in chair conformation then). It makes more sense now.
What Jorriss said is also making sense now, but I wish he would be a little more elaborate in what he says..lol.
Thanks everyone.

[AWK]

I get what you mean, cyclohexane is not planar and is slightly distorted(existing in chair conformation then)

None of your compounds are planar and they aren't only slightly distorted.
Sometimes approximation to plane works but this is rather exception, not a rule.
Organic chemistry in space is considered almost one and half century

[Jorriss]

That's very smart of you Jorriss, but surely you could have a little foresight and think that it had occurred to me earlier?

When you have a question on planes of symmetry and your drawings aren't in 3-d, it makes sense to tell someone they should draw it in 3-d.

Your search phrase is so vague that Google would give me thousands of pages, none of which would have 1,2-dimethylcyclohexane or how to draw a plane of symmetry.
My problem is with why I cannot draw a plane of symmetry as I have drawn in 1,2-dimethylcyclohexane. My query is regarding how a plane of symmetry is drawn. I'm not asking about axial and equitorial configurations(which I have addressed as "cis" and "trans") and not about chair and boat conformations.
Jesus..

I didn't need to find 1,2-dimethylcyclohexane myself, I just found cyclohexane and stuck those methyl groups in in my head.

[SVXX]

Yeah, but not everyone is as good as you are. Now I understand that planes of symmetry are drawn taking into account planarity(or lack thereof) of the substrate.

[Borek]

Seems to me like in boat conformation 1,2-dimethylcyclohexane will have a plane of symmetry. But I can be easily wrong.

[Jorriss]

http://web.pdx.edu/~wamserc/C334F06/images/E1_2c.gif

[orgopete]

Ah, an argument about what Solomons said. What did he say? It does not seem to me that he meant a conformational argument. 2,3-Butanediol has a plane of symmetry in the meso isomer, but I doubt it has a conformation that has a plane of symmetry. (The other isomers do not have a plane of symmetry.) The preferred conformation of cis-1,2-dimethylcyclohexane does not have a plane of symmetry, yet it can be written as though it does and this will correctly predict the conformational effects (that is you cannot isolate one of the isomers).

The trans-1,2-dimethylcyclohexane as noted by Jorriss does not have a plane of symmetry. This would be true for trans-1,2-, trans-1,3-dimethylcyclohexane or cyclopentane. Perhaps a larger quote of Solomons would resolve this issue?

[SVXX]

@orgopete :
I'll gladly scan and put up the exact text later...am a little busy right now!