Please clarify some confusion about stereoisomer

[Bernoull]

I have two lingering confusions about stereoisomers. The best compound to help explain the first confusion is tartaric acid, so I've included a link that shows its both the d,l and meso stereoisomers.
http://en.wikipedia.org/wiki/Tartaric_acid

1. the 2 carbons (bound to the OHs) are connected by a sigma bond and we all know rotation around sigma bonds happens (which can cause conformational isomerism etc). If you have 180* rotation about that C-C bond for either the d or l stereoisomer won't you get the corresponding meso-compound if not why? and if you do get the mesocompound then are the different stereoisomers of tartaric acid also conformers?

2. For (cis/trans) geometric isomerism in cyclic compounds.

CIS = similar substituents on same side of the ring
TRANS = similar substituents on opposite sides of the ring.

My confusion here is what does same side of the ring mean.
For dimethylcyclohexane is the cis isomer when the CH3 groups are on C1 & C2 respectively and trans would have them on C1 and C4? Or does cis mean that the CH3 are both above or below the plane of the ring (irrespective of which C that are bound to) and trans means that one CH3 is above the plane of the ring and the other below the plane. In other words for cyclic compounds, do you only need 2D to determine cis/trans or do you need 3D?

---

Members don't see this ad.

 

[FutureScaresMe]

1. the 2 carbons (bound to the OHs) are connected by a sigma bond and we all know rotation around sigma bonds happens (which can cause conformational isomerism etc). If you have 180* rotation about that C-C bond for either the d or l stereoisomer won't you get the corresponding meso-compound if not why? and if you do get the mesocompound then are the different stereoisomers of tartaric acid also conformers?

If you were to build these models, you would clearly see why a 180 degree rotation along the sigma bond you're talking about would not generate a meso compound. I'll try my best to describe why. In a fischer projection, the right and left substituents all point up and out, right? Like diagonally away from the carbon they're attached to. If you rotate the sigma bond 180 degrees, the the hydroxyl group which originally was pointing up and out is now down and out. So now the two hydroxyl groups which appear to make the compound meso in the fischer projection aren't aligned. One is still pointing up and out, and the other is pointing down and out. There is no internal plane of symmetry.

2. For (cis/trans) geometric isomerism in cyclic compounds.

CIS = similar substituents on same side of the ring
TRANS = similar substituents on opposite sides of the ring.

My confusion here is what does same side of the ring mean.
For dimethylcyclohexane is the cis isomer when the CH3 groups are on C1 & C2 respectively and trans would have them on C1 and C4? Or does cis mean that the CH3 are both above or below the plane of the ring (irrespective of which C that are bound to) and trans means that one CH3 is above the plane of the ring and the other below the plane. In other words for cyclic compounds, do you only need 2D to determine cis/trans or do you need 3D?

As I describe this, think of the chair conformation.

Same side means if the two substituents are above the plane. If you have cis-1,2-dimethylcyclohexane, then one methyl is axial, and the other is equitorial. If it's trans-1,2-dimethylcyclohexane, both methyl groups are either axial or both are equitorial. Neither of these compounds are meso, even though the cis one appears to be when you draw it out in the simple 2 dimensional form.

cis-1,3-dimethylcyclohexane. Both methyl groups are either axial or both groups are equitorial. This compound is meso

trans-1,3-dimehtylcyclohexane. One group must be axial, the other must be equitorial. This compound is not meso.

make sense?

so yeah, your second reasoning is the correct one, and you must consider the 3D chair structure in order to determine cis/trans and meso compounds

 

[Bernoull]

Many thanks for the very insightful explanation.

Question#1:
Your explanation makes absolute sense!! I didn't consider the implications of fischer projection (horizontal lines come out of the plane of the page etc) I was looking at the diagram of tartaric acid as purely 2 dimensional!!

Question 2:
I'm realizing the magnitude of my misconceptions here and it's scary... I always looked at the cycloalkane rings sorta like the plane z=0 (in x,y,z coordinate system). Therefore if identical substituents (CH3) were both above or below the plane (z>0 or z<0) then it's CIS, if one CH3 was above (z>0) and the other below (z<0) then its TRANS. But this does not even account for equatorial substituents and is obviously wrong.

Your explanation is very good but I don't understand it because I can't see an underlying rationale:

cis-1,3-dimethylcyclohexane has the same criteria as trans-1,2-dimethylcyclohexane - namely both methyl groups are either axial or both groups are equatorial - but yet this same criteria results in cis for (1,3) but trans for (1,2). It seems almost arbitrary to me, so my question now is, is there any logic behind cis/trans designation of di-substituted cyclohexane or should I just memorize them and move on?

Once more, thanks for the help!!

 

[FutureScaresMe]

Many thanks for the very insightful explanation.

No problem

Therefore if identical substituents (CH3) were both above or below the plane (z>0 or z<0) then it's CIS, if one CH3 was above (z>0) and the other below (z<0) then its TRANS. But this does not even account for equatorial substituents and is obviously wrong.

Well, that's technically correct, but remember that a substituent that's above the plane can be either axial or equatorial depending on which carbon in the chair conformation we're talking about.

cis-1,3-dimethylcyclohexane has the same criteria as trans-1,2-dimethylcyclohexane - namely both methyl groups are either axial or both groups are equatorial - but yet this same criteria results in cis for (1,3) but trans for (1,2). It seems almost arbitrary to me, so my question now is, is there any logic behind cis/trans designation of di-substituted cyclohexane or should I just memorize them and move on?

Once more, thanks for the help!!

look at my attached image. It may help with your confusion. I hope you can read it. Wrote it on a napkin in starbucks...

 

[Bernoull]

Bro ur freakin amazing!!!! I can't believe that diagram was on a starbucks napkin.

I finally got it and thanks alot for ur patience, it's really appreciated.

My problem was that I wrongly visualized the equatorial bonds as being exactly on the plane z=o, whereas in reality they alternate between z>0 and z<0 (just like the axial ones) and now the cis-trans designations make absolute sense!!

Kudos on that drawing, I still can't tell it apart from a textbook's diagram!!!