[Bubleeshaark]

Hey guys! (first post )

I just got back my third test for organic chemistry 1. The following question is multiple choice:

Which of the following is a meso compound?
A) cis-1,3-dimethylcyclohexane
B) trans-1,3-dimethylcyclohexane
C) cis-1,4-dimethylcyclohexane
D) trans-1,4-dimethylcyclohexane
E) trans-1,2-dimethylcyclohexane

The correct answer (according to him) was A. I agree this is a correct answer. However, I answered C. I think this is also a meso compound. Can somebody correct or affirm me?

Thank you!

P.S. I also think D is a meso compound. Am I right? Are A, C and D all meso compounds?

[Honclbrif]

Its late and I'm tired, but don't A, C, and D all have internal mirror planes?

[Bubleeshaark]

Its late and I'm tired, but don't A, C, and D all have internal mirror planes?

And that makes A, C and D all meso compounds then, correct?

[pfnm]

Yep, all three are meso because of the internal planes of symmetry

[fledarmus]

From the wikipedia definition of a meso compound:

"A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active.[1][2] This means that despite containing two or more stereocenters (chiral centers) it is not chiral. "

The problem with both answers c and d is that there are no stereocenters in either of these molecules. The 1 and 4 carbons themselves contain a plain of symmetry, so they are not chiral centers and there is no stereoisomer.

[Bubleeshaark]

To above:

Wikipedia also says, quote: "A meso compound need not even have a chiral center."

It seems to conflict with your previous quote though.

Now idk what to think?

[Dan]

Wikipedia should be used with caution.

The IUPAC definition is here: http://www.chem.qmul.ac.uk/iupac/stereo/FM.html#33

"the achiral member(s) of a set of diastereoisomers which also includes one or more chiral members"

There are no chiral stereoisomers of 1,4-dimethylcyclohexane, so none are meso - they are just all achiral.

[fledarmus]

To above:

Wikipedia also says, quote: "A meso compound need not even have a chiral center."

It seems to conflict with your previous quote though.

Now idk what to think?

This is true, but it is an even more unlikely scenario. There are a few structural classes of compounds which have stereoisomers but do not have chiral centers. These include things like allenes and spiranes. It is possible to form the equivalent of diastereoisomers with two of these structures rather than two chiral centers, and then there would be the possibility of meso structures in this form. I probably should have ended my quote with the first sentence, which actually reflects the IUPAC definition as given by Dan. The second half of the sentence is a simplification (since it doesn't include the rare classes of stereoisomers that don't contain chiral centers) but it is a very useful one that will cover almost everything you will see in organic chemistry. If you don't have at least two chiral centers, (okay, or at least two of the other weird stereoisomer structures), you can't have a meso structure. The 1,4-dimethylcyclohexanes don't contain any chiral centers, and there are no stereoisomers for those compounds, meso or otherwise.