[Monium101]

I was drawing the structures for the following compounds such as 1,3-dichlorocyclohexane, 1,4-dichlorocyclohexane, and 1,2-dichlorocyclohexane but I was wondering why the 1,3 and 1,2 have a stereoisomer that is a meso compound but not the 1,4, why is that? any help is appreciated, thanks!

[discodermolide]

Look here:
http://chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/Meso_Compounds

[Dan]

I was drawing the structures for the following compounds such as 1,3-dichlorocyclohexane, 1,4-dichlorocyclohexane, and 1,2-dichlorocyclohexane but I was wondering why the 1,3 and 1,2 have a stereoisomer that is a meso compound but not the 1,4, why is that? any help is appreciated, thanks!

Not true - both stereoisomers of 1,4-dichlorocyclohexne are meso.

[spirochete]

I was always under the impression that the 1,4 disubstituted cyclohexanes and analogous 1,3 disubstituted cyclobutanes are not meso. The UC Davis website posted by Disco even says that in a related page: http://chemwiki.ucdavis.edu/Organic_Chemistry/Fundamentals/Functional_Groups/Stereoisomers/Stereoisomerism_in_Disubstituted_Cyclohexanes

It says, among other things:   "The cis & trans-1,4-dichlorocyclohexanes do not have any chiral centers, since the two ring groups on the substituted carbons are identical"

The entire molecule can be thought of as having a stereogenic unit, although I'm not sure if this is in the IUPAC definition: http://goldbook.iupac.org/S05980.html

A good check for meso is to see if the molecule has any chiral diastereomers.

[zsinger]

Spiro,
What do you mean when you speak of checking for "chiral diastereomers".  Sounds like a good way, but I just quite get what that implies?  Thanks,
                 Zack

[Dan]

My mistake, you are right Spirochete. 1,4-disubstitued cyclohexanes are achiral but not meso (since there are no stereocentres - a point I missed). Thanks for posting the correction.

[spirochete]

Spiro,
What do you mean when you speak of checking for "chiral diastereomers".  Sounds like a good way, but I just quite get what that implies?  Thanks,
                 Zack

It really means simply that. For example the trans diastereomer of cis 1,2 dimethylcyclohexane is chiral. The fact that the trans diastereomer is chiral means that the cis isomer must be meso.

[Mitch]

@spirochete Great answers. I never knew that about cyclohexanes.