[tashkent]

Greetings everyone!

I would like to ask a diastereomer problem.  What would be the diastereomers of cis- and trans- 1, 2 dimethyl cyclohexane?  Would it be: cis- 1, 3 dimethylcyclohexane, trans-1,3 dimethylcyclohexane, cis- 1,4 dimethylcyclohexane, trans-1,4 dimethylcyclohexane, cis-1,1 dimethylcyclohexane, or trans-1,2 dimethylcyclohexane?

Are there helpful online links in practicing exercises about enantiomers and diastereomers?

Hope to hear from u guys.  Thanks.

Regards,
Tashkent

[movies]

Diastereomers refer only to compounds that have the same connectivity and are not enantiomers.  Therefore 1,2-dimethylcyclohexane and 1,1-dimethyl cyclohexane are not diastereomers because they have different connectivity of bonds.

So in your example of 1,2-dimethylcyclohexane, there are four stereoisomers.  The two trans forms are enantiomers of one another.  The cis form actually consists of two ring-flipped isomers which are enantiomers of one another.  The ring flip of these compounds is much too fast to separate the two enantiomers, even at very low temperatures.  Compounds with this kind of enantiomeric relationship are called "conformational enantiomers."  Out of these four possible compounds, each has two possible diastereomers.  For example, the two diastereomers of (R,R)-trans-1,2-dimethylcyclohexane are the two cis forms.

I hope this helps.  Just remember that by definition a diastereomer is a stereoisomer that is not an enatiomer.