Question

Can you account for the stereochemistry of `"1,2-dimethylcyclohexane"`?

Answer 1

Well, as far as I know, ONLY `"B"` can exhibit cis-trans isomerism.

Explanation:

Both molecules can support optical isomerism; but ONLY `"1,2-dimethylcyclohexane"` can support `"cis-trans isomerism"`.

For `"(b)"`, `"1,2-dimethylcyclohexane"`, CLEARLY, the pendant methyl groups can lie on the same side of the ring, or on opposite sides of the ring.

And this cis isomer, clearly has a plane of symmetry bisecting the `"MeCH-CHMe"` bond.

On the other hand, for `"trans-1,2-dimethylcyclohexane"`, this has a `C_2` axis that maps the pendant methyl group. And, thus, for a fact, this structure can support `"RR"` and `"SS"` enantiomers.

For `"1,2-diethyl-1-methylcyclohexane"`, this generates 4 stereoisomers, because there are 2 chiral centres on the ring.