Exercise 5 | Cycloalkanes

Here are two of the four stereoisomers of 1,2,3-trimethylcyclohexane.

Find the remaining two stereoisomers and identify which of the four is the most stable isomer.

These four stereoisomers are equivalent to the stereoisomers below by a horizontal rotation of 180°:


The most stable isomer is the one with the three methyl substituents in the equatorial positions. We can draw all the chair conformations to determine the most stable isomer. However, we previously saw that there is an alternation of up and down positions for successive equatorial positions. 

The only stereoisomer with an alternation of up and down methyl substituents is:

We can verify that this stereoisomer is the most stable isomer by drawing its chair conformations: