INTOCHEMISTRY / CONFORMATION LESSON
Cis/Trans Disubstituted Cyclohexanes
Translate flat-ring stereochemistry into two consistent chairs.
Distinguish cis/trans from axial/equatorial positions for 1,2-, 1,3- and 1,4-disubstituted rings.
1Assign ring faces first
Cis substituents lie on the same face (both up or both down); trans substituents lie on opposite faces, independently of chair conformation.
2Use axial direction alternation
In chair A used here, axial C1, C3, C5 are up; axial C2, C4, C6 are down. At each carbon, equatorial points in the opposite up/down sense.
3Predict both chair patterns
Cis-1,2 and trans-1,3 and cis-1,4 have one axial and one equatorial substituent in either chair. Trans-1,2, cis-1,3 and trans-1,4 can interconvert diaxial and diequatorial forms.
4Worked example
For trans-1,2-dimethylcyclohexane, C1 methyl up and C2 methyl down are diaxial in chair A; a ring flip makes both equatorial while keeping C1 up and C2 down. The diequatorial chair is generally more favorable.
Check your reasoning
Can cis-1,3-dimethylcyclohexane have both methyl groups equatorial?
Reveal the explanation
Yes. One chair is diequatorial; its ring flip is diaxial.