Question

Which one of the following isomeric structures has the lowest energy?

• A.
• B.
• C.
• D. None of the above

Answer 1

The correct option is C

When a substitutent is present at axial position, the conformer will be less stable because it has 1, 3 diaxial interaction which is a steric interaction of axial group. This will increase the energy of the conformer and make it less stable.
When a substituent is present at equatorial position, the conformer is stable because the substituent has more room and fewer steric interactions when it is in an equatorial position

Compound (a) is trans 1, 3 dimethyl cyclohexane, when it is drawn in chair conformer we can see one methyl at axial and one methyl at equatorial position. Thus it is less stable.

Compound (b) is cis 1, 2 dimethyl cyclohexane, when it is drawn in chair conformer we can see again one methyl at axial and one methyl at equatorial position. Thus it is less stable.

Compound (c) is trans 1, 4 dimethyl cyclohexane, when it is drawn in chair conformer we can see both methyl groups at equatorial position. Thus it is more stable and having lowest energy.