Cyclohexanecarboxylic acid

The carboxyl group has a conformational energy slightly lower than that of the methyl group. Let's try to understand why.

The carboxyl group is planar because of the C=O bond and its sp² hybridization. By the rotation of the C-C bond -curved arrow-, the planar carboxyl can always find an arrangement where its interaction with the ring's axial hydrogens at positions 3 and 5 is lower than that of a methyl group.