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Why is equatorial methylcyclohexane more stable than axial methylcyclohexane?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

In methylcyclohexane, the chair conformation allows for two distinct positions for the methyl group: axial and equatorial. These two forms are conformational isomers that interconvert through a process known as a ring "flip."

The equatorial methylcyclohexane isomer is more stable than the axial isomer because of the spatial arrangement of the atoms. In the axial position, the methyl group experiences unfavorable steric interactions with nearby methylene groups within the ring. These destabilizing interactions are absent when the methyl group is in the equatorial position, which places the substituent roughly in the plane of the ring. Consequently, the axial methyl isomer is higher in energy—specifically by 1.8 kcal/mol—making the equatorial form the more stable isomer.