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Why is the diequatorial form of trans-1,4-dimethylcyclohexane more stable than its diaxial form?

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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 the study of stereoisomerism, the stability of cyclohexane derivatives is determined by the orientation of substituents relative to the ring's chair conformation.

In chair cyclohexane, substituents can occupy either axial or equatorial positions. An equatorial position is more stable than an axial position because axial substituents experience unfavorable steric interactions with nearby methylene groups within the ring, which increases the molecule's potential energy.12

For trans-1,4-dimethylcyclohexane, the diequatorial form places both methyl groups in these more stable equatorial positions. When the ring undergoes a "flip," both methyl groups move to axial positions, creating the diaxial form. Because an equatorial methyl group is approximately 1.74 kcal/mol more stable than an axial one, the diaxial form is destabilized by roughly twice that amount—about 3.5 kcal/mol—compared to the diequatorial form. Consequently, the diequatorial conformation is significantly more stable and predominates at equilibrium.3