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Question 4.62: The diaxial conformation of cis-1,3-dimethylcyclohexane is 2......

The diaxial conformation of cis-1,3-dimethylcyclohexane is 23 kJ mole^{−1} less stable than the diequatorial conformation. Why is this value larger than twice the steric strain of a methyl group?

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The steric strain results from repulsion between an axial group and the axial hydrogen atoms at C-3 and C-5.
Repulsion between an axial group and larger atoms at the C-3 and C-5 positions must be substantially larger. In the diaxial conformation of cis-1,3-dimethylcyclohexane, that repulsion is between two methyl groups.

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