Methylcyclohexane, 1,3-diaxial interactions conformations

Methyl-3-buten-l-ol, NMR spectrum of, 647 Methylcyclohexane, 1,3-diaxial interactions in, 123 conformations of, 123 mass spectrum of, 4H molecular model of, 123, 293... 

Figure 4.20 (a) The conformations of methylcyclohexane with the methyl group axial (1) and and equatorial (2). (b) 1,3-Diaxial interactions between... 

As a simple example of the importance of 1,3-diaxial interactions, let us consider methylcyclohexane. In estimating relative stabilities of various conformations of this compound, we must focus our attention on methyl, since it is the largest substituent on the ring and hence the one most subject to crowding. There are two... 

Consider methylcyclohexane (Figure 2.21). When —CHj is axial, it is parallel to the axial C—H bonds on carbons 3 and 5. Thus, for axial methylcyclohexane, there are two unfavorable methyl-hydrogen diaxial interactions. No such unfavorable interactions exist when the methyl group is in an equatorial position. For methylcyclohexane, the equatorial methyl conformation is favored over the axial methyl conformation by approximately 7.28 kj (1.74 kcal)/mol. 

Two chair conformations of methylcyclohexane.The steric strain introduced by two diaxial interactions makes the axial methyl conformation less stable by approximately 7.28 kj (1.74 kcal)/mol. 

A gauche conformer of butane and the axially substituted conformer of methylcyclohexane are compared in Figure 3.15. Notice that the gauche interaction in butane is the same as a 1,3-diaxial interaction in methylcyclohexane. 

CHa-ring interactions might also be expected to control the conformational preferences of dimethylcylohexanes. It might even be anticipated that the energy differences between a diequatorial conformer and a diaxial conformer will be twice the equatorial-axial energy difference in methylcyclohexane. 

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