Methylcyclohexane, 1,3-diaxial conformations

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. 

The steric strain of the diaxial conformation is twice that of the axial conformation of methyl-cyclo-hexane or 2 X 7.6 = 15.2 kJ mole. However, interconversion of this conformation by a ring flip changes both axial methyl groups into equatorial methyl groups. The steric energy of this conformation is zero, like the equatorial conformation of methylcyclohexane. Thus, the diaxial conformation is 15.2 kJ mole" less stable than the diequatorial conformation. Because of this energy difference, approximately 99.5% of the compound exists in the diequatorial conformation at 25 °C. 

The diaxial conformation of c -l-chloro-3-methylcyclohexane is 16 kJ mole less stable than the diequatorial conformation. Why is this value larger than the sum of the steric strains of a chlorine atom and a methyl group ... 

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. 

Again, a good strategy for addressing conformational issues is to build models. First constract a model for cis-l,3-dimethylcyclohexane andpaformthe ring flip fromdiequatorial to diaxial. What is different in this system compared with methylcyclohexane taken twice ... 

Draw the chair conformations of cw-l-chloro-2-methylcyclohexane and its trans isomer. For E2 elimination reactions to occur, there must be a H and X trans diaxial to each other. 

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