Methylcyclohexane, 1,3-diaxial

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... 

The more stable diastereomer in each case is the one having both methyl groups equatorial. The free-energy difference favoring the diequatorial isomer is about the same for each case (about 1.9 kcal/mol) and is close to the — A(j value of the methyl group (1.8 kcal/mol). This implies that there are no important interactions present that are not also present in methylcyclohexane. This is reasonable since in each case the axial methyl group interacts only with the 3,5-diaxial hydrogens, just as 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. 

Figure 4.14 The origin of 1,3-diaxial interactions in methylcyclohexane. The steric strain between an axial methyl group and an axial hydrogen atom three carbons away is identical to the steric strain in gauche butane. Note that the -CH3 group in methylcyclohexane moves slightly away from a true axial position to minimize the strain.

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

Under similar conditions the equatorial 4-C — H bond in cw-l-feri-butyl-4-methylcyclohexane (1) is observed10 to be six times as reactive as the axial 4-C—H bond in the traw-isomer. As the axial C—H bond in the latter is flanked by the two 1,3-diaxial C—H bonds, this may be simply a steric effect. 

Wltat IS true fur methylcyclohexane is also true for other monosubsti tuted cycMiexanes A substituent is more strd>le in an equatorial position than in an axial position, the exact amount of 1.3-diaxial steric strain i n a spedfic compound depends on the nature and sire of the substituent, as... 

We saw in Problem 4.20 that c/.s-decalin is less stable than fm/rs-decalin. Assume that the l/t-diaxial interactions in frnn.s-decaiin are simiiai to those in axial methylcyclohexane [that is, one Cl h —>11 interaction costs 3.8 kj/rnol (0.9 kcal/mol), and calculate the magnitude of the energv difference between d.s- and /wns-decalin. 

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... 

Figure 9.10. 1,3-Diaxial interaction in methylcyclohexane. Axial —CH3 more crowded than equatorial —CH3.

One of the two chair structures of cis-l-chloro-3-methylcyclohexane is more stable than the other by 15.5 kJ/mol (3.7 kcal/mol). Which is it What is the energy cost of a 1,3-diaxial interaction between a chlorine and a methyl group ... 

Assume that the 1,3-diaxial interactions in tn / .s-decalin are similar to those in axial methylcyclohexane [that is, one interaction costs 3.8 kj/mol... 

---