1, 2-dimethylcyclohexanes

This conformational change does not alter the cis relationship of the methyl groups. Moreover, the two conformations are equivalent because each one has an equatorial and an axial methyl group. The steric energy of the cis isomer is 7.6 kj mole , like that of the axial conformation of methylcydohex-ane. 

Because cit-1,4-dimethylcyclohexane has an axial methyl group in both of its chair conformations, and because rr 5 t-l,4-dimethylcyclohexane can exist in a conformation in which both methyl groups are equatorial, the trans isomer is more stable than the cis isomer. The two geometric isomers differ in energy by 7.6 kJ mole . This energy difference is due solely to the axial methyl group 

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. 

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Wedge and dash drawings fail to show conformation and it s important to remember that the rings of cis and trans 1 2 dimethylcyclohexane exist m a chair conformation This... 

CIS 1 2 Dimethylcyclohexane trans 1 2 Dimethylcyclohexane Axial-equatorial Diequatorial (1248 3) (1246 8) (1 5) trans... 

A reaction that introduces a second chirality center into a starting material that already has one need not produce equal quantities of two possible diastereomers Con sider catalytic hydrogenation of 2 methyl(methylene)cyclohexane As you might expect both CIS and trans 1 2 dimethylcyclohexane are formed... 

Could the fact that hydrogenation of 2 methyl(methylene)cyclo hexane gives more as 1 2 dimethylcyclohexane than trans be explained on the basis of the relative stabilities of the two stereoisomeric products ... 

No The major product as 1 2 dimethylcyclohexane is less stable than the minor product trans 1 2 dimethylcyclohexane... 

Like the 1,4-dimethyl derivatives, trani-1,2-dimethylcyclohexane has a lower heat of combustion (see Table 3.2) and is more stable than d5-l,2-dimethylcyclohexane. The cis ster eoisomer has two chair conformations of equal energy, each containing one axial and one equatorial methyl group. 

The heats of combustion of the more and less stable stereoisomers of the 1,2-, 1,3-, and 1,4-dimethylcyclohexanes are given here. The values are higher for the 1,2-dimethylcyclohexanes than for the 1,3- and 1,4-isomers. Suggest an explanation. 

Active Figure 4.15 Conformations of c/s-1,2-dimethylcyclohexane. The two chair conformations are equal in energy because each has one axial methyl group and one equatorial methyl group. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Figure 4.16 Conformations of rrans-1,2-dimethylcyclohexane. The conformation with both methyl groups equatorial is favored by 11.4 kJ/mol (2.7 kcal/mol) over the conformation with both methyl groups axial.

Dimethylallyl diphosphate, biosynthesis of, 1077 geranio biosynthesis and. 382 cis-1,2-Dimethylcyclohexane. 

Since compounds with alkyl equatorial substituents are generally more stable, trans-1,2 compounds, which can adopt the ee conformation, are thermodynamically more stable than their cis-1,2 isomers, which must exist in the ae conformation. For the 1,2-dimethylcyclohexanes, the difference in stability is about 2kcal moP (8 kJ mol" ). Similarly, trans-1,4 and cis-1,3 compounds are more stable than their stereoisomers. 

Relative rate studies of cyclic cis- and trans-l 2-diols give no clear pattern of results. C -cyclohexane-l 2-diol is oxidised by V(V) faster than the trans isomer, but the reverse is found for the isomers of 1 2-dimethylcyclohexane-1 2-diol. 

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