Cis-l,3-Dimethylcyclohexane

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

Figure 6 Energy transfer rate coefficients from excited cyclohexane molecules to solutes as a function of the A energy difference (1) isooctane, (2) w-heptadecane, (3) cyclooctane, (4) tram-, 2-dimethylcyclohexane, (5) isopropylcyclohexane, (6) cis-l,2-dimethylcyclohexane, (7) decalin, (8) 1-decene, (9) 2-hexene, (10) tetramethylethylene, (11) benzene. (From Ref. 135.)...

Figure 8 Connection between primary C-H and C-C bond ruptures during radiolysis and photolysis. Alkanes (1) propane, (2) w-butane, (3) -pentane, (4) -hexane, (5) w-heptane, (6) n-octane, (7) w-decane, (8) isobutane, (9) neopentane, (10) 3-methylpentane, (11) 2,2-dimethylbutane, (12) isooctane, (13) cyclopentane, (14) cyclohexane, (15) cycloheptane, (16) cyclooctane, (17) cyclodecane, (18) methylcyclopentane, (19) methylcyclohexane, (20) ethylcyclohexane, (21) 1,1-dimethylcyclohexane, (22) cis-l,2-dimethylcyclohexane, (23) fraw5-l,2-dimethylcyclohexane, (24) cis-1,3-dimethylcyclohexane, (25) trarw-l,3-dimethylcyclohexane, (26) cw-l,4-dimethylcyclohexane, (27) trawi-l,4-dimethylcyclohexane. (From Refs. 18, 29, 91, 92, 99, 100, 108, 110, 111, 113, 114, and 160.)...

Figure 6.34 Data for structure H hydrates of methane with methylcyclohexane, 1,1-dimethylcyclohexane, cis-l,2-dimethylcyclohexane, and ethylcyclohexane.

How can you explain the observation that ci -l,2-dimethylcyclohexane is optically inactive even though it has two chirality centers ... 

Let s look at 1,2-dimethylcyclohexane as an example. There are two isomers, cis-l,2-dimethylcyclohexane and trons-l,2-dimethylcyclohexane, which must be considered separately. In the cis isomer, both methyl groups are on the same side of the ring, and the compound can exist in either of the two chair conformations shown at the top of Figure 4.22. (It s often easier to see... 

The stable chair-chair conformation of gauche interactions compared to one in the chair form of frans-1,2-dimethyl-cyclohexane and three in the chair conformations of cis-decalin and cis-l,2-dimethylcyclohexane. The various factors affecting cis-trans equilibrium of the decalins are given in Table III. Both isomers have an... 

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