Dimethylcyclohexane isomers

It was an important observation from the point of view of the opening mechanism that in dimethylcyclohexane isomers the yield of transformation from cis into tram and from tram into cis also takes place. In the 7.6-eV photolysis of 1,2-dimethylcyclohexanes the cis form isomerizes to tram with a yield of 0 c t) = 0.12, while the reversed reaction has a yield of 0.07. In the photolysis of 1,3-dimethylcyclohexanes (c f) = 0.07 and (t c) = 0.15,... 

Build handheld molecular models of the 1,4-, 1,3-, and 1,2-dimethylcyclohexane isomers discussed here and examine their stereochemical properties. Experiment with flipping the chairs and also switching between cis and trans isomers. 

As m the 1 4 dimethylcyclohexanes the 6 kJ/mol (1 5 kcal/mol) energy difference between the more stable (trans) and the less stable (cis) stereoisomer is attributed to the strain associated with the presence of an axial methyl group m the cis isomer... 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

FIGURE 1.9 Two isomers of 1,4-dimethylcyclohexane. (a) Trans isomer (b) cis isomer. 

Certain dimethylcycloalkanes contain a plane of symmetry. For example, both chair conformers of crs-l,3-dimethylcyclohexane possess a plane of symmetry bisecting the molecule through C-2 and C-5. The trans isomer does not have any element of symm and is chiral. 

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. 

Johnson and Whitehead have further shown that the reductive elimination of the pyrrolidine group from the pyrrolidine enamine of 2,4-dimethyl-cyclohexanone (16), which involved treating it with a mixture of lithium aluminum hydride and aluminum chloride (9), gave the trans isomer of 3,5-dimethyl-/l -cyclohexene (17) which on subsequent hydrogenation on a platinum catalyst led to the // onr-3,5-dimethylcyclohexane (18). 

Let s look at 1,2-dimethy]cyclohexane as an example. There are two isomers, c/s-l,2-dimethylcyclohexane and trcinsA, 2-dimethylcyclohexane. which... 

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. 

Miscible with cyclopentane, cyclohexane, methylcyclohexane, dimethylcyclohexane, and trimethylcyclohexane isomers... 

Although analysis of the consequences of ring flip in a monosubstituted cyclohexane is pretty straightforward, the presence of two or more substituents requires careful consideration to decide which conformer, if any, is the more favoured. Let us illustrate the approach using 1,4-dimethylcyclohexane. Now, two configurational isomers of this structure can exist, namely trans and... 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

The situation in trans- and ciG-decalin is complicated by the symmetry elements. If this symmetry is destroyed, e.g. by introducing dimethyl substituents, we get back to reassuringly familiar territory in which two chiral centres lead to four configurational isomers. The same is true in the trans- and cw-1,2-dimethylcyclohexane series. 

Problem 9.4 Give the names, structural formulas and stereochemical designations of the isomers of (u) bromochlorocyclobutane, (ft) dichlorocyclobutane, (c) bromochlorocyclopentane, (d) diiodocyclopentane, (e) dimethylcyclohexane. Indicate chiral C s. ... 

Problem 9.17 Give your reasons for selecting the isomers of dimethylcyclohexane shown in Figs. 9-8 to 9-12 that exist as (a) a pair of configurational enantiomers, each of which exists in one conformation (ft) a pair of conformational diastereomers (c) a pair of configurational enantiomers, each of which exists as a pair of conformational diastereomers (d) a single conformation (c) a pair of conformational enantiomers. ... 

Problem 9.23 The planar structure of c -l,2-dimethylcyclohexane, which is meso, shows a plane of symmetry [Problem 9.4(e)). (a) Is the chair conformer achiral (f>) Why is this isomer optically inactive -4... 

A similar situation has been observed by Rossini and Pitzer. After a careful study of 1,3-dimethylcyclohexanes (b. 120.09° and 124.45° C.) they found the cis configuration to be less strained and accordingly assigned to the low boiling isomer. 

Figure 19.14. Construction and performance of thermal diffusion columns, (a) Basic construction of a thermal diffusion cell, (b) Action in a thermogravitationai column, (c) A commercial column with 10 takeoff points at 6 in. intervals the mean dia of the annulus is 16 mm, width 0.3 mm, volume 22.5 mL (Jones and Brown, 1960). (d) Concentration gradients in the separation of cis and trans isomers of 1,2-dimethylcyclohexane (Jones and Brown, 1960). (e) Terminal compositions as a function of charge composition of mixtures of cetane and cumene time 48 hr, 50°C hot wall, 29°C cold wall (Jones and Brown, 1960).

Pelosi and Miller have reported the results of an elegant study of the photolytic and thermal isomerizations of C CFj) isomers.42 The anti-dimer of perfluoro(tetramethylcyclobutadiene). anti-21, gives a mixture of perlluoro(octamethylcubane) (22) and perfluoro(octamethylcuneane) (23) in 20% and 15 % yield, respectively, when irradiated for 36 hours in a fluorocarbon solvent [a mixture of perfluoro(1,3- and perfluoro(1,4-dimethylcyclohexane)]. Cubane 22 is presumably formed via xyn-21, whereas 23 is postulated to be formed via intermediates per-lluoro(oclamcthyleyclooctatetraene) (24) and the corresponding scmibullvalene. 

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

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