Conformations of Disubstituted Cyclohexanes


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Structure 1 (cis12DMCH.mol)


Structure 2 (cis13DMCH.mol)


Structure 3 (cis14DMCH.mol)


Structure 1 (cis12DMCH.mol) Dimethylcyclohexanes (1,2-, 1,3-, and 1,4-) exist as cis and trans isomers. Due to steric factor one isomer is preferred over the other. The chemical reactivity of these compounds also differ due to the different sterochemistry of the groups in the molecules. cis 1,2 dimethylcyclohexane has one methyl group in equatorial position and one methyl in axial position (a,e). The axial methyl group creates 1,3 diaxial steric interactions, this makes it unstable. The rapid interconversion of both forms of cis isomer (a,e and e,a) creates a meso form. Hence cis 1,2-dimethylcyclohexane is achiral. The trans 1,2 dimethylcyclohexane, one chair conformer has both methyl groups axial (a,a) and the other conformer has both methyl groups equatorial (e,e). The most stable conformer is the one which has both the methyl groups in the equatorial position (e,e). Both the trans forms, a,a and e,e are chiral. Each axial methyl group contributes about 1.74 kcal/mol towards steric energy of the molecule, therefore the enegy of trans a,a conformer is about 3.48 kcal/mol. The trans e,e conformer has two methyls gauche to each other, the steric energy therefore will be that of gauche butane, 0.74 kcal/mol. Therefore The trans e,e conformer has (3.48-0.74 = 2.74) 2.74 kcal/mol less steric energy than a,a isomer.

Structure 2 (cis13DMCH.mol) The cis and trans isomers of 1,3-dimethylcyclohexane exist in single conformations. For the trans isomer with one axial and one equatorial methyl groups (a,e) the two possible conformations (a,e and e,a) are supeposable. The cis isomer can exist in two conformations, one with both methyl groups axial (a,a) and the other with both methyl groups equatorial (e,e). Due to two synaxial methyl groups the a,a conformer is very unstable. Hence predominantly it exists in e,e conformation. The cis 1,3-dimethylcyclohexane e,e is achiral. The trans isomer is chiral.

Structure 3 (cis14DMCH.mol) The cis and trans isomes of 1,4-dimethylcyclohexane are achiral. They have mirror plane symmetry. For the cis isomer one methyl is axial and another is equatorial (a,e). The two conformers are indistinguishable, in equimolar proportion and are superposable. The trans isomer consists of two conformers, less abuntant (0.3%) a,a where both the methyl groups are axial and more predominant (99.7%) e,e, both the methyls are equatorial. Each axial methyl group contributes to about 1.74 kcal/mol towards steric energy of the molecule, therefore the enegy level of a,a isomer is about 3.48 kcal/mol more than the e,e conformer.

, atomno=20, atomno=21; color bonds white