Axial groups, cyclohexane

Diaxial interaction (Section 4.8) The strain energy caused by a steric interaction between axial groups three carbon atoms apart in chair cyclohexane. 

Steric strain (Sections 3.7) The strain imposed on a molecule when two groups are too close together and try to occupy the same space. Steric strain is responsible both for the greater stability of trans versus cis alkenes and for the greater stability of equatorially substituted versus axially substituted cyclohexanes. 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

Diaxial interaction (Section 6.7) An interaction that destabilizes two axial groups on the same face of a cyclohexane ring because of steric crowding between them. 

Trans-diaxial elimination (Section 9.3) Elimination of axial groups on adjacent atoms on a cyclohexane ring. This is the preferred geometry for the E2 reaction with cyclohexane derivatives. 

The strong steric strain between two axial groups on cyclohexane carbons with one carbon between them. (p. 119)... 

In reality, however, steric interactions in oxane and in a cyclohexane derivative are not the same. Because C-O bonds are shorter than C-C bonds, repulsive interactions of an axial group on a pyranoid ring are likely to be larger than those of the same group on the cyclohexane ring, and the... 

Information on disubstituted cyclohexanes with substituents other than methyl and on more highly substituted cyclohexanes may be found elsewhere. Of particular interest are the interaction energies of syn-axial groups other than the two methyl groups shown in Figure 8 for the minor conformer of cis -1,3 -di methylcyclohexane. 

Ring inversion in methylcyclohexane differs from that of cyclohexane m that the two chair conformations are not equivalent In one chair the methyl group is axial m the other It IS equatorial At room temperature approximately 95% of the molecules of methylcyclohexane are m the chair conformation that has an equatorial methyl group whereas only 5% of the molecules have an axial methyl group... 

The most stable conformation of trans 1 4 dimethylcyclohexane has both methyl groups in equatorial orientations The two chair conformations of trans 1 4 dimethyl cyclohexane are not equivalent to each other One has two equatorial methyl groups the other two axial methyl groups... 

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

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