Energy difference, between axial and

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial methyl group on Cl is too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kj/mol of steric strain (Figure 4.13). 

Table 6.1 A Values Free-Energy Differences between Axial and Equatorial Conformations of Monosubstituted Cyclohexanes (kcal/mol)...

Now we can compare the relative stabilities of the cis and trans isomers of 1,3-dimethylcyclohexane. The most stable conformation of the cis isomer has both methyl groups in equatorial positions. Either conformation of the trans isomer places one methyl group in an axial position. The trans isomer is therefore higher in energy than the cis isomer by about 7.6 kJ/mol (1.8 kcal/mol), the energy difference between axial and equatorial methyl groups. Remember that the cis and trans isomers cannot interconvert, and there is no equilibrium between these isomers. 

X Equilibrium constant, K Energy difference between axial and equatorial conformers, kJ mol-1 % with substituent equatorial... 

Table 4.1 shows that an axial hydroxyl group causes 2 x 2.1 kJ/mol of steric strain. Thus, the energy difference between axial and equatorial cyclohexanol is 4.2 kJ/mol. 

Table 4.1 shows that an axial bromine causes 2 x 1.0 kJ/mol of steric strain. Thus, the energy difference between axial and equatorial bromocyclohexane is 2.0 kJ/mol. According to Figure 4.12, this energy difference corresponds approximately to a 75 25 ratio of more stable less stable conformed Thus, 75% of bromocyclohexane molecules are in the equatorial conformation, and 25% are in the axial conformation at any given moment. 

Figure 4. The important interactions in glycopyranosyl halides with axial C-Cl bond, and the corresponding expression for the energy difference between axial and equatorial anomers...

The anomeric effect is quoted as a free energy difference between axial and equatorial forms in tetrahydropyrans fTHPs) with account taken of the greater steric preference of a substituent to be equatorial in a THP with respect to a cyclohexane. 

Figure 8 Calculated conformational energy differences between axial and equatorial methyl-cyclohexane in kcal/mol. The dashed line shows the experimental value.

When cyclohexane is substituted by an ethynyl group, —C=CH, the energy difference between axial and equatorial conformations is only 1.7 kj (0.41 kcal)/mol. Compare the conformational equilibrium for methylcyclohexane with that for ethynylcyclohexane and... 

The energy differences between axial and equatorial conformers for 7.101 are given in the below table in comparison with those for the related cyclohexanes. Explain this observation. 

TABLE 26.6 Gibbs Energy Differences Between Axial and Eq uatorial Conformers of Mono-Substituted Cyclohexanes... 

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