Conformers of Monosubstituted Cyclohexanes

A substituent is in an equatorial position in one chair conformer and in an axial position in the other. The conformer with the substituent in the equatorial position is more stable. 

Butane has one gauche interaction between a methyl group and a hydrogen, whereas methylcyclohexane has two 1,3-diaxial interactions between a methyl group and a hydrogen. 

In Section 3.10, we saw that the gauche interaction between the methyl groups of butane causes a gauche conformer to be 0.87 kcaFmol (3.6 kJ/mol) less stable than the anti conformer. Because there are two such interactions in the chair conformer of methylcyclohexane when the methyl group is in an axial position, this conformer is 1.74 kcaFmol (7.2 kJ/mol) less stable than the chair conformer with the methyl group in an equatorial position. 

Because of the difference in stability of the two chair conformers, a sample of methylcyclohexane (or any other monosubstituted cyclohexane) will, at any point in time, contain more chair conformers with the substituent in an equatorial position than with the substituent in an axial position. The relative amounts of the two chair conformers depend on the substituent (Table 3.9). 

The substituent with the greater bulk in the vicinity of the 1,3-diaxial hydrogens will have a greater preference for an equatorial position because it will have stronger 1,3-diaxial interactions. For example, the experimental equilibrium constant (A gq) for the 

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Ring-flip in chair conformation of monosubstituted cyclohexane... 

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

In opposition to a central principle of alicyclic conformational analysis, Kang and Yin reported that a complex O-cyclohexyl nitronate and the corresponding O-cyclohexyloxime constitute the first stable axial conformer of monosubstituted cyclohexanes at ambient temperature (97JA8562). Snyder and co-workers (99JA11864) reevaluated the corresponding... 

Problem-Solving Strategy Drawing Chair Conformations 116 3-14 Conformations of Monosubstituted Cyclohexanes 117 3-15 Conformations of Disubstituted Cyclohexanes 120... 

Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

As Table 1.1 shows, fluorine is the second smallest element, with size approximately 20% larger than the smallest element, hydrogen. Table 1.2 summarizes four steric parameters for various elements and groups (i) Taft steric parameters Es [44], (ii) revised Taft steric parameters E [45], (iii) Charton steric parameters o [46], and (iv) A values [47], The steric parameters, Es, E, and u are determined on the basis of relative acid-catalyzed esterification rates, while the A values are derived from the Gibbs free energy difference calculated from the ratios of axial and equatorial conformers of monosubstituted cyclohexanes by NMR. 

We have seen that when a cyclohexane ring flips, all equatorial bonds become axial and all axial bonds become equatorial. Now lets consider the consequences of flipping a substituted cyclohexane ring. The chair—chair interconversion of monosubstituted cyclohexanes occurs very rapidly. However, the two conformations of monosubstituted cyclohexanes, unlike those of cyclohexane, are not equally stable. 

Compare models of the chair conformations of monosubstituted cyclohexanes in which the substituent alkyl groups are methyl, ethyl, isopropyl, and tert-butyl. 

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