Axial and equatorial conformations

For the equilibrium between the axial and equatorial conformations of a monosubstituted cyclohexane. 

It was assumed above that the methyl group adopts an equatorial conformation. Actually, methylcyclohexanone exists as a mixture of axial and equatorial conformations. 

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). 

Look at Figure 4.12, and estimate the percentages of axial and equatorial conformers present at equilibrium in bromocyclohexane. 

The chloro- and bromo-cyclohexane inclusion compounds have been extensively examined by infrared (4000-30 cm-1) spectroscopy65-68 and by Raman (< 1000 cm-1) spectroscopy 69). In the canals both guests are found to exist exclusively in the chair conformation with an axial halogen substituent, while iodocyclohexane 65, 68,69) acj0pts both axial and equatorial conformations in the canal. These results should be contrasted with the familiar situation in the liquid phase where the equatorial arrangement is the lowest energy conformer and is present to the extent of about 65-70% at room temperature. 

The conformational properties of mono-substituted cyclohexanes, C V, 1111X, in their thiourea inclusion compounds have been studied102. Variable-temperature MAS spectra demonstrate that a chair-chair ring inversion process occurs in the thiourea tunnel, in which the axial and equatorial conformers are interconverted. Predominance of the equatorial conformer is found when X = NH2. 

Conformational analysis in connection with determinations of ffee-energy differences (AG°) between axial and equatorial conformers is still attracting interest. Schneider and Hoppen (114) discussed A values ( —AG°) and preferred orientations of axial substituents with lone pairs at heteroatoms directly attached to C (e.g., -OR, -NR2, and -N3), as well as of some other nonspherical substituents (X = -NC, -NCS, -CN, -C CH). Phenyl and vinyl groups were investigated by Eliel and Manoharan (277), who found A values of 2.87 0.09 kcal/mol for phenyl and 1.68 0.06 kcal/mol for vinyl. The latter value was essentially confirmed by Buchanan (196) the formyl group A = 0.84 0.08 kcal/mol) in axial position adopts a predominant (93%) conformation (305) with the plane of the axial CHO group nearly perpendicular to the plane of symmetry of the cyclohexyl residue (Scheme 71) (196). 

Chemical equilibration of conformationally fixed model compounds. Diastereomeric analogs of the distinct axial and equatorial conformations... 

The NMR parameters Pax and Peq must be sufficiently different in the axial and equatorial conformers to have an accurate measure of K. The following are most commonly used ... 

At this point, it probably will be helpful to construct models of cis- and trans-decalins to appreciate the following (a) The two compounds cannot interconvert unless C-C or C-H bonds first are broken, (b) traw -Decalin is a relatively rigid system and, unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the substituent is fixed in the chair-chair conformation of trans-decalm, (c) The chair-chair forms of cw-decalin are relatively flexible, and inversion of both rings at once occurs fairly easily (the barrier to inversion is about 14 kcal mole-1). A substituent therefore can interconvert between axial and equatorial conformations (Figure 12-24). 

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

If substituents are placed in the 3 and 4 positions, axial and equatorial conformations may arise. In 3-chlorocyclohexene (52)327 80% is axial and 20% equatorial,... 

Figure 6. Crystal structures of the ternary complexes of [Yb.la]3 with (R)-alanine (left) and (

The rotational spectra of the thietane/HCl complex have been registered by Fourier transform microwave spectroscopy <2001AGE935, 2002CEJ4265>. It was found that the equivalence of the nonbonding pairs at sulfur is broken by complexation as a consequence of the appearance of axial and equatorial conformers in the thietane-HC1 complex. Several aspects on the axial and equatorial structures have been discussed <2001AGE935, 2002CEJ4265>. 

Methylcyclohexane exists at room temperature as a rapidly interconverting mixture of axial and equatorial conformers. These conformers are not superposable, and at low temperatures a spectrum of each conformer exists. 

Given that the equilibrium constant for the interconversion of the axial and equatorial conformations of methylcyclohexane is 18, show how to calculate the percentage of each that is present at equilibrium. 

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

The conformational behaviour of 2-phosphoryl-l,3-dioxanes and dithianes continues to be an area of interest. Molecular mechanics methods have been applied to 2-phosphoryl-l,3-dioxane (34), the corresponding dithiane (35) and 2-thiophosphoryl-l, 3-dithiane (36).15 The relative stabilities of the axial and equatorial conformers in each case vary due to a combination of differing 1,3-axial interactions and anomeric effects. The conformer calculated to be the most stable for (34) and (35) is the structure given in each case. In the case of (36) the isomers are estimated to be of... 

Use SpartanBuild to construct models of axial and equatorial conformations of methylcyclohexane and (er/-butylcyclohexane. Minimize each structure, and use the energy differences to predict the relative conformational preferences of methyl and tert-butyl groups. 

Table 28-1 shows the three possible axial and equatorial conformations. Substitution in Equation 28-27 generates the calculated ratio of each conformcr. Because MMFF94 was parameterized to reproduce the quantum mechanical calculations. it is illustrative to look at the ratio calculated with MMFF94. The Boltzmann-averaged distribution may then be compared with the experimental data as well as the other force field results. 

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