Fluorocyclohexane equatorial

It is believed that equatorial substituents such as chlorine or bromine would increase the guest diameter beyond the allowed values (assuming that the guest molecules stack roughly parallel to the canal68)). Support for this comes from the study of fluorocyclohexane where the population of the axial conformer is not enhanced to any major extent70. Nitro-71) and cyano-cyclohexane, trans-l,2-dichloro-, trans-1,2-dibromo-, tram-1,4-dichloro-, trans-1,4-dibromo-, and trans-l-bromo-4-chloro-cyclohexane all pack most efficiently in the thiourea canals as the axial or diaxial conformer 68,72. Tram-2,3-dichloro-1,4-dioxane behaves similarly73. In contrast isocyanato-, tram-1,4-diiodo-, trans-1 -bromo-4-iodo-, and tram-1 -chloro-4-iodo-cyclohexane are present as mixtures of the axial/equatorial or diaxial/diequatorial conformations as appropriate 68,72). The reason for this anomalous behaviour of the iodosubstituted cyclohexanes is not clear. 

Fooking more closely at fluorocyclohexane systems, it has been observed that a fluorine substituent in the axial position is much more highly shielded than one in the equatorial position. Of course,... 

More evidence for the existence of several conformational isomers, at least in liquid and gaseous substances comes from infrared and also Raman spectra. For example each conformer has its own I.R. spectrum, but the peak positions are often different. Thus the C-F bond in equatorial fluorocyclohexane absorbs at 1062 Cm-1, the axial C-F bonds absorbes at 1129 Cm . So the study of infrared spectrum tells, which conformation a molecule has. Not only this, it also helps to tell what percentage of each conformation is present in a mixture and since there is relationship between configuration and conformation in cyclic compounds the configuration can also be frequently determined. 

Molecular motion in solids has been the object of many studies in the field of physical chemistry of polymers , but dynamic processes in molecular crystals of organic and inorganic compounds are less well investigated. In fact, the average chemist is not aware of the fact that processes like internal rotation or ring inversion proceed in solids quite often with barriers which are not very different from those found for these types of internal motion in the liquid state. Thus, for the equatorial axial ring inversion of fluorocyclohexane values of 42.4 and 43.9 kJ mol have been measured in the liquid and the solid, respectively. The familiar thermal ellipsoids of individual atoms obtained from X-ray studies are qualitative indicators of molecular motion in the crystal, but a more quantitative study of such processes is only possible after appropriate solid state NMR techniques are applied. 

All models (except STO-3G and semi-empirical models) reproduce the significant reduction in dipole moment observed in going from equatorial to axial fluorocyclohexane, but all underestimate the even... 

There are many possible sources of error in the calculation of equilibrium from dipole moments. It is difficult to calculate accurately the moments of model compounds. As has been pointed out,124 the carbon skeleton itself contributes to the moment, as shown by the different dipole moments of ethyl bromide (2.069 D) and n-hexyl bromide (2.156 D) and of axial and equatorial fluorocyclohexane (1.81 D and 2.11 D, respectively). For nonsym-metrical molecules in general, the estimation of the precise direction of a dipole moment is subject to considerable error. 

Looking more closely at fluorocyclohexane systems, it has been observed that a fluorine substituent in the axial position is much more highly shielded than one in the equatorial position. Of course, ordinarily one will observe a single, time-averaged 19F signal for fluorocyclohexanes, because of the relatively fast interconversion of the two chair conformations. A chemical shift of -171 (broad singlet) has been reported for dynamically equilibrated fluorocyclohexane at room temperature. See Chapter 4 for more details about the conformational dynamics of fluorocyclohexane systems. 

At 25 °C, the energy difference (AG°) for the conversion of axial fluorocyclohexane to its equatorial conformation is -0.25 kcal/mol. (a) Calculate K q for this equilibrium, (b) Calculate the percentage of eixial and equatorial conformations present at equilibrium. 

A. Compound 72 is drawn in 73 in its most stable chair conformation, with the bulkier C02H groups equatorial and the smaller F axial. The name is wj.v-5-fUiorocyclohexane-r-l,c7j-3-dicar-boxylic acid or 5[3-fluorocyclohexane-lot,3a-dicarboxylic acid. A plane of symmetry through C 2) and C 5) of 72 rules out chirality. 

The chair conformer of fluorocyclohexane is 0.25 kcal/mol (1.0 kJ/mol) more stable when the fluoro substituent is in the equatorial position than when it is in the axial position. How much more stable is the anti conformer of 1-fluoropropane compared with a gauche conformer ... 

Write a conformational structure for l-bromo-3-chloro-5-fluorocyclohexane in which all PRACTICE PROBLEM 4.17 the substituents are equatorial. Then write its structure after a ring flip. 

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