Equatorial fluorine cyclohexane

In terms of its steric impact, fluorine is the smallest substituent that can replace a hydrogen in a molecule, other than an isotope of hydrogen. Table 1.1 provides insight as to the comparative steric impact of various fluorinated substituents on the equilibrium between axial and equatorial substitution in cyclohexane.14... 

Mono- and 1,1-disubstituted cyclohexanes. There are several electron diffraction investigations of mono-substituted cyclohexanes. Cyclohexylfluoride301 exists as 57% e and 43% a which means that the equatorial position is favored by about 710 J/mol. A microwave study302 yielded 1.6+ 1.2 kJ/mol for this energy difference. Cyclohexylchloride303 is very similar to the fluorine derivative in that the gas phase... 

As was the case for their fluorine spectra, the carbon NMR spectra of trifluoromethyl cyclohexanes show some slight differences depending on whether the CF3 group is equatorial or axial, with the axial CF3 appearing at slightly ( 1 ppm) higher field. 

In the static TBP shown in Fig. 13 there are two types of methine protons, two types of cyclohexane-equatorial and two types of cyclohexane-axial methylene protons, and two types of fluorines. Yet, the n.m.r. spectra of this compound in solution at + 30°, which is summarized in Table 13, exhibits only one signal (tCh) for the methine protons, one signal for the equatorial and one signal for the axial methylene protons of the cyclohexane (rCHl), and one signal for the fluorines. These spectra... 

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. 

Discussion of the physicochemical sizes of substituents on the basis of conformational analysis of the torsional process of substituted cyclohexanes, cyclophanes, and diaryl derivatives have been summarized by Schlosser [5]. The energies required for the conformational exchange of an equatorial position to the axial position for each substituent in cyclohexane are listed in Table 1.47 [6]. For fluorine, the energy difference between the axial and equatorial environment is negligibly small as compared with methyl, while that of CF3 seems larger than that of z-Pr so far observed in the cyclohexane torsional system. From the... 

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