Cyclohexanes conformational equilibrium

The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

Valuable information on conformational equilibrium can be obtained particularly by N.M.R. technique. When a molecule can exist in several conformations which rapidly interchange, then any proton which assumes all possible positions in a very short time, the n.m.r. spectrum would show only one peak. This happens in most open chain compounds and even in cyclohexanes where the interconversion is very rapid. But if the interconversion is slowed or prevented, either by cooling or due to the inherent structure in the molecule, the hydrogens of each conformer appear separately and so more than one peak would appear. For example by cooling cyclohexane to -110°C, two peaks appear, one due to equatorial and the other to the axial hydrogens. 

Taft steric values. A, values from the axial-equatorial conformational equilibrium in cyclohexane v, Charton steric parameters. ... 

H-NMR studies on 2,6-dimethylmorpholine314 and N-substituted 2,6-dimethylmorpholines315 are consistent with the single diequatorial conformation 391 for the cis isomers and with the equilibrium 392 393 for the trans isomers. The 13C-NMR spectrum of the monomethyl-, 2,3-, 2,5-, 2-6-, 3-5-dimethyl-, 2,3,4-, 2,3,6-trimethyl-, and 2,3,5,6-tetramethylmorpholines are largely consonant with expectations based on conformational principles derived from cyclohexane conformational analysis.316 The all-cis 2,3,5,6-isomer, which did not prove amenable to analysis by H-NMR spectro-... 

Other substituted cyclohexanes are similar to methylcyclohexane. Two chair conformations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative amounts of the two conformations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane conformations, is related to the degree of branching at the atom connected to the ring. A single... 

Actually, the conformational equilibrium of cyclohexane-based 1,3-di-podands 27 changes from a conformational mixture biased towards the... 

The preference of a bulky substituent for the equatorial position in a cyclohexane ring often results in the establishment of a conformational equilibrium for the mobile mono-cyclic system, but the rigidity of a steroid molecule prevents conformational inversion. Thus the. epimeric 5a-cholestan-3j0-ol (i) and 5a-cholestan-3a-ol (2) have conformations in which the hydroxyl group is respectively held in the equatorial and... 

Because of the shorter C-0 bond distance (C-0 1.43 A), the 1,3 R H interaction is expected to increase. This is especially evident when two heteroatoms are present in the cyclohexane ring, as in 1,3-dioxanes. For example, the conformational equilibrium in the following example favors the axial t-butyl group It should be noted that interference by bonded atoms or groups is more severe than by nonbonding (lone pairs) electrons. 

There may be some traps in these deductions. Complexation can bring about configurational inversion of the ring which, in turn, leads to a variation of the signal positions, much greater than complexation. Owing to fact that 11.7 is in conformational equilibrium with 11.8, this cyclohexane polyol can present two efficient complexation sites, one being aea and the other triaxial, which do not exist in its most stable conformation. 

Figure 2.14 Proportions of axial conformer of 2-aryloxytetrahydropyrans in deutero-chloroform solution. The conformational equilibrium is invariant with X in cyclohexane (84-87% axial).

The acetal RCH(OMe)2 can have a total of nine conformers, 30a-30i. We may ignore the broken red bonds, which are included to allow a quick conformational match with that of the cyclohexane chair and, thus, ascertain the geometrical relationships rather easily. The conformers 30a and 30e have two methyl groups within van der Waals distance and, hence, their contributions to the overall conformational equilibrium will be small, if not zero. We can therefore eliminate these conformers from further discussion. The conformers 30b and 30d, 30c and 30 g, and 30f and 30 h are mirror images and, thus, we need to consider only one conformer of each pair. Thus, we are left with four distinct conformers, namely 30b, 30c, 30f, and 30i, to consider for acid hydrolysis. The relative contributions of these conformers could be estimated from the understanding that they are laced with two, one, one and zero stereoelectronic effects, respectively. The conformers 30b and 30i are, respectively, the most contributing and the least contributing. The conformers 30c and 30f contribute at the medium level. 

Uvarov et al. demonstrated the suitability of the Saunders isotopic perturbation method, applied to vicinal C-D couplings, in order to measure the conformational equilibrium of deuterated cycloalkanes. To this end, the secondary isotope effect on "7(C,D) was determined as shown in Eq. (89), where I represents (Di)cyclohexane, and II represents (l,I-D2)cyclohexane. 

If steric repulsion experienced by the methyl and dimethylamino groups increases by the same factor on going from the cyclohexane to dioxane, then the equilibrium constants for the ring inversion of the two compotmds should be the same. At 185°K, cyclohexane conformation in which the NMc2 is axially... 

For substituted cyclohexanes, the slow-exchange condition is met at temperatures below about -50°C. Table 3.5 presents data for the half-life for conformational equilibrium of cyclohexyl chloride as a function of temperature. 

At temperatures where the ring inversion (47) is fast, 1,1,3,3-tetramethyl-cyclohexane shows only one averaged methyl signal in its C nmr spectrum. The degeneracy of the fast conformational equilibrium is lifted in [52] which has one deuteriated methyl group. This leads to a splitting of the averaged... 

Gunther and Aydin (1981) have investigated the conformational equilibrium (59) in di-cyclohexane. The two conformational isomers [65a] and [65b] with deuterium in the axial and in the equatorial position can be frozen out in the 100 MHz C spectrum below — 80 C. The deuteriated carbons in the two isomers have different intrinsic isotope shifts which are upheld compared to the nondeuteriated carbons. The triplet with the smaller coupling constant was assigned to the carbon with axial deuterium and was shifted 0.0482 ppm to higher held than the triplet caused by the carbon with equatorial deuterium. 

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... 

Conformational Analysis. - Freitas et al studied the conformational equilibrium of tra 5-l,2-difluoro-, tra s -l,2-dichloro-, and trani-l,2-dibromo-cyclohexanes, in which the halogen atoms have the two possible conformations of the axial-axial (aa) conformer and the equatorial-equatorial (ee) conformer. The observed /(H, H) coupling constant, is obtained as the weighted average of the aa conformer s coupling and the ee conformer s coupling Jee as follows ... 

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