Cyclohexane ring inversion

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

The activation energy for cyclohexane ring inversion is 45 kJ/mol (10 8 kcal/mol) It IS a very rapid process with a half life of about 10 s at 25°C... 

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

A more detailed discussion of cyclohexane ring inversion can be found in the July 1997 issue of the Journal of Chemical Education pp 813-814... 

Ring inversion in methylcyclohexane differs from that of cyclohexane m that the two chair conformations are not equivalent In one chair the methyl group is axial m the other It IS equatorial At room temperature approximately 95% of the molecules of methylcyclohexane are m the chair conformation that has an equatorial methyl group whereas only 5% of the molecules have an axial methyl group... 

Ring inversion (Section 3 9) Process by which a chair conforma tion of cyclohexane is converted to a mirror image chair All of the equatonal substituents become axial and vice versa Also called ring flipping or chair-chair interconversion... 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

Fig. 3.4. Energy diagram for ring inversion of cyclohexane. [For a rigorous analysis of ring inversion in cyclohexane, see H. M. Pickett and H. L. Strauss, J Am. Chem. Soc. 92 7281 (1979).]...

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Figure 6-1. Arrhenius plot for (he chair-chair ring inversion of cyclohexane. ...

One after the other, step through (or animate) the sequence of structures depicting ring inversion in cyclohexane, methylcyclohexane and trans-l,2-dimethylcyclohexane. Is the overall motion involved in the ring inversion similar in all three Describe any differences. 

The barrier for the ring inversion of cyclohexane (i.e. the barrier of the chair/twist interconversion) has been determined experimentally (by n.m.r.-techniques) by Anet and... 

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. 

Another approach to synthesize stable Diels-Alder adducts of Cjq was introduced by Mullen and co-workers [41—43], The use of o-quinodimefhane derivatives as dienes, prepared in situ, leads to the formation of thermally stable cycloadducts (Scheme 4.5). As with the isobenzofuran addition product [13], a cycloreversion of these adducts would need to overcome the stabilization provided by the aromatic system and would also give the unstable o-quinodimefhane intermediate. A fast ring inversion, at elevated temperatures, of the cyclohexane moiety causes a 2 -symmetry of the cycloadduct, leading to 17 lines for the fuUerenyl carbons in the NMR spectra [41]. 

The energy profile for ring inversion in cyclohexane may be rationalized given what has already been said about single-bond rotation in -butane. Basically, the interconversion of chair cyclohexane into the twist-boat intermediate via the transition state can be viewed as a restricted rotation about one of the ring bonds. 

In the case of bond rotation in ethane, the reactants and products are the same and the reaction is said to be thermoneutral . This is also the case for the overall ring-inversion motion in cyclohexane. 

This chapter assesses the ability of molecular mechanics and quantum chemical models to properly assign preferred conformation, and to account quantitatively for differences in conformer energy as well as for barriers to rotation and inversion. The chapter ends with a discussion of ring inversion in cyclohexane. 

Another important conformational process, ring inversion , is best typified by cyclohexane. This molecule undergoes motion in which axial and equatorial ring positions interconvert. 

Semi-empirical models do not provide good descriptions of the energy barrier to ring inversion in cyclohexane. The MNDO model underestimates the barrier by a factor of three, and the AMI and PM3 models by almost a factor of two. This behavior is consistent with previous experience in dealing with single-bond rotation barriers. 

Special cases of these involving transition states for rotation about single bonds, inversion of pyramidal nitrogen and phosphorus centers and ring inversion in cyclohexane, have been discussed in the previous chapter. The only difference is that these conformational processes are typically well described in terms of a simple motion, e.g., rotation about a single bond, whereas the motion involved in a chemical reaction is likely to be more complex. 

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