Ring inversion of cyclohexanes

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).]...

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

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... 

Another example is the ring inversion of cyclohexanes. These are usually nonresolvable equilibrium mixtures of rapidly interconverting conformers at room temperature, the configuration of substituents changing frori) axial to equatorial. 

It s clear from the diagram that the barrier to ring inversion of cyclohexane is 43 kjmol, or a rate at 25 °C of about 2 x 105 s-1. Ring inversion also interconverts the axial and equatorial protons, so these are also exchanging at a rate of 2 x 105 s-1 at 25 °C—too fast for them to be detected individually by NMR, which is why they appear as an averaged signal. 

To illustrate the role of quantum energy flow on rates of conformational isomerization, we calculate the rate of ring inversion of cyclohexane. Measurements of rates of ring inversion by NMR in the vapor phase and in nonpolar liquids reveal an interesting pattern in the variation of the rate over a broad range of collision frequency with solvent. In the vapor phase, the rate increases with... 

If one examines the H NMR spectrum of cyclohexane at temperatures in the range in which the changeover takes place from a single average absorption to separate axial and equatorial absorptions, it is possible to estimate the rate constant for the ring inversion of cyclohexane. One can translate this result to room temperature and say that here cyclohexane is undergoing ring inversion more than 100,000 times... 

Ring inversion of cyclohexane occurs by a route that involves twist-boat, and possibly boat, conformations and which is not reproduced accurately in the manual inversion outlined below. 

A large number of studies employing different NMR techniques have been devoted to the investigation of the temperature dependence of the ring inversion of cyclohexane. This is not surprising in view of the fact that the problem of cyclohexane inversion represents the seminal problem in conformational analysis. What is surprising, however, that all previous studies have used a single solvent - carbon disulfide, and, that only a limited pressure study (up to 2 kbar) of cyclohexane in quaternary mixture has been performed by Liidemann et al. ( 7)... 

Ring flipping Synonymous with ring inversion of cyclohexane and related compounds. 

One much-studied subject is the ring inversion of cyclohexane and polymethylcyclo-hexanes, as shown in Table 8. The subject has been reviewed " but, very generally, compounds with flattened chair conformations due to methyl-methyl 1,3-diaxial interactionshave lowered barriers. Simply substituted compounds where rotation about substituted bonds can take place in the boat-twist manifold of conformations have barriers similar to that of cyclohexane, while more highly substituted compounds, where rotation about substituted bonds must take place on the way to the transition state have markedly higher barriers than in cyclohexane. 

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