Cyclohexane, chair-boat equilibrium

CyclohexanethioL In substituted cyclohexanes the number of possible conformations increases considerably, since the equatorial-axial equilibrium has to be considered in addition to that of the chair-skew-boat equilibrium. For thermodynamic calculations the contributions of these two equilibria have to be treated separately. ... 

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... 

Exhaustive catalytic hydrogenation of triptycene affords an equilibrium mixture of perhydrotriptycene isomers. As expected, Boyd s force field (37) calculations, with a modified torsional constant, reproduced the observed composition fairly well (Table 6). All important conformations were taken into account for each isomer. The most stable conformations agree with the results of the X-ray analysis (131) and have the characteristic that the cyclohexane rings are invariably either boat or deformed chair. The most stable conformation of all is 20 (ttt). The predominant conformation of ccc, in which all cyclohexane rings are boat, has an enthalpy only 2.56 kcal/mol above that of 20. The difference is virtually all due to angle and torsional terms. 

Energy changes for the conversion of the chair to the boat conformation of the cyclohexane ring can be estimated from a study of the equilibrium between cis- and tra i-l,3-di-t-butylcyclohexane. Some analytical results of Allinger and Freiberg [9] are listed in Table 12.7. 

Time-dependent effects The NMR signals are sometimes influenced by time-dependent phenomena such as conformational or prototropic changes, which take place at a rate faster than the line width and comparable to (or faster than) the inverse of the differences between the frequencies of the transitions of the different sites. This means that kinetic phenomena may be studied by the NMR technique, especially if the temperature of the sample can be adapted. As an example we mention the two chair forms of cyclohexane, which are energetically stable there is a fast inversion from the one into the other via the boat form. At room temperature the PNMR spectrum exhibits one sharp peak, corresponding to the mean of the two chemical shifts. At -120 °C the dynamic equilibrium is frozen and the spectrum exhibits two sharp peaks, whereas at -60 °C the inversion is slow and the spectrum exhibits one broad peak. 

A substituent on a cyclohexane ring (in the chair conformation) can occupy either an axial or an equatorial position. In many cases, the reactivity of the substituent depends on whether its position is axial or equatorial. The two possible chair conformations for methylcyclohexane are shown in Figure 3-23. These conformations are in equilibrium because they interconvert at room temperature. The boat (actually the twist boat) serves... 

A. The chair conformation is shown in 74. One consequence of a chair conformation is that a pronounced l,3-diaxial interaction exists between a /-butyl group and a hydroxyl group. This can be avoided if the ring adopts a twist-boat conformation 75. The number of molecules that exist to any extent in the twist-boat conformation is small, and 75 has the benefit of a hydrogen-bonded interaction between the two hydroxyl groups. The twist-boat conformation is ca, 6 kJ mol 1 more stable than the boat conformation. In this work, Stolow8 claims an equilibrium between 74 and 75. Another molecule known to exist in a twist-boat conformation is cyclohexane-1,4-dione (76) (see Hoffmann and Hursthouse9). 

A big problem in molecular mechanics is that the final geometry is very close to the starting one. We start from a boat (chair) conformation of cyclohexane and obtain a boat (chair) equilibrium geometry. The essence of molecular mechanics however, is that when started from a distorted boat (chair) conformation, we obtain the perfect, beautiful equilibrium boat (chair)... 

Cyclopentanes, cyclohexanes, and all larger cycloalkanes exist in dynamic equilibrium between a set of puckered conformations.The lowest energy conformation of cyclopentane is an envelope conformation.The lowest energy conformations of cyclohexane are two interconvertible chair conformations. In a chair conformation, six bonds are axial and six are equatorial. Bonds axial in one chair are equatorial in the alternative chair, and vice versa. A boat conformation is higher in energy than chair conformations. The more stable conformation of a substituted cyclohexane is the one that minimizes axial-axial interactions. 

For a more complicated case, consider the structure of cyclohexane, which exists in two conformational mimima, the chair structure of D d symmetry and the twist-boat structure of D2 symmetry. The chair conformer is stabilized with respect to the twisted boat by about 25 kJ/mole in a minimal basis. Starting at the D d equilibrium structure, the TRIM method reaches a first-order saddle point of C2 symmetry in 16 iterations and one backstep, again with no symmetry constraints applied. The activation energy is 49 kJ/mole. Minimizing from this structure, we reach the D2 twist-boat conformation in 10 second-order iterations. Thus we have been able to walk between the two stable conformers of cyclohexane in an automated manner. The uphill and downhill walks both require only a modest number of iterations, considering that they started far away in the global region. 

The chair—chair interconversion of cyclohexane passes through twist boat conformations that are in equilibrium with a boat conformation. 

---