The isomers of cyclohexane

Abstract

The equilibrium between the rigid and the flexible isomer of cyclohexane is calculated. The results indicate that a t ordinary temperatures the flexible isomer occurs a t most to a very slight extent. At higher temperatures it may occur in concentrations which are large enough t o allow an experimental confirmation.Cyclohexane resembles in many respects the saturated paraffinic hydrocarbons. This has led to the hypothesis that the valence state of the carbon atom in cyclohexane is similar to the valence state in saturated molecules where, according to Van't Hoff and Le Bel, one has to assume a tetrahedral arrangement of the four valencies. On the basis of this hypothesis, Sachse derived by geometrical considerations that two isomers of the cyclohexane molecule might be possible. The angles between two consecutive carbon-carbon bonds being less than IzoO-in our case 1 0 g O 28' 16"--it is possible to construct an infinite number of configurations which can be divided into two groups. In one group the six-membered ring can pass continuously through an infinite number of configurations.

Investigations of the conformation of non-aromatic ring compounds. XIII. The crystal structure of cyclohexane-1,3-dione at –140°C

“…The molecular structure of cyclohexane-l,4-dione has been under discussion for several years. This compound shows in solution a dipole moment of 1.3 D (Le F~vre & Le F@vre, 1935) which has been interpreted in terms of an equilibrium between chair and boat forms (Le F~vre & Le F~vre, 1955) as well as a predominant existence of flexible forms (Oosterhoff, 1949;Hazebroek & Oosterhoff, 1951;Allinger & Freiberg, 1961). The similarity of the infrared spectra in the solid state and in solution seems to indicate that the compound has the same conformation in both the solid and the solution.…”

The Dihedral Angles of Cyclohexane

“…Klyne and Prelog (1960). The former values are obtained also by straightforward operations on Hazebroek and Oosterhoff's (1951) equations (9), (S),…”

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Some 10 years ago Hazebroek and Oosterhoff (1951) thoroughly analysed the complex geometrical forms which the cyclohexane ring can assume, the mechanical rigidity of the chair form and the contrasting flexibility of the boat form. One single variable suffices to define completely the geometry of the flexible form ; this variable may be any one dihedral angle between three successive carbon-carbon bonds, any one 1-4 carbon-carbon distance, or, as in Hazebroek and Oosterhoff's case, a mathematical variable selected to suit their purpose of describing in a symmetrical way the rotation between staggered and eclipsed conformations.

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Thermodynamic Properties

“…13.3 represents the equilibrium conformations C s and C 2 of the ring and an intermediate non-equilibrium conformation. Hazebrock and Oosterhoff 51 have pointed out the fact that the chair form is rigid and cannot be converted into the boat form without stretching of bonds, whereas the boat form is flexible. The coordinate z is given by z = Λ / 2 7 5 Q cos 2(7Γ?/5 + φ) (13.80) where j is 1, 2, 3, 4, or 5, depending on the index number of the atom, and φ is an angle such that change by π brings the conformation back to itself.…”