The conformation of cyclic systems

The conformational behavior of cyclic molecules is conveniently discussed separately from that of acyclic molecules. Ring closure indeed imposes conformational constraints and limitations not existing in linear systems. 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and 

The two conformers LXXVlIIa and LXXVIIIb are extremes of symmetry in what is known as the pseudorotational circuit of cyclopentane. If in structure LXXVlIIa, the out-of-plane carbon (arbitrarily designated C-l here) is pushed down together with C-2, a half-chair is obtained (C-l above, C-2 below the plane). If the motion is continued, another envelope is reached (C-2 below the plane). The process then repeats with C-2 and C-3, and so on 10 envelope forms and 10 half-chair forms interconvert by this process, which is not unlike a wave on a water surface ([100], and refs, therein). 

The pseudorotation circuit of cyclopentane is essentially of constant strain, and therefore without maxima and minima in contrast, the fully planar conformer is less stable by ca. 5 kcal/mol [101]. An extensive treatment of the conformational aspects of cyclopentane and of many five-membered-ring compounds has been published by Legon [92]. 

The preferred conformer of cyclohexane is the chair form (LXXIXa), in which all 

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