Saturated Rings and Ring Assemblies

Although cydopentane is non-planar, it is impossible for it to adopt a stable conformation with alternate CH groups tilted towards staggmng, as in cyclobutane (or cydohexane), simply because it has an odd number of 

Unsaturated Rii] —Num ous cyclic olefins have berai studied recently. The simplest of these, cyclopropene, has C=C = 1.305 0.003 A, C-C = 1.521 0.002 A, ZHC=C = 133 3°. It thus possesses one of the shortest C=C bonds known C=C is 0.03 A shorter than in ethylene, presumably because non-bonded interactions again are reduced by the enforced bending back of the third carbon atom. The shortening is even more than observed in cyclopropane, where C-C is only 0.02 A less than in ethane. For comparison, microwave studies show cyclobutene to be planar with C=C = 1.342 A, =C-C = 1.517 A, C-C = 1.566 A (r values see Chapter 4). Cydopentene has a non-planar static conformation, Uke that of a partly opened envelope (14a), with flap angle, a = 29 1°. The bond lengths are C=C = 1.343 0.003 A, =C-C = 1.521 0.010 A, C-C = 1.547 0.012 A. The angle ZC=C-C is 111.0 0.4°, other 

CXX 103i. Cyclohexene has been the subject of three determinations. - It has Cj symmetry, with its two-fold axis passing through the centres of the double bond and the bond opposite (14b). Carbon atoms 1,2, 3, and 6 lie in a plane. The results are compared with Table 6. The double-bond Imgth is 

Like cyclohexene, 1,3- and 1,4-cyclohexadiene have been the subject of several studies. The 1,3-isomer (18) has symmetry and a shape comparable to that of cyclohexene (14b), but in (18) it is the bond linking the double bonds which is bisected by the two-fold axis. In the 1,4-isomer (19), the two moieties C-C=C C are planar, and the two planes meet at an obtuse angle. The results are listed in Table 7. In general, the results in 

In cyclohepta-l,3-diene, which has C, symmetry, all the carbon atoms except the one remotest from the double bonds are coplanar, giving the molecule a partly open envelope configuration (20), with flap angle a = 73 out-of-plane. Butadiene-like coi jugation is observed. Like cyclohexa-1,3-diene, cyclo-octa-l,3-diene has its strain relieved by a twist (here 38°) about the =C-C= bond, but the molecule appears to possess no elements of symmetry. Formula (21) shows a projection down the =C-C= bond. 

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