Show below are the ring structures and conformations of some simple cycloalkanes such as cyclopentane, cyclobutane, cyclopropane and cyclopropane and cycloethane or ethene.
Ring Structures and Conformations of Cyclopentane
The five —CH2— groups of cyclopentane theoretically could form a regular planar pentagon (internal angles of 108°) with only a little bending of the normal C-C-C bond angles. Actually, cyclopentane molecules are not flat. The planar structure has completely eclipsed hydrogens, which make it less stable by about 10 kcal mole”1 than if there were no eclipsed hydrogens. The result is that each molecule assumes a puckered conformation that is the best compromise between distortion of bond angles and eclipsing of hydrogens. The best compromise conformations have the ring twisted with one or two of the —CH2— groups bent substantially out of a plane passed through the other carbons (Figure 12-14). The flexibility of the ring is such that these deformations move rapidly around the ring.
Ring Structures and Conformations of Cyclobutane
Formation of a four-membered ring of carbon atoms can be achieved only with substantial distortion of the normal valence angles of carbon, regardless of whether the ring is planar or nonplanar. In cyclobutane, for example, if the valence bonds are assumed to lie along straight lines drawn between the carbon nuclei, each C-C-C bond angle will be 19.5° smaller than the 109.5° tetrahedral value:
Of course, the angle distortion will be still greater if the ring is nonplanar. Nonetheless, the energy of eclipsing the hydrogens in cyclobutane is sufficient to cause the ring to be nonplanar. Substituents are located most favorably in what might be called the “quasi-equatorial” positions (Figure 12-15).
Figure 12-15 Nonplanar cyclobutane conformation with a substituent R in the less hindered, quasi-equatorial position. The dihedral angle between the two halves of the bent ring usual ly is 25° to 30°, that is, a 25° to 30° deviation from planarity.
Ring Structures and Conformations of Cyclopropane
The three carbon atoms of the cyclopropane ring lie in a plane. Therefore the angle strain is expected to be considerable because each C-C-C valence angle must be deformed 49.5° from the tetrahedral value. It is likely that some relief from the strain associated with the eclipsing of the hydrogens of cyclopropane is achieved by distortion of the H-C-H and H-C-C bond angles:
Ring Structures and Conformations of “Cycloethane” (Ethene)
If one is willing to consider a carbon-carbon double bond as a two-membered ring, then ethene, C2H4, is the simplest possible cycloalkane (“cycloethane”). As such, C2H4 has C-C-C valence angles of 0° and therefore an angle strain of 109.5° at each CH2 group compared to the tetrahedral value:
Shown above is the actual ring and conformations of cycloethane or ethene where each carbon atom is Sp2 hybridized which makes the molecules arranged in trigonal order and 120o apart.
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