Cyclopentane
Since the internal angle of the pentagon is 108o, which is very close to that of the tetrahedron 109.5o, the cyclopentane ring can exist in a planar conformation with little angular strain. However, if the cyclopentane is planar, all 10 hydrogen atoms would disappear and the torsional energy would be 10 × 4.2 = 42 kJ mol-1. This undesirable situation can be minimized by twisting the cyclopentane ring into a lower energy non-planar, conformationally shifted conformation known as the envelope conformation (Fig. 4.8). The "twisting" occurs by rotation around the C—C bond in the ring.
Substituted cyclopentanes and cyclopentanones
Although cyclopentane itself does not have a single highly favorable conformation, substituents on the cyclopentane ring disrupt the strain balance in favor of the envelope or half-chair. In monosubstituted cyclopentanes (e.g., methylcyclopentane), the envelope conformation is preferred at 0.9 kcal mol−1 when the substituent occupies a sterically favorable equatorial position at the tip or wing of the envelope (Fig. 3.55A) . 111 In the case of 1,2-disubstituted cyclopentanes, the trans diastereomer (envelope e,e) is more stable than the cis isomer (envelope e,a); when two When the substituents are all Me, the energy difference is 1.8 kcal mol−1 (Fig. 3.55A). Here the cis isomer (envelope e,a) is mainly destabilized by the Me/Me steric interaction, which is stronger than usual because the torsion angle of the two Me is significantly lower than 60°. 112 For trans 1,2-dihalocyclopentanes, when the two halos are anti-axial and biaxial, the preferred conformation is presumably half-chair (Fig. 3.55B). This preference is consistent with the low dipole moment of trans 1,2-dihalocyclopentanes. For 1,3-disubstituted cyclopentanes, no clear order of stability for the cis and trans isomers was observed.
Cyclopentane in the planar conformation has no angular strain but is affected by shadowing strain. This leads to a preference for two non-planar conformations, the envelope and the half-chair conformation. The envelope conformation with a symmetry plane belongs to the CS point group, and the half-chair conformation belongs to the C2 point group. Enveloping conformers have four atoms in one plane with the fifth carbon atom out of plane. Whereas the half-chair conformation has three carbons in the same plane and the remaining two alternate above and below the plane. The C–C–C bond angles of the non-planar conformation are between 102° and 106°. The envelope conformation is 0.5 kcal/mol more stable than the half-chair conformation (Pitzer & Donath, 1959). The envelope is also 5.0 kcal/mol more stable than the planar conformation (Dragojlovic, 2015; Kolossváry & Guida, 1993). In cyclopentane, the interconversion between different conformations occurs via pseudo-rotation. It may be recalled that conformational exchange in cyclohexane involves ring inversion. Envelope or half-chair preference depends on substituents. For example, methylcyclopentane prefers an envelope conformation. There are several intermediate conformers associated with pseudo-rotations. Figure 8.19 shows the envelope and half-chair conformations of cyclopentane with twist angles (Bucourt, 1974).