Cyclopentane, eclipsing strain

Cyclopentane with a planar ring would have five pairs of eclipsed H s producing considerable eclipsing strain. This strain is reduced, at the expense of some increase in angle strain, when one CHj pushes out of the plane of the ring (Fig. 9-4). The puckering is not fixed but alternates around the ring. 

If cyclopentane were planar, it would have ten hydrogen-hydrogen interactions with a total energy cost of 40 kJ/mol. The measured total strain energy of 26 kJ/mol indicates that 14 kJ/mol of eclipsing strain in cyclopentane (35%) has been relieved by puckering. 

Solution Eclipsing strain has a valu e of approximately 1 Kcal/mole for two adjacent, eclipsed bonds. In planar cyclopentane 10 pairs of eclipsed hydrogens are found, thus 10 Kcal of eclipsing strain is present. 

As planar cyclopentane folds, the number of eclipsed hydrogens decreases, and thus eclipsing strain decreases. 

Therefore, as planar cyclopentane becomes puckered, a loss of 8 Kcal of eclipsing strain enables the puckered form to become more stable. 

Problem 4,10 How many H eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kj/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have Since the measured total Strain of cyclopentane is 26 kj/mol, how much of the torsional strain is relieved by puckering ... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

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