How many H H eclipsing interactions would be present if cyclopentane were planar

Question

How many H 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? Two conformations of cis-1, 3-dimethylcyclobutane are shown. What is the difference between them, and which do you think is likely to be more stable? Draw two different chair conformations of cyclohexanol (hydroxycyclohexane), showing all hydrogen atoms. Identify each position as axial or equatorial. Draw two different chair conformations of trans-1, 4-dimethylcyclohexane, and label all positions as axial or equatorial.

Explanation / Answer

4.10 A planar structure for cyclopentane has ten H---H eclipsing interactions.

1 eclipsing interaction has 4 kJ/mol hence, 10 eclipsing interaction will exert 4 x 10 kJ/mol = 40 kJ/mol for a planar molecule.

The strain relieved on puckering = 40 - 26 = 14 kJ/mol.

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