Beginning Organic Chemistry (BOC)
8. Isomerism.
a. Conformational Isomers.
1. Cyclic Compounds.

Return to BOC Index. | Cyclohexane Conformations.

Ring Strain: Angle and Torsional Strain.

When bond angles are forced by non-bonding interactions to be different from the normal bond angle (109.5 for tetrahedral geometry), the bond strength decreases and thus the molecule energy increases. Such an increase in energy is termed angle strain.

Ring strain is a combination of angle strain and eclipsing (torsional strain) and both these contribute to the shapes and energies of cyclic molecules.

Cyclopropane: large angle and torsional strain.

Strain energy: 114 kJ/mol (per C atom: 38kJ/mol)

Cyclobutane: a balance between torsional and angle strain gives the molecule its non-planar shape.

Strain energy: 111 kJ/mol (per C atom: 28 kJ/mol)

Cyclopentane: a flat structure would have an angle strain and a large torsional strain, the molecule is bent (like an envelope) to relieve some of the torsional strain.

Strain energy: 26 kJ/mole (per C atom: 5 kJ/mol)

Cyclohexane: this ring is able to form two conformations with no angle strain. One, the so-called chair conformation also has an all-staggered structure. The illustration here is the chair conformation.

Strain energy (chair): 0 kJ/mol

More discussion of Cyclohexane conformations

Cycloheptane: this ring must balance angle and torsional strain.

strain energy: 26 kJ/mol (per C atom: 4 kJ/mol)

Cyclooctane: again, a balance of angle and torsional strain.

Strain energy: 41 kJ/mol (per C atom: 5 kJ/mol)

Date created: 2005 06 12.