Which of the Following Cycloalkanes Has the Least Ring Strain?

(A) Cyclopropane: its approximately 60° ring angles create severe angle strain; eclipsing also contributes.
(B) Cyclopentane: puckering reduces eclipsing, but residual torsional strain remains.
(C) Cyclohexane: the chair is essentially strain-free in the usual textbook comparison.
(D) Cycloheptane: its preferred conformations retain torsional and nonbonded interactions.

Question

Which of the following cycloalkanes has the least ring strain?(A) cyclopropane(B) cyclopentane(C) cyclohexane(D) cycloheptane

Answer (C) cyclohexane, in its lowest-energy chair conformation, has the least ring strain among these four choices. The chair allows carbon bond angles close to the tetrahedral value of 109.5° and staggered bonds around the ring, minimizing angle and torsional strain. (A) Cyclopropane: its approximately 60° ring angles create severe angle strain; eclipsing also contributes. (B) Cyclopentane: puckering reduces eclipsing, but residual torsional strain remains. (C) Cyclohexane: the chair is essentially strain-free in the usual textbook comparison. (D) Cycloheptane: its preferred conformations retain torsional and nonbonded interactions. A flat polygon drawing is not the actual geometry. In particular, cyclohexane is not forced to have the 120° angles of a planar regular hexagon.

This answer applies to the four named, unsubstituted rings in Georgia Tech Quiz 3-1, question 4, compared in their preferred conformations. It does not mean every cyclohexane conformation is unstrained: boat and twist-boat forms have higher energy. No numerical strain energies are inferred from a flat ring drawing.

These references support the concepts and methods used in the explanation above.