Which of the Following Cycloalkanes Has the Most Ring Strain? Cyclopropane

Answer

A. Cyclopropane has the most ring strain among the four choices.

Ring strain is the total energetic penalty from angle strain, torsional strain, and, where relevant, steric interactions. Cyclopropane is forced into a three-membered ring with C–C–C angles near 60°, far from the roughly 109.5° angle preferred by an $sp^3$ carbon. Its neighboring C–H bonds are also eclipsed, so it has substantial torsional strain as well as angle strain.

OpenStax reports total strain energies of about:

• Cyclopropane: 115 kJ/mol
• Cyclobutane: 110 kJ/mol
• Cyclopentane: 26 kJ/mol
• Cyclohexane: essentially strain-free in its chair conformation

Cyclobutane is therefore a close second, not an unstrained ring. It puckers slightly to reduce eclipsing interactions, but that conformational change cannot eliminate its strain. Cyclopentane puckers more effectively, and chair cyclohexane avoids nearly all angle and torsional strain.

The correct answer is A, cyclopropane.