ring strain

Meaning: Ring strain is the extra potential energy of a cyclic molecule compared with a strain-free open-chain analog, so more strain means less stability. Sources are angle strain (bond angles forced away from the ideal 109.5° of sp³ carbon), torsional strain (eclipsed bonds), and steric strain (crowding, such as 1,3-diaxial interactions). Cyclopropane is the most strained: it has 60° angles and fully eclipsed bonds. Cyclobutane and cyclopentane pucker to relieve some torsional strain. The cyclohexane chair has near-tetrahedral angles and staggered bonds, so it is essentially strain-free.

Equation: Strain energy = ΔH°comb(ring) − n × ΔH°comb(per CH₂ of an unstrained alkane), where n is the number of CH₂ groups and ΔH°comb is the heat of combustion in J/mol (usually quoted in kJ/mol).

On the MCAT: Expect questions that rank cycloalkanes by stability or by heat of combustion per CH₂, explain why small rings open readily, or predict why a chair with a bulky group equatorial is favored.

Common confusion: Students assume rings are flat, so large rings should have severe angle strain. Rings larger than cyclopropane can pucker, so cyclohexane is not strained. Compare heats of combustion per CH₂, because the total rises with ring size simply from having more carbons.