strain

Meaning: Strain is the extra energy a molecule carries when its geometry is forced away from the lowest-energy arrangement, so more strain means lower stability. Sources: Angle strain: bond angles deviate from the ideal, 109.5° for sp³ carbon (cyclopropane has 60°). Torsional strain: bonds on adjacent atoms are eclipsed rather than staggered. Steric strain: nonbonded groups are pushed close enough for van der Waals repulsion, as in gauche butane or 1,3-diaxial interactions.

Equation: Ring strain = |ΔH°comb(measured)| − n × |ΔH°comb(per CH₂, strain-free)|, where ΔH°comb is the heat of combustion in kJ/mol, n is the number of CH₂ groups (unitless), and the strain-free value is about 658.6 kJ/mol per CH₂. Use magnitudes because combustion enthalpies are negative.

On the MCAT: Expect Newman projection rankings (anti is lower in energy than gauche, which is lower than eclipsed) and chair versus boat comparisons: the boat has eclipsing and flagpole crowding, and the twist-boat is somewhat lower. Larger substituents favor equatorial positions. Given heats of combustion per CH₂, the larger value means more strain.

Common confusion: Students treat rings as flat polygons and predict 120° for cyclohexane. Real cyclohexane is a chair with angles near 109.5° and staggered bonds, so it has little angle or torsional strain. Cyclobutane and cyclopentane also pucker, easing torsional strain at some cost in angle strain.