1. Ring strain has two sources: bent bond angles and eclipsed bonds.
Angle strain grows as bond angles depart from the ideal 109.5 degrees, and torsional strain grows when C-H bonds eclipse.
2. Cyclopropane is the most strained because its angles are forced to 60 degrees.
Its three-membered ring squeezes every C-C-C angle to 60 degrees and eclipses all C-H bonds, making it reactive.
3. Strain drops sharply as the ring grows from four to six carbons.
Cyclobutane puckers to ease eclipsing, cyclopentane adopts a slight envelope, and cyclohexane reaches near-perfect tetrahedral geometry.
4. Cyclohexane is essentially strain-free because the chair gives ideal, staggered geometry.
The puckered chair restores 111-degree angles and staggers every neighboring C-H bond, erasing both angle and torsional strain.
5. A ring blocks rotation, so a disubstituted cycloalkane splits into cis and trans diastereomers.
Cis places both substituents on the same face and trans on opposite faces, giving two different compounds with different stabilities.
6. Chair stability decides which isomer wins: the one that puts both groups equatorial.
For trans-1,4-dimethylcyclohexane both methyls can sit equatorial, so it is more stable than the cis isomer.
Summary
Angle strain bends bond angles from 109.5° · torsional strain eclipses C-H bonds · cyclopropane and cyclobutane are strained and reactive · cyclohexane chair is strain-free · rings block rotation · cis = same face, trans = opposite faces · the diequatorial chair is most stable.
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Cyclopropane, because its rigid three-membered ring forces every C-C-C angle to 60°, far from the ideal 109.5°, and eclipses all C-H bonds.
The puckered chair restores near-tetrahedral (~111°) bond angles and staggers all adjacent C-H bonds, so it has essentially no angle or torsional strain.
The ring prevents bond rotation, locking substituents on a fixed face; cis has both groups on the same face and trans on opposite faces, so they are non-interconverting diastereomers.
Only the trans isomer can place both methyl groups equatorial on the chair, avoiding 1,3-diaxial strain; the cis isomer must put one methyl axial.
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