Advanced Topic 758 of 1102 Organic Chemistry
Ring Strain
Instability in small rings due to angle and torsional strain.
In-depth explanation of Ring Strain
Ring strain is the extra energy a cyclic molecule has compared with an unstrained reference, arising from angle strain, torsional strain and steric strain. Angle strain occurs when the bond angles deviate from the ideal tetrahedral 109.5 degrees, which is severe in small rings. Torsional strain comes from eclipsing bonds on adjacent atoms, and steric strain from crowding between non-bonded groups. Cyclopropane has the most strain of common rings, about 115 kJ/mol, and this energy makes it considerably more reactive and prone to ring opening than cyclohexane.
Going deeper: Ring Strain
Key ideas: Total ring strain equals angle strain plus torsional strain plus steric strain, estimated from heats of combustion per CH2 relative to 658.6 kJ/mol for strain-free alkanes. Cyclopropane has about 115 kJ/mol total strain, cyclobutane about 110, cyclopentane about 26, cyclohexane 0. Baeyer's theory assumed planar rings and fails for rings above five.
Common mistakes and connections: Mistakes include applying Baeyer's angle strain to cyclohexane and predicting high strain, and assuming cyclopentane is perfectly planar. Another is forgetting that cyclopropane has bent (banana) bonds which make it behave a bit like an alkene. Links to conformations, ring-opening reactions and epoxides.
Visualization of Ring Strain
Ring strain peaks for cyclopropane and cyclobutane (angle plus torsional strain) and vanishes at cyclohexane. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Cyclobutane puckers out of planarity to relieve some of its torsional strain.
Frequently asked questions about Ring Strain
What is Ring Strain?
Instability in small rings due to angle and torsional strain.
Is Ring Strain a beginner, intermediate or advanced topic?
Ring Strain is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Ring Strain?
Study these earlier topics in Organic Chemistry first: Conformations, Newman Projections, Cyclohexane Chair Conformation.
What comes after Ring Strain?
The next topics in Organic Chemistry are Inductive Effect and Resonance Effect.
Can I practise Ring Strain with quizzes?
Yes. The interactive lesson includes a self-check question on Ring Strain, and the Organic Chemistry quiz draws on every topic in this area.
Where Ring Strain fits in Organic Chemistry
Ring Strain is topic 30 of 148 in Organic Chemistry.