Ring strain

Ring strain is the increase in a molecule’s energy when a ring’s geometry or electronic structure constrains its bonds and atoms. It can affect stability, reactivity, and preferred molecular shapes.

Angle strain: The energy increase caused when bond angles depart from their preferred values. Small rings force bond angles away from the geometry preferred by their bonded atoms.
Cyclopropane: A three-carbon cyclic alkane with unusually compressed bond angles and bent carbon–carbon bonds. Its compressed geometry makes it a classic example of severe angle strain.
Strain-release reaction: A reaction driven in part by converting a highly strained molecule into a less strained product. Opening or rearranging a strained ring can release energy and promote bond formation.
Conformational strain: An energy increase caused by a molecule adopting a less favorable three-dimensional conformation. It can occur in open-chain molecules, whereas ring strain specifically involves constraints from a ring.
Torsional strain: The energy increase caused by eclipsing or otherwise unfavorable orientations of bonds on adjacent atoms. Ring closure can prevent bonds from adopting low-energy staggered orientations.
Cyclobutane: A four-carbon cyclic alkane that adopts a puckered shape to reduce eclipsing interactions. Its conformation shows how puckering relieves torsional strain without eliminating angle strain.
Ring-opening polymerization: A polymerization in which cyclic monomers open and join to form a polymer chain. Strained cyclic monomers can gain stability as their rings open during polymer growth.
Steric hindrance: Reduced reactivity or increased energy caused by spatial crowding between atoms or groups. Steric crowding can contribute to ring strain, but the terms describe different features.
Transannular strain: Repulsion or crowding between atoms or bonds positioned across the interior of a ring. In medium-sized rings, atoms across the ring can approach and interact unfavorably.
Cyclopentane: A five-carbon cyclic alkane that adopts nonplanar conformations to reduce torsional strain. Its envelope and twist forms illustrate conformational relief in a relatively low-strain ring.

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