In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that deviate from their normal values as a result of being part of a ring. This type of strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealized value of approximately 109°. Because of their high strain, the heat of combustion for these small rings is elevated. Ring strain results from a combination of angle strain, conformational strain or Pitzer strain (torsional eclipsing interactions), and transannular strain, also known as van der Waals strain or Prelog strain. The simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane. Ring strain energy can be attributed to the energy required for the distortion of bond and bond angles in order to close a ring. The avoidance or reduction of ring strain can help direct or accelerate chemical reactions.
History Ring strain theory was first developed by German chemist Adolf von Baeyer in 1890. Previously, the only types of strain believed to exist were torsional and steric; however, Baeyer's theory became based on the interactions between the two strains. Baeyer's theory was based on the assumption that the rings in cyclic compounds were flat, and the simple geometry required for atoms in a planar structure. Around the same time, Hermann Sachse postulated that rings were not flat, and potentially existed in other folded or twisted conformations. Ernst Mohr later combined the two theories to explain the stability of six-membered rings and their frequency in nature, as well as the energy levels of other ring structures.
Angle strain (Baeyer strain)
Alkanes In alkanes, optimum overlap of atomic orbitals is achieved at 109.5°, the mathematically ideal angle for tetrahedral geometry. The most common cyclic compounds have five or six carbons in their ring. Adolf von Baeyer received a Nobel Prize in 1905 for the discovery of the Baeyer strain theory, which was an explanation of the relative stabilities of cyclic molecules in 1885. Angle strain occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation. Angle strain typically affects cyclic molecules, which lack the flexibility of acyclic molecules. Angle strain destabilizes a molecule, as manifested in higher reactivity and elevated heat of combustion. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond. A quantitative measure for angle strain is strain energy. Angle strain and torsional strain combine to create ring strain that affects cyclic molecules.
C n H 2 n + 3 n 2 O 2 ⟶ n CO 2 + n H 2 O − Δ H combustion {\displaystyle {\ce {C}}_{n}{\ce {H}}_{2n}+{\tfrac {3n}{2}}{\ce {O2}}\longrightarrow n{\ce {CO2}}+n{\ce {H2O}}-\Delta H_{\text{combustion}}}
Normalized energies that allow comparison of ring strains are obtained by measuring per methylene group (CH2) of the molar heat of combustion in the cycloalkanes.
ΔHcombustion per CH2 − 658.6 kJ = strain per CH2 The value 658.6 kJ per mole is obtained from an unstrained long-chain alkane.
Cycloalkanes generally have less ring strain than cycloalkenes, which is seen when comparing cyclopropane and cyclopropene.
Angle strain in alkenes
Cyclic alkenes are subject to strain resulting from distortion of the sp2-hybridized carbon centers. Illustrative is C60 where the carbon centres are pyramidalized. This distortion enhances the reactivity of this molecule. Angle strain also is the basis of Bredt's rule which dictates that bridgehead carbon centers are not incorporated in alkenes because the resulting alkene would be subject to extreme angle strain.
Small trans-cycloalkenes have so much ring strain they cannot exist for extended periods of time. For instance, the smallest trans-cycloalkane that has been isolated is trans-cyclooctene. Trans-cycloheptene has been detected via spectrophotometry for minute time periods, and trans-cyclohexene is thought to be an intermediate in some reactions. No smaller trans-cycloalkenes are known. On the contrary, while small cis-cycloalkenes do have ring strain, they have much less ring strain than small trans-cycloalkenes. In general, the increased levels of unsaturation in alkenes leads to higher ring strain. Increasing unsaturation leads to greater ring strain in cyclopropene. Therefore, cyclopropene is an alkene that has the most ring strain between the two mentioned. The differing hybridizations and geometries between cyclopropene and cyclopropane contribute to the increased ring strain. Cyclopropene also has an increased angle strain, which also contributes to the greater ring strain. However, this trend does not always work for every alkane and alkene.
Torsional strain (Pitzer strain)
In some molecules, torsional strain can contribute to ring strain in addition to angle strain. One example of such a molecule is cyclopropane. Cyclopropane's carbon-carbon bonds form angles of 60°, far from the preferred angle of 109.5° angle in alkanes, so angle strain contributes most to cyclopropane's ring strain. However, as shown in the Newman projection of the molecule, the hydrogen atoms are eclipsed, causing some torsional strain as well.
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