text: In cycloalkanes, each carbon is bonded nonpolar covalently to two carbons and two hydrogen. The carbons have sp hybridization and should have ideal bond angles of 109.5°. Due to the limitations of cyclic structure, however, the ideal angle is only achieved in a six carbon ring &mdash; cyclohexane in chair conformation.  For other cycloalkanes, the bond angles deviate from ideal.
  Molecules with a high amount of ring strain consist of three, four, and some five-membered rings, including: cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, [1,1,1]propellanes, [2,2,2]propellanes, epoxides, aziridines, cyclopentenes, and norbornenes. These molecules have bond angles between ring atoms which are more acute than the optimal tetrahedral (109.5°) and trigonal planar (120°) bond angles required by their respective sp and sp bonds. Because of the smaller bond angles, the bonds have higher energy and adopt more p-character to reduce the energy of the bonds. In addition, the ring structures of cyclopropanes/enes and cyclclobutanes/enes offer very little conformational flexibility. Thus, the substituents of ring atoms exist in an eclipsed conformation in cyclopropanes and between gauche and eclipsed in cyclobutanes, contributing to higher ring strain energy in the form of van der Waals repulsion.
  monocycles
  *cyclopropane (29 kcal/mol), CH &mdash;  the C-C-C bond angles are 60° whereas tetrahedral 109.5° bond angles are expected. The intense angle strain leads to nonlinear orbital overlap of its sp orbitals. Because of the bond's instability, cyclopropane is more reactive than other alkanes. Since any three points make a plane and cyclopropane has only three carbons, cyclopropane is planar. The H-C-H bond angle is 115° whereas 106° is expected as in the CH groups of propane.
  *cyclobutane (26.3 kcal/mol), CH &mdash; if cyclobutane were completely square planar, its bond angles would be 90° whereas tetrahedral 109.5° bond angles are expected. However, the actual C-C-C bond angle is 88° because it has a slightly folded form to relieve some torsional strain at the expense of slightly more angle strain. The high strain energy of cyclobutane is primarily from angle strain.
  *cyclopentane (7.4 kcal/mol), CH &mdash; if it was a completely regular planar pentagon its bond angles would be 108°, but tetrahedral 109.5° bond angles are expected. However, it has an unfixed puckered shape that undulates up and down. 
  *cyclohexane (1.3 kcal/mol), CH &mdash; Although the chair conformation is able to achieve ideal angles, the unstable half-chair conformation has angle strain in the C-C-C angles which range from 109.86° to 119.07°.
  Bicyclics
  *[[Bicyclobutane|bicyclo[1.1.0]butane]]  (66.3 kcal/mol), 
  *[[housane|bicyclo[1.2.0]pentane]] (54.7 kcal/mol), 
  *[[bicyclo[1.3.0]hexane]] (26 kcal/mol), 
  *norbornane (16.6 kcal/mol), 
  Ring strain can be considerably higher in bicyclic systems.  For example, bicyclobutane, CH, is noted for being one of the most strained compounds that is isolatable on a large scale; its strain energy is estimated at 63.9 kcal mol (267 kJ mol).
  Cyclopropane has a lesser amount of ring strain since it has the least amount of unsaturation; as a result, increasing the amount of unsaturation leads to greater ring strain. For example, cyclopropene has a greater amount of ring strain than cyclopropane because it has more unsaturation.