In Adolf Baeyer proposed a theory to explain the relative stability of the first few cycloalkanes. Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring. He also assumed that all cycloalkanes were planner and thus calculated the angles through which each of the valency bond was deflected from the normal direction in the formation of the various rings. This he called Angle Strain, which determined the stability of the ring. In cyclopropane, the three carbon atoms occupy the corners of an equilateral triangle. The angle strain for other cycloalkanes can be calculated in the same way.

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Illustrative is C60 where the carbon centres are pyramidalized. This distortion enhances the reactivity of this molecule. This rule is a consequence of angle strain. Examples[ edit ] In cycloalkanes, each carbon is bonded nonpolar covalently to two carbons and two hydrogen. The carbons have sp3 hybrization and should have ideal bond angles of Due to the limitations of cyclic structure, however, the ideal angle is only achieved in a six carbon ring — 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 Because of the smaller bond angles , the bonds have higher energy and adopt more p-character to reduce the energy of the bonds. 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.

Other cycloalkanes that do not have 3- and 4-membered rings can be strained. These include cyclophanes , Platonic hydrocarbons , pyramidal alkenes , and cyclic alkynes. For example, bicyclobutane , C4H6, is noted for being one of the most strained compounds that is isolatable on a large scale; its strain energy is estimated at Examples of such reactions are Ring opening metathesis polymerisation , photo-induced ring opening of cyclobutenes , and nucleophilic ring-opening of epoxides and aziridines.


Strain theory

See Article History Strain theory, in chemistry , a proposal made in by the German chemist Adolf von Baeyer that the stability of carbocyclic compounds i. The amount of deviation is the measure of the strain of the ring: the greater the strain, the less stable is the ring. Baeyer postulated that these rings are planar and concluded that strain exists in three- and four-membered rings and in rings of six or more atoms, the strain increasing with the size of the ring. Another German chemist, H. Sachse, in suggested that in rings of six or more atoms the strain can be relieved completely if the ring is not planar but puckered, as in the so-called chair and boat conformations of cyclohexane.


Ring strain



Baeyer strain theory | Ring Strain |




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