Cyclic Molecules, Configurational Assignment and Strain

Acyclic molecules can readily minimize strain simply by changing conformation. There is less freedom to make large changes of conformation in cyclic molecules and so the incidence of strain is much more 

Cyclic molecules are frequently strained. Three types of strain, (1) angle strain, (2) torsion strain and (3) steric strain, are encountered. 

Strain has been encountered previously, e.g. in Chapter 1, and it is appropriate to collate the three types since cyclic molecules that are strained will experience one or more of the types outlined below. 

Angle strain (formerly known as Baeyer strain) occurs when C-C-C single bond angles deviate significantly from the ideal value of 109°28. These deviations occur either as a compression, or an expansion, of this angle. 

Torsion strain (formerly known as Pitzer strain) is introduced when two substituents X and Y, bonded to adjacent carbons C(l) and C(2), are such that the dihedral angle X-C(l)-C(2)-Y is less than 60°. Torsion strain reaches a maximum when the above dihedral angle is 0°. 

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