Baeyer strain theory

Baeyer strain theory or strain theory explains specific behaviour of chemical compounds in terms of bond angle strain.

It was proposed by Adolf von Baeyer in 1885 to account for the unusual chemical reactivity in ring opening reactions of cyclopropanes and cyclobutanes where this angle strain is relieved.

On ring strain he noted in 1885:

The four valences of the carbon atom act in the directions that connect the center of a sphere with the corners of a tetrahedron and that form an angle of 109 28’ with each other. The direction of the attraction can experience a deviation that will, however, cause an increase in strain correlating with the degree of this deviation.

— Armin de Meijere, Angew. Chem.

William Henry Perkin, Jr. was the first chemist to synthesize a cyclopropane ring as a doctoral student in the group of Baeyer. A few years earlier another student of Baeyer, Viktor Meyer, had doubted whether a three-membered ring could exist at all. For a long time Baeyer held the belief that even 6- and 7-membered rings were planar and subjected to ring strain.