Stretching, Bending, and Twisting the Rules of Bonding

The rarity and reactivity of three- and four-membered rings led Adolph von Baeyer to enunciate his theory of ring strain in 1885. Alkanes normally favor tetrahedral geometry with bond angles close to 109.5°. fii 

Julius Bredt (1855-1937) observed that molecules containing twisted carbon-carbon double bonds are prohibited because alkenes favor planar double bonds with bond angles near 120°. Alkynes, such as acetylene (HC=CH), favor linear geometry (180°) about their triply bonded carbons. The benzene ring was shown in the 1930s to be flat. 

Some representative strained organic molecules in dijferent classes A) simple, spiro, and fused cycloalkanes B) distorted and twisted alkenes C) strained polycyclic compounds 

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