Angle strain

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The presence of angle strain in a molecule indicates that in a specific chemical conformation there exist bond angles that deviate from the ideal bond angles required to achieve maximum bond strength. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond.

Angle strain typically affects cyclic molecules because non-cyclic molecules will thermodynamically conform to the most favorable stable state. Angle strain in cyclic molecules is also called Baeyer strain.

As an example of angle strain, take cyclic alkanes, in which each carbon is equally bonded two carbons and two hydrogens. Since each of the four bonds of a carbon is equivalent, it is sp³ hybridized and ideally should have cos⁻¹(−1/3) ≈ 109.5° (the angle that maximizes the distance between atoms) bond angles. Due to the limitations of cyclic structure, however, the ideal angle is only achieved in a six carbon ring - cyclohexane, and only when it is in its chair conformation. For other cyclic alkanes, the bond angles deviate from ideal. In cyclopropanes (3 carbons) and cyclobutanes (4 carbons) the C-C bonds will be 60° and ~90° respectively.

A quantitative measure for angle strain is strain energy. Having higher angle strain makes a molecule more unstable and reactive.

Angle strain and torsional strain combine to create ring strain that affects cyclic molecules.

Examples of molecules with angle strain cycloalkanes, cyclophanes, platonic hydrocarbons and pyramidal alkenes.

Examples

• cyclopropane, C₃H₆
• ethylene oxide CH₂OCH₂
• chemiluminescence of CSPD