Proposed by Adolf von Baeyer in 1885, Baeyer's Strain Theory is one of the earliest theories explaining the stability of cycloalkanes.
Key Concepts of Baeyer’s Strain Theory:
Angle Strain:
The theory is based on the idea that carbon atoms in a cycloalkane are sp³ hybridized, and therefore, they should ideally have bond angles of 109.5° (the tetrahedral angle).
Deviation from Ideal Angle:
In cyclic structures, the internal bond angles often deviate from 109.5°, leading to strain in the molecule, known as angle strain or Baeyer strain.
Explanation of Stability:
Cyclopropane: In cyclopropane, the bond angles are 60°, far from the ideal 109.5°, causing significant angle strain and making it highly unstable.
Cyclobutane: In cyclobutane, the bond angles are 90°, also leading to considerable strain but less than in cyclopropane.
Cyclopentane: The bond angles in cyclopentane are 108°, close to 109.5°, resulting in minimal strain.
Cyclohexane: Baeyer originally predicted cyclohexane to be strained because he assumed it to be planar with 120° bond angles. However, in reality, cyclohexane adopts non-planar conformations to relieve strain.
Limitations of Baeyer’s Strain Theory (Shortened)
Planarity Assumption: Assumes all cycloalkanes are planar, which is inaccurate; cyclohexane, for example, adopts a non-planar chair conformation.
Cyclohexane Anomaly: Predicts cyclohexane to be highly strained with 120° angles, but cyclohexane is actually strain-free in its chair form.
Neglect of Conformational Flexibility: Fails to account for the conformational flexibility of larger rings, which can adopt non-planar structures to reduce strain.
Inadequacy for Larger Rings: Doesn't explain why cycloalkanes with more than six carbon atoms can be strain-free despite larger deviations from the ideal bond angle.