Baeyer’s Strain Theory -Stabilities of Cycloalkanes: Key Concepts

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Proposed by Adolf von Baeyer in 1885, Baeyer's Strain Theory is one of the earliest theories explaining the stability of cycloalkanes.

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).

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.

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.

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.

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