Baeyer's Strain Theory l Study Pharma l

Baeyer strain theory:
In 1885 Baeyer proposed the Strain theory that states the stability of cycloalkanes are depends on bond angle deviation from tetrahedral angle 109 degree 28 minutes or 109.5 degree.
According to Baeyer:
1. Cycloalkanes are closer to 109.5 degree, it is more stable.
2. Cycloalkanes are far to 109.5 degree, it is less stable.
This theory is known as" Baeyer's Strain Theory".

What is the importance of 109.5° in Baeyer strain theory?
Reasons are:
1. All the cyclic hydrocarbons or cycloalkanes are single bonded.
2. Single bonded molecules are sp3 hybridized in nature.
3. sp3 hybridized molecules are tetrahedral and bond angle is 109.5 degree.

By all these reasons, Baeyer follow according to angle strain.

Angle strain:
Angle strain is the strain in which increase in the energy with increase in the deviation from 109.5 degree.

Assumption:
1. All cycloalkanes are planar.
2. Positive angle strain cycloalkanes are exists if bond angle either far or closer to 109.5 degree.
3. Negative angle strain cycloalkanes are not exists because of high angle strain.

Stability and reactivity:

1. According to Baeyer: Order of Stability is:

cyclopentane > cyclohexane > cyclobutane > cyclopropane

2. According to Baeyer: Order of Reactivity is:

                                         cyclopropane > cyclobutane > cyclohexane > cyclopentane

Limitation:
1. Unable to explain the effect of angle strain on higher members of cycloalkanes.
2. Unable to explain why negative angle strain compounds not exists.
3. According to Baeyer, Stability order

cyclopentane > cyclohexane > cyclobutane > cyclopropane

but in reality, it is opposite.

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