Baeyer Strain Theory

 Semester-3   Unit-2    Fundamentals of Organic Chemistry

Baeyer Strain Theory

Adolf von Baeyer (a Nobel prize winner) proposed a theory in 1885 to explain the relative stability of the first few cycloalkane.


The theory based on following facts:

1. Cyclo-alkanes are saturated compounds. So, all the carbons should have normal tetrahedral angle of 109 o281 (or 10951).

2. Any deviation of bond angles from the normal tetrahedral value would, impose a condition of internal strain on the ring called bond angle strain or Beayer strain

3. Higher value of angle strain of cycloalkane shows instability. As instability increases reactivity also increase.

4. If all the rings in cycloalkanes assumed to be planar, the bond angle strain for each ring can be calculated below .

5. According the above table smaller ring compounds cyclo propane and cyclobutane suffer large distortion and believed to be the cause of grater instability and enhanced reactivity.

6. Higher angle strain present in cyclopropane (therefore least stable & more prone to undergo ring opening reactions). C
yclopentane is more stable due to low angle strain value.

Stability α 1/Angle strain

7. Bayer concludes that rings smaller or larger than cyclopentane are unstable.

According to Baeyer, the relative order of stability to some common cycloalkanes is as under:

Cyclopentane > Cyclohexane > Cyclobutane > Cyclopropane

Actual observed order of stability for these cycloalkanes is

Cyclohexane > Cyclopentane > Cyclobutane > Cyclopropane

LIMITATIONS OF BAEYER’S STRAIN THEORY

·        ✔   Baeyer was not able to explain the effect of angle strain in larger ring systems

·         ✔  According to Baeyer Cyclopentane should be much stable than cyclohexane but practically it is reversed.

·          ✔   Larger ring systems are not possible according to Baeyer as they have negative strain but they exist and much stable.

·          ✔   Larger ring systems are not planar but wrinkled (puckered) to eliminate angle strain.




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