Bayer Strain Theory – Easy Explanation for Pharm.D Students
Baeyer Strain Theory is an important concept in organic chemistry. It explains why some cycloalkanes (cyclic organic compounds) are stable, while others are less stable. The theory was introduced by Adolf von Baeyer in 1885. His idea was simple: the stability of a ring depends on how much its bond angles are strained.
What is “Strain” in a Ring?
In an open-chain alkane (like methane or ethane), the ideal bond angle is 109.5°. This angle allows carbon atoms to stay comfortable without stress on bonds.
But when carbon atoms join in a ring, they cannot always maintain this 109.5° angle. If the angle becomes smaller or bigger, the bond feels “strained”. This strain reduces stability.
Baeyer’s Main Idea
Baeyer said:
Smaller and very large rings are unstable because their bond angles deviate too much from 109.5°.
Therefore, according to his theory:
- 3-membered rings (cyclopropane) → high strain → very unstable
- 4-membered rings (cyclobutane) → strained → unstable
- 5-membered rings (cyclopentane) → almost ideal → quite stable
- 6-membered rings (cyclohexane) → most stable
- 7 or more membered rings → strain increases again
Bond Angle Comparison
Baeyer assumed all rings are flat (planar), so he calculated bond angles as follows:
- Cyclopropane → 60°
- Cyclobutane → 90°
- Cyclopentane → 108°
- Cyclohexane → 120°
The ideal angle is 109.5°, so rings close to this value are more stable.
Problems With Baeyer’s Theory
Although Baeyer’s theory was helpful, it had one major mistake:
He assumed all cycloalkanes are flat (planar), but they actually adopt 3D shapes to reduce strain.
For example:
- Cyclohexane is NOT flat — it forms “chair” and “boat” shapes.
- These 3D shapes help remove almost all angle strain.
Sachse-Mohr Theory (Correction to Baeyer’s Theory)
Sachse and Mohr later corrected Baeyer’s mistake. They said:
Cycloalkanes adopt non-planar (3D) conformations that remove strain.
Example:
- Cyclohexane forms a stable chair form with almost no strain.
- Cyclopentane forms an envelope shape to reduce strain.
Therefore, even if Baeyer predicted some rings to be unstable, many are actually stable due to 3D conformations.
Summary of Baeyer Strain Theory
- Explains stability of cycloalkanes based on bond angle strain.
- More deviation from 109.5° → more strain → less stability.
- Small rings (3, 4) are highly strained.
- Medium rings (5, 6) are most stable.
- Large rings (7+) again show strain.
- The theory assumes rings are flat (planar), which is not true.
Final Note
Even though Baeyer Strain Theory is not perfect, it is a very important concept for understanding the basics of ring structure and stability in organic chemistry.
Detailed Notes:
For PDF style full-color notes, open the complete study material below:
PRO VERSION:
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/BAYER STRAIN THEORY