Baeyer’s Strain Theory and its limitations were proposed by Adolf von Baeyer in order to explain the relative stability of various alicyclic rings (aliphatic cyclic rings).

## Baeyer’s strain theory and its limitations

Let’s discuss postulates of Baeyer’s strain theory of cycloalkanes first of all, and its limitations later.

### Baeyer’s strain theory of cycloalkanes

When a carbon atom is bonded to four other atoms, its four valencies are directed towards the four corners of a regular tetrahedron, and therefore, any two of its bonds form an angle of 109^{0} 28’ the ideal tetrahedral angle at the center of the tetrahedron.

Baeyer’s Strain Theory is based on the following assumptions or postulates.

- An alicyclic ring having all carbon atoms lie in the same plane. Hence, one pair of bonds can not assume tetrahedral angle i.e., 109
^{0}28’. - Any deviation of carbon-carbon bond angle of the ring form the ideal tetrahedral angle develops a strain in the ring. Such strain is called angle strain which makes the ring unstable.
- Greater the angle strain in the ring, the more unstable is the ring.
- The more stable a ring, more easily it can be synthesized.

The angle strain developed in the ring due to deviation forms regular tetrahedron angle can be expressed in terms of valence angle deviation (d).

**d = (109 ^{0} 28’ – α)/2** where

**α**is the bond angle in the cycloalkane.

For example: Let’s consider cyclopropane ring.

In this ring, the three carbon atoms are situated at the corners of an equilateral triangle. Therefore, the bond angle (α) is 60^{0}. This indicates that during the formation of cyclopropane, the regular tetrahedron angle 109^{0} 28’ is compressed to 60^{0}.

Now the valence angle deviation (d) =**(109 ^{0} 28’ – α)/2** = 24

^{0}44’

The angle of strain in cyclopropane is 24^{0}44’.Similarly, the angle strain calculated in various ring sizes of cycloalkane can be calculated.

From this table, the following conclusion can be drawn.

- The ‘+’ ve and ‘-’ ve values of ‘d’ indicates that whether the bond angle is less than and greater than the regular tetrahedron bond angle unstable.
- Cyclobutane has less value of angle of strain as compared to cyclopropane.
- Cyclopentane has least value of angle of strain. Hence, it must be stable.
- Cyclohexane has slightly higher strain than cyclopentane.
- Cyclopentane and other cycloalkanes have higher values of angle strain and are highly stable
- At last, rings smaller or greater than cyclopentane or cyclohexane possess higher angle strain and hence these are unstable.

### Baeyer’s strain theory limitations

According to Baeyer’s Strain Theory of cycloalkanes, a ring system greater than cyclopentane and cyclohexane should be unstable. But experimentally, it is found that the heat of combustion of cycloheptane is about 158.3 kcal/ mole which is very close to that of cyclopentane or cyclohexane.

Thus, Baeyer’s Strain Theory explains the relative stabilities of ring size up to 5 and 6 in a better way. It can’t explain the exact stability of the higher ring system. This is the failure of Baeyer’s strain theory.

## Baeyer’s strain theory video:

## References:

- Morrison, R. T., & Boyd, R. N.,
*Organic chemistry*, Allyn and Bacon, Inc. 1987 - Michael B. Smith,
*March’s Advanced Organic Chemistry*, (7th Edition), John Wiley and Sons, Inc., 2013. - J. March,
*Advanced Organic Chemistry*, (4th Edition), John Wiley and Sons, 1992