Stability of cycloalkanes

Q.Discuss the stability of cycloalkanes and various theories.

The stability of cycloalkanes is explained by various scientists by their respective theories.

Some of the most prominent theories that can explain the stability of cycloalkanes are

  • Bayer’s strain theory
  • Coulson and Moffitt’s modification
  • Sasche Mohr’s theory of strain less ring

Bayer’s strain theory

It is the first theory that explained the difference in the relative stability of cycloalkanes.

The main postulates of this theory are:-

  • 1. In cycloalkanes each carbon atom is SP3 hybridized and bonded to the other four carbon atoms. The angle between any pair of the adjacent bond should be tetrahedral (109.5°)
  • 2. Due to cycle Nature, cycloalkanes are planar in nature, as a result, the bond angles between adjacent carbon atoms in the ring are no longer equal to the normal tetrahedral angle that is 109.5°.
  • The angle deviation varies with the size of the ring.
  • 3. Any distortion or deviation from the normal tetrahedral angle causes a strain in the ring and produces instability to the resultant molecule. This strain is known as angle strain.
  • 4. The greater the deviation from the normal angle greater will be angle strain and hence the greater the reactivity of cycloalkanes.
  • 5. The more the stability of the ring system the more easily it is formed or in other words the ease of formation of cycloalkanes is directly related to the stability of the ring.
  • The stability of the ring is indirectly proportional to angle strain.

Evidence in favor of Bayer’s strain theory

  • 1.The statement “as the angle strain decreases the relative stability and ease of formation increases” can be explained based on heat combustion per methylene group in each cycloalkane.
  • 2. It is observed that cyclopropane and cyclobutane cause more heat of combustion per CH2 group than corresponding straight-chain alkanes. Therefore they are less stable and tend to undergo additional reactions through ring fission.
Note- smaller the value of heat of combustion per CH2 group more stable is the ring as in the case of cyclopentane and cyclohexane which are having heat combustion of 664 kilojoules per mole and 658.5 KJ /mole respectively. Show they are stable and do not undergo ring fission or addition reactions. 

Limitations of Bayer’s strain theory

  • 1. In alkenes angle strain is very large, therefore, it should be formed with difficulty. But, in alkenes, the double bond is formed easily by dehydration of alcohols korma dehydrogenation of alkyl halides. So Bayer’s strain theory could not explain the formation of a carbon-carbon double bond. 
  • 2. According to this theory, due to large angle strain, cyclic rings of the higher carbon atoms should not exist but these rings are found to be more stable than cyclopentane rings. Thus, Bayer’s strain theory fans with higher rings containing 6 or more carbon atoms and applicable only to three, four, and five-membered rings.

Coulson and Moffitt’s modification

Cyclopropane has a triangular planner structure with an interval bond angle between carbon-carbon bonds are 60°. The SP3 hybrid orbitals of carbon atoms in cyclopropane cannot undergo complete overlap with each other because it requires a significant deviation of the tetrahedral angle of 109°- 5°. this is the reason, it shows a significant amount of ring strain in cyclopropane. In addition to angular strain cyclopropane also suffers from a decent torsional strain. This torsional strain is due to the coplanar arrangement of the carbon atoms whereas leading to the eclipsed arrangement of C-H bonds.

In cyclopropane re hybridization of course and bonding between the carbon center exist in terms of bent bonds in carbon-carbon bonds are bent outwards so that inter orbital angle is 104 degrees which consequently reduces the level of bond strength.

So it behaves something like between Sigma and Pi bonding. These bonds look like banana bonds. So the C-C bonds have more p-character than normal whereas C-H bonds have S-character. Thus we can see bond strain weakens the C-C bonds of the cyclopropane ring. This is why cyclopropane is clearly more reactive than alkanes or other higher ring systems.

Sasche mohr’s theory of strain less ring

According to Sasche mohr’s theory of strainless rings, the carbon atoms of five-membered and smaller rings lie in a single plane but in six-membered and larger rings, the carbon atoms lie in a different plane. These rings do not lie in a single plane. They are puckered. In these puckered rings the normal tetrahedral angle remains unaltered and therefore angle strain is negligible hence these rings are called stainless rings.

Sasche mohr proposes the existence of two types of multiplanar or puckered stainless rings known as boat and chair forms of cyclohexane.

Both these forms are free from angle strain and can be interconverted just by rotation around a single bond.

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