Formation of cyclic compounds by interamolecular cyclisation depends on the following factors:
1. Thermal stability, which is determind by ring strain. The decreasing order of thermal stability of cyclic ring is: 6 gt 7, 5 gt 8, 9gtgt4gt3.
2. Probability of bringing together the terminal C atoms (the entropy factor) decreases with increase in chain length. The decreasing order of entropy (probability) of ring closure is:
3 gt 4 gt 5 gt 6 gt 7 gt 8 gt 9.
Ease of synthesis (net effect of two facrors):
5 gt 3, 6 gt 4, 7, 8, 9.
a. Therefore, in the synthesis of cyclopropane, the entropy factor is more favourable than thermal stabililty.
b. In the synthesis of ring with more than six C atoms, thermal or ring stability factor is outweighed by the more important and unfavourable entropy factor, hence it is difficult to synthesise. When the concention of reactants is vry low (i.e., dilute solution), the collision between chains decreases which minimises the intermolecular side reactions of chain lengthening and cyclisation leading to the formation of rings with more than six C atoms. For example,
