Question

is open chain more stable or closed chin more stable ?

Answer 1

The stability of open and closed structure depends on the type and magnitude of strain introduced in a molecule. The different types of strain and their effect of molecular structure are:
1. Angle Strain: This strain arises in molecules when the actual bond angles in the molecule deviates from the ideal bond angles of their geometry. In alkanes, carbon is sp³ hybridized and H-C-H bond angle is 109.5°. In 3-4 membered rings, to form the closed ring structure the bond angle is reduced to 60° and 88° respectively. The strain caused by the distorted angle destabilizes the structure and for hydrocarbons containing 3 to 4 carbon atoms an open chain is more stable.
2. Torisonal strain arises due to repulsion between electrons in between groups that overlap each other during rotation around a sigma bond. Torisonal strain is seen in cyclobutane and eclipsed conformations.
3. Steric or VanderWaal’s strain is observed when the distance between non-bonding atoms is less than their Van der Waal’s radii. Steric hindrance is seen in 7-14 membered rings and between methyl groups in axial positions of methylcyclohexane.

So, depending upon the number of carbon atoms and type of substituents, the stability can vary between open and closed chain molecules.