Question

Which is more stable and why?

Answer 1

Hint: Cyclohexane is a saturated compound while benzene is an unsaturated compound. Cyclohexane exists mostly in chair form and benzene is an aromatic compound.

Complete answer:
Cyclohexane is more stable compared to benzene. Although everyone would be thinking its benzene which is more stable but it’s not the case. The reason for thinking benzene to be more stable is due to the following reason:
- benzene is less reactive
- It is an aromatic compound.
- it obeys Huckel rule
- It has many resonance structures.
But it’s not the case. If we learn about Heat of formation / combustion graph we would get a clear idea. What is standard heat of formation? Standard Heat of formation is also known as standard enthalpy of formation. Standard enthalpy of formation is the change in enthalpy during formation of one mole of the substance. Lower the heat of formation, more stable is the compound.

From the above graph, it's very clear that cyclohexane is more stable compared to benzene. Because cyclohexane has 50 kcals/mol less energy compared to benzene. As we know lower the heat of formation more stable the compound.

Additional information:
Comparison of benzene and cyclohexane.

BENZENE	CYCLOHEXANE
Molecular formula- $$C_6{H}_6$$	Molecular formula- $$C_6{H}_{12}$$
Hybridization- $$s{p}^2$$	Hybridization- $$s{p}^3$$
Aromatic structure	Not aromatic, but cyclic structure
Presence of α and β bonds	Presence of only α bonds.

Note: - Due to benzene’s aromaticity, resonance structure and less reactive nature it was thought to be very stable that cyclohexane.
- The stability of the compounds is given in the increasing order:
Cyclohexane>Cyclohexene>Benzene>1,3-Cyclohexadiene