Question

Question: Compounds A and B are both hydrogenated to methylcyclohexane. Which compound has the larger heat of hydrogenation? Which compound is more stable?

Short answer

Answer

Compound B has larger heat of hydrogenation.

Compound A is more stable.

Step-by-step solution

Significance of heat of hydrogenation

Heat of hydrogenation is used to measure the relative stability of compounds. Greater the release of the heat of hydrogenation, less stable is the reactant.

Comparing stability and heat of hydrogenation

Compound A has a benzene ring.

Benzene ring has a conjugated double bond that can undergo resonance.

Resonance brings extra stability to the molecule.

Therefore, compound A will be more stable and will have less $∆H°$value.

Compound A

Compound B does not have a benzene ring.

Therefore, will be less stable and have a larger $∆H°$value.

Compound B