Which has the greatest heat of hydrogenation?

Question

Which of 1,3-cyclohexadiene and 1,4-cyclohexadiene has the greatest heat of hydrogenation (i.e., the most exothermic reaction with H2 in the presence of a catalyst). Provide a brief explanation for your choice.

Answer 1

Hi Kat S.,

Why don't you sketch these two molecules. That should convince you that one of them has extended conjugated double bonds, and the other doesn't. Conjugated systems are extra stable, their bonds require more energy to break and reform (e.g. by hydrogenation). Therefor, if there is energy to be liberated upon hydrogenation, the conjugated one will liberate less. I think this effect dominates over any considerations of stereochemistry, bond strain, etc.

-- Cheers, --Mr. d.