Hydrogenation of the first double bond in a conjugated diene evolves less heat than hydrogenation of an isolated double bond. Similarly, the enthalpy change of the catalytic hydrogenation of benzene is significantly lower than would be expected with a hypothetical cylohexatriene that is not aromatic. What leads to the lower than expected internal energy of resonant and aromatic systems is resonance stabilization. Aromatic systems are especially stable. The 2p orbitals of benzene carbon atoms are in alignment for π overlap, which generates the π system of delocalized electron density above and below the plane of the ring. The MCAT is fond of testing whether students are aware of the connection between greater stability and reduced heat of hydrogenation.
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