Integrated SequencePhysics Chemistry Organic Biology

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The Chemical Bond


Conjugated π Systems and Aromaticity

Reactions of Alkenes

Reactions of Conjugated Species

Through the process of hydrogenation, the electron pair originally in the π bond becomes distributed between two σ bonds formed with hydrogen atoms. This process is generally exothermic, and the heat evolved is a useful index of the relative strength of π bonds. The stronger the initial bond, the lower the energy of the initial state of the system, so the less heat will ultimately be liberated through the reaction.

Comparison of heats of hydrogenation is a useful analytical tool and often finds its way into MCAT passages. Heat of hydrogenation allows us to measure energy differences among isomeric alkenes (internal vs. terminal double bonds, cis vs. trans, etc.), and resonance stabilization can be measured. Resonance stabilization is especially great with the bonds between atoms in an aromatic ring. Hydrogenation of aromatic rings yields substantially less heat than the π bonds of ordinary alkenes.

Because the activation energy is so high, hydrogenation of alkenes does not occur at a measurable rate without a catalyst. The mechanism of heterogeneous catalysis (gaseous reagents on the surface of a solid metal catalyst) involves syn addition exclusively.

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