Ch_322b_15.18

Ch_322b_15.18 - Chapter 15.18 lecture note

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Reductions of Aromatic Compounds Although the aromatic ring is more difficult to reduce than alkenes and other unsaturated compounds, it can be done. Catalytic hydrogenation proceeds under vigorous conditions of high pressure of H 2 and high temperature. Dissolving metal reduction is a more standard reaction that has important synthetic utility. Catalytic Hydrogenation The overall enthalpy change for the hydrogenation of benzene to cyclohexane is energetically very favorable: overall + 3H 2 ! H o H 2 = -49.6 kcal/mol Ni pressure and high temp yet, the rate of the reaction is very slow compared with the facile hydrogenation of alkenes. This general resistance of benzene towards hydrogenation is understandable, when the individual steps in the reduction are analyzed.
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Step-wise Reduction of Benzene Catalytic hydrogenation of benzene occurs under pressure using a nickel catalyst. The intermediate products, 1,3-cyclohexadiene and cyclohexene, cannot be isolated because they hydrogenate much more rapidly than benzene. + H 2 ! H o H 2 = +5.8 kcal/mol (1) + H 2 ! H o H 2 = -26.5 kcal/mol (2) + H 2 ! H o H 2 = -28.3 kcal/mol (3) overall + 3H 2 ! H o H 2 = -49.6 kcal/mol Ni pressure and high temp The hydrogenation of a C=C bond in an alkene is exothermic (~-28
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Ch_322b_15.18 - Chapter 15.18 lecture note

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