Ch_322b_14.02

Ch_322b_14.02 - The Special Stability of Benzene Benzene...

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Unformatted text preview: The Special Stability of Benzene Benzene does not show the chemical reactivity of an alkene. The aromatic ring structure shows an unusual stability, a resistance to change. rapid hydroxylation no reaction rapid addition no reaction rapid addition no reaction hydrogenation 25 o C, 20 psi very slow hydrogenation >100 o C, 1500 psi Comparative Chemical Reactivity of Cyclohexene and Benzene Reagents cold, dilute KMnO 4 Br 2 /CCl 4 HI H 2 /Ni Aromatic Substitution Reactions The chemical reactivity of benzene is characterized by substitution rather than addition , which preserves the aromatic ring . X This pattern of reactivity suggests the aromatic ring is unusually stable . A Quantitative Measure of Stability: Heats of Hydrogenation of Benzene and Model Cycloalkenes The heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, and benzene are compared in the table below. The expected values for ! H o are based on the observed ! H o for cyclohexene. H 2 H 2 + H 2 ! H o H 2 (kcal/mol) observed expected Heats of Hydrogenation-28.6----- + 2H 2 Pt Pt + 3H 2 Pt-55.4-57.2 As a conjugated diene, 1,3-cyclohexadiene shows resonance stabilization of 57.2-55.4 = 1.8 kcal/mol....
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_14.02 - The Special Stability of Benzene Benzene...

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