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Unformatted text preview: Chapter 20-1 Chem 62 Aromaticity Aromaticity and Electrophilic Aromatic Substitution Reactions Stability of Aromatic Rings No Reaction + HX, X 2 , B 2 H 6 , KMnO 4 , OsO 4 b) Heat of hydrogenation of benzene + 49.8 kcal/mole + 28.6 kcal/mole Pt/ 3 H 2 35 atm, 225° Pt/H 2 25° The heat of hydrogenation of benzene would be expected to be approximately 3 x 28.6 kcal/mole (the value of a single C=C bond in cyclohexene) or 85.8 kcal/mole. Therefore benzene is 85.8 — 49.8 = 36 kcal/mole more stable than would be predicted. Difference in energy (36 kcal/mole) is the resonance energy of benzene...the energy lost (or stability gained) by complete delocalization of the electrons in the π-system. a) Chemical reaction...It has been noted that the C=C bonds of the benzene ring do not react with the usual E + A – reagents. Br Br Br 2 Br + Br + δ Br δ− + Br – Alkenes readily undergo electrophilic addition reactions Chapter 20-2 Chem 62 Aromaticity Molecular Orbital Picture of Benzene...
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