2.3 Aromaticity1Aromaticitycyclohexene1,3-cyclohexdiene1,3,5-cyclohexatrieneH2/NiH2/NiH2/NiΔH = -28.6 kcal/molΔHcalc= 2 x –28.6=-57.2 kcal/molΔHobs= - 55.4 kcal/molDiff = 1.8 kcal/molDiff = 36 kcal/molΔHobs= - 49.8 kcal/molΔHcalc= 3 x –28.6 = - 85.5 kcal/molThe delocalization of the six πelectrons makes the electrons more stable. The difference of 36 kcal/mol is a measure of the added stability of the aromatic system compared to that of the localized systemBenzene is unusually stable and not very reactive
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2.3 Aromaticity2Molecular Orbitals (MOs) of Benzene6 atomic p orbitals will combine to form 6 molecular orbitalsone nodetwo nodesthree nodesTwo molecular orbitals that have the same potential energy are called degenerateAntibonding OrbitalsBonding OrbitalsPotential Energyπ1π3π2π4 ∗π6 ∗π5 ∗