Chapter 15 (2) Conjugated

Chapter 15 (2) Conjugated - REACTIVE ALLYLIC INTERMEDIATES...

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REACTIVE ALLYLIC INTERMEDIATES Before examining the reactions of allylic compounds, a review of reaction-energy diagrams is in order. Hammond’s postulate states that related species that are similar in energy are also similar in structure. The structure of a transition state resembles the structure of the closest stable species. In an endothermic reaction, the transition state is closer in energy to the products . In an exothermic reaction, the transition data is closer in energy and in structure to the reactants . Allyl Cations When allyl bromide is heated in a good ionizing solvent, it ionizes to the allyl cation. All allylic cations are stabilized by resonance because the double bond can delocalize this charge over two carbon atoms. This delocalization may be expressed in terms of resonance. Br 1 2 3 1 2 3 heat or H 2 C C H CH 2 or ionizing solvent Stability of carbocations H 2 C C H CH 2 CH 3 < primary < secondary, allyl < tertiary, substituted allylic is about as stable as CH 3 -CH-CH 3 CH C H CH 2 H 3 C is about as stable as (CH 3 ) 3 C The primary allylic cation is about as stable as the isopropyl cation and more substituted allyl cations are about as stable as tertiary cations.
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Molecular Orbital Description of Allylic Reactive Intermediates ! 3 * ! 2 ! 1 Two nodes One node No nodes A S S Energy A = antisymmetric S = symmetric cation radical anion isolated p orbitals molecular orbitals electron occupancy HOMO LUMO HOMO HOMO LUMO LUMO antibonding nonbonding bonding energy of isolated p orbitals symmetry Conjugated unbranched ions and radicals have an odd number of carbon atoms . The molecular orbitals of the cation , radical , and anion of the allyl system show important differences from those of simple alkenes.
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