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Unformatted text preview: Conjugated Systems 1 Copyright, Arizona State University Copyright, Arizona State University Conjugated Systems Return to M.O. Theory (more...) 1 Conjugated Systems and Resonance Requirements for Conjugation : Same as resonance! • Parallel array of atomic p orbitals on adjacent atoms (adjacent sp2 or sp hybridized atoms) C C C C C C C C C C C C H H H H H H H H H H H H H H H H H H conjugated non-conjugated NOT conjugated cumulated Significance of Conjugation • Allows reactions to be explained which cannot using simple Lewis structures, since under these conditions simple two-atom Lewis bonding does not occur, pi-bonding can occur over the entire length of the conjugated system, bonds including multiple atoms Example: allyl radical BDE (kcal/mol) ~98 ~88 ~91 similar stability CH 2 H 3 C H CH 2 CH H 2 C H CH CH 3 H 3 C H CH 2 H 3 C CH 2 CH H 2 C CH CH 3 H 3 C H 2 C CH CH 2 2° 1° allylic + H + H + H • lower BDE means higher radical stability • RESONANCE DELOCALIZATION in the CONJUGATED SYSTEM stabilizes the radical due to delocalization of electrons (the electrons “see” more nuclei) Example: dienes + H 2 + 2 H 2 + 2 H 2-30-60 (-30 x 2)-53 H reaction Δ (kcal/mol) most stable conjugated non-conjugated • the LESS EXOTHERMIC the reaction, the MORE STABLE the diene (since it starts lower in energy) • the conjugated diene has overall lower energy electrons due to delocalization compared to the non-conjugated diene. (a molecular orbital explanation will be coming soon…) Conjugated Systems 2 Copyright, Arizona State University Conjugation and Resonance Are Different Ways of Describing the Electron Delocalization resonance contributors conjugated, p orbitals parallel and adjacent ~1/2 ~1/2 = "actual" structure All conjugated systems involve resonance delocalization, and vica versa; you can draw resonance contributors for any conjugated system • This discussion of conjugated systems is simply a more detailed and complete look at resonance systems, their underlying structural properties, and their chemical reactions, AND a look at some reactions that can NOT be explained using only the resonance model (remember chemical models!) 2 Reactions of Allyl Systems : Conjugated Intermediates Cl allyl chloride O allyl methyl ether allyl group = allyllic position CH 3 H C H 2 C • Allyl systems often react via conjugated (resonance stabilized) intermediates Allylic Substitution via S N 2 (Review, seen before!) Recall : S N 2 reactivity: 3° < 2° < 1° < allylic (fastest of all, because the transition state is conjugated!) C C H H H H Nu X C R H H Nu X C R H H C Nu X primary ‡ ‡ ‡ = C C H H H H Nu X C allylic ‡ H = conjugated ‡ • conjugation (resonance) stabilizes the electrons in the transition state in allylic S N 2 One consequence X R' MgX R' X + Grignard primary halide slow reaction, Grignard not a strong enough nucleophile R' MgX + allylic halide X R' new C-C bond, important!...
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This note was uploaded on 01/07/2012 for the course CHM 234 taught by Professor Iangould during the Spring '12 term at Appalachian State.
- Spring '12