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Unformatted text preview: Aromaticity 1 Copyright, Arizona State University Copyright, Arizona State University Aromaticity Stability and Instability in Cyclic pi-Systems 1 Hückel Rule (Review) (4 n + 2) cyclic electrons are stable (4 n) cyclic electrons are unstable works for both transition states AND stable species Example + 3 H 2 Δ H r (kcal/mol)- 50 + H 2- 29 3 x -29 = –87 • benzene more stable than expected for cyclohexatriene by ca 37 kcal/mol • can not be explained merely by conjugation or resonance, because....... aromatic conjugated but not cyclic > more stable than > 6 electrons ( 4 x 1 + 2) anti-aromatic < less stable than < 4 electrons ( 4 x 1) non-aromatic = equally stable = conjugated but not cyclic conjugated but not cyclic 2 Aromatic Ions Example in a surprisingly fast SN1 reaction H Br heat CH 3 OH H + Br – H H CH 3 OH –H + OCH 3 H • 2 electrons in a cyclic cation system, • aromatic: stable ! Examples in Bronsted acidity H H H + Br – – Base • 4 electrons in a cyclic system • anti-aromatic: unstable ! Aromaticity 2 Copyright, Arizona State University H H H 2 O H + H 3 O + pKa ~ 19 • 6 electrons, aromatic stable conjugate base anion • cyclopentadiene is even more acidic than a terminal alkyne!! H 2 O + H 3 O + pKa ~ 40 MUCH higher pKa, very weak acid H H H • 8 electrons, anti-aromatic unstable conjugate base anion • cycloheptatriene is MUCH less acidic than a terminal alkyne!! H + H OH H OH 2 H tropylium ion • 6 electrons, aromatic • the tropylium ion is a very stable cation 3 Aromatic Heterocycles A Heterocycle: a cyclic system containing N, O, S etc....
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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