Bài giảng Organic Chemistry - Chapter 15 Benzene and Aromaticity

1825: Faraday discovers benzene by pyrolysis of whale oil: Colorless liquid bp ~80°C. Very unreactive.Analysis::CH:11Benzene and AromaticityCyclic structure: Kekulé 1865Michael Faraday 1791-1867Resonance:NomenclatureSubstituted benzenes: alkylbenzene, halobenzene, nitrobenzene, etc. (Functional groups have priority).Disubstituted:orthometaparaFunctional groups take over: General term for benzene derivatives: Arene. C6H5- is phenyl; general aryl.C6H5CH2- is phenylmethyl or benzyl. IUPAC acceptedBecause many benzene derivatives exhibit a nice smell, compounds containing the benzene ring were called historically “aromatic”.Structure: Regular HexagonBoth the π and σ frame symmetrize the structureBenzene is unusually unreactive. Does this mean that it is also especially stable thermodynamically? Look at ΔH° of hydrogenation:Special stability is now called aromaticity. All cyclic 6e arrangements are aromatic, including transition states.(2x28.6) – 54.9 = 2.3= the resonance in the conjugated dieneCalculated ΔH°(H2) = (3x28.6)–6.9 = -78.9Aromatic Transition StatesAll are six electron transition statescis-1,3,5-Hexatriene electrocyclic closurePolycyclic Benzenoid HydrocarbonsFusion: Linear and angularNaphthalene: AromaticClar’s rule: Full electron sextets are “good”Fullerenes: “Curved Graphite”Harry Kroto, Robert Curl, Richard SmalleyNP 19961985Nobel Prize in Physics 2010: GrapheneConjugated Cyclopolyenes: [N]AnnulenesAll aromatic? No! Hückel’s rule: Systems with [4n +2] π electrons are aromatic (stabilized relative to acyclic analog). Systems with [4n] π electrons are antiaromatic (destabilized) n = integer number: 1, 2, 3, etc. 4n +2 = 6, 10, 14, 18..... 4n = 4, 8, 12, 16... Interruption of cyclic conjugation: Nonaromatic. Evidenced by chemistry and 1H NMR (Laboratory Lecture)Erich Hückel1896-1980Cyclobutadiene ([4]Annulene)Unstable, very reactive (Diels-Alder dimerization at 35K), rectangular (not square). Bulky substituents allow isolation.Tetra(tert-butyl)cyclobutadiene X-ray structure:AntiaromaticCyclooctatetraene ([8]Annulene)Nonplanar, which disrupts delocalization: Nonaromatic (like a polyene).Barrier to ring flip 11 kcal mol-1[10]Annulene: Aromatic?HHPlanar structure too crowded by the inside hydrogens. Other isomers suffer bond angle strain: Undergo electrocyclic ring closures!trans,trans Isomerall-cis Isomer154ºSolution to this problem: Bridged annulenes: 1,6-Methano[10]annulene.Stable; behaves much like benzene[18]AnnuleneFranz Sondheimer1926-1981The first unstrained aromatic annulene9.2 ppm-3.0 ppm[16]Annulene: AntiaromaticThe AnnulenesCharged AnnulenesBut, the cyclopentadienyl cation is antiaromatic, very unstable, much worse than allyl cation: 4 electrons!Six eBromocycloheptatriene spontaneously dissociates: 6 electrons. But, the cycloheptatrienyl anion is unstable: pKa of cycloheptatriene ~ 39!, 8 electrons.(propene = 40)Six eHückel’s Rule in PolycyclesAromaticPeriphery aromatic: 14 eIsolatedPeriphery aromatic: 18 e Can also be viewed as 3 benzenes.Good approximation: Count peripheral electrons10 e 14 e 18 e 14 e 8 eAntiaromaticBut: 10 eAromaticWhat are these?Aromaticity is an important concept, includes heterocycles(Chapter 25). >300,000 papers since 1981!likeElectrophilic Aromatic Substitution (EAS)(Not addition)Mechanism:Not nucleophilic trapping by X- Potential Energy Diagram of Progress of ReactionHalogenation: F2 violent; Cl2, Br2 need catalyst; I2 endothermic-10 kcal mol-1Electron poorOther catalytic Lewis acids: BF3, AlCl3, etc.Stops. Br is e-withdrawing, deactivates ring-HBr3 Resonance forms2. Nitration with nitric acidHNO3 = HO—NO2Mechanism:Color:Nu, E, L3. Sulfonation: Reagent is fuming sulfuric acid: H2SO4 + SO3. Process is reversible: dilute aqueous H2SO4. SulfonationZwitterion4. Friedel-Crafts reactions: Alkylation and acylation A. AlkylationMechanism:Charles Friedel1832-1899James Craft1839-1917Lewis acidProduct contains an e-pushing alkyl group, making the benzene more reactive, causing overalkylation 2. Lewis acid activation generates carbocationic species prone to rearrangements and polymerization.Often low yields. Why?AlkanolAlkeneB. Acylation - selectiveWith acyl halides:With carboxylic anhydrides:e-Withdrawing: deactivates ringMechanism:Lewis acid needed in equimolar amounts, because it binds to product (before aqueous work-up):Electrophilic aromatic substitution synthetic take home lessons: functional groups!1. Halobenzenes make Grignards, lithium reagents2. Acylbenzenes have carbonyl functionWe shall see next (Chapter 16) how to use the nitro and sulfonyl functions.