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Resonance Energy of Benzene

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Resonance Energy of Benzene AromaticsAromatics H H H H H H SomeSome HistoryHistory 18251825 MichaelMichael FaradayFaraday isolatesisolates aa newnew hydrocarbonhydrocarbon fromfrom illuminatingilluminating gas.gas. 18341834 EilhardtEilhardt MitscherlichMitscherlich isolatesisolates samesame substancesubstance andand determinesdetermines itsits empiricalempirical formulaformula toto bebe CCnHHn.. CompoundCompound comescomes toto bebe calledcalled benzenebenzene.. 18451845 AugustAugust W.W. vonvon HofmannHofmann isolatesisolates benzenebenzene fromfrom coalcoal tar.tar. 18661866 AugustAugust KekuléKekulé proposesproposes structurestructure ofof benzene.benzene. GreatestGreatest ScientistScientist inin History??History?? Michael Faraday (1791-1867) WhatWhat inin thethe WorldWorld isis Benzene??Benzene?? z C6H6 discovered by Michael Faraday in 1825 – Synthesized in 1834 from benzoic acid – Remarkable chemical stability – Unsaturation number is very high but…. z Does not add Bromine z Substitutiuon with Br2 / FeBr3 z Not oxidized by Permanganate or ozone z No reaction with strong HBr (aq) z No reaction with Hydrogen on Pd..?????? Addition vs Substitution…a Mystery Br + Br2 Br Addition of Bromine to Cyclohexene Br FeBr3 + Br2 + HBr Substitution of Bromine for Hydrogen in Benzene C6H6 + Br2 C6H5Br + HBr ∆∆H°H° ofof HydrogenationHydrogenation ∆H° Name Structural Formula (kcal/mol) ethylene CH2 =CH 2 -32.8 propene CH3 CH=CH 2 -30.1 1-butene CH3 CH2 CH=CH 2 -30.3 cis-2-butene CH3 CH=CHCH 3 -28.6 trans-2-butene CH3 CH=CHCH 3 -27.6 2-methyl-2-butene (CH 3 ) 2 C=CHCH 3 -26.9 2,3-dimethyl-2-butene (CH 3 ) 2 C=C(CH 3 ) 2 -26.6 A quantitative measure of the special stability of benzene Friedrich August Kekulé (1829-1896) Friedrich August von Kekule had a dream of whirling snakes, and the structure of benzene. He reported the dream in the following words in 1890, in a speech at a dinner commemorating his discovery that reportedly occurred during a “reverie” on a London bus. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen. Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows. H H H H H H QUESTION: Can you tell me more about this vision to which you have referred? KEKULÉ: Surely! Let me read to you the remarks I am about to make to the assembly today: During my stay in London I resided in Clapham Road....I frequently, however, spent my evenings with my friend Hugo Mueller....We talked of many things but most often of our beloved chemistry. One fine summer evening I was returning by the last bus, riding outside as usual, through the deserted streets of the city....I fell into a reverie, and lo, the atoms were gamboling before my eyes. Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion. Now, however, I saw how, frequently, two smaller atoms united to form a pair: how a larger one embraced the two smaller ones; how still larger ones kept hold of three or even four of the smaller: whilst the whole kept whirling in a giddy dance. I saw how the larger ones formed a chain, dragging the smaller ones after them but only at the ends of the chains....The cry of the conductor: "Clapham Road," awakened me from my dreaming; but I spent a part of the night in putting on paper at least sketches of these dream forms. ContributorsContributors toto ourour understandingunderstanding ofof thethe StructureStructure ofof BenzeneBenzene 1791-1867 1829-1896 1901-1994 KekuléKekulé FormulationFormulation ofof BenzeneBenzene NoteNote bondbond lengthslengths H H H H H H H H H H H H KekuléKekulé FormulationFormulation ofof BenzeneBenzene Later,Later, KekuléKekulé revisedrevised hishis proposalproposal byby suggestingsuggesting aa rapidrapid equilibriumequilibrium betweenbetween twotwo equivalentequivalent structures.structures. HH HH HH HH HH HH HH HH HH HH HH HH KekuléKekulé FormulationFormulation ofof BenzeneBenzene However,However, thisthis proposalproposal suggestedsuggested isomersisomers ofof thethe kindkind shownshown werewere possible.possible. Yet,Yet, nonenone werewere everever found.found. XX XX HH XX HH XX HH HH HH HH HH HH AllAll CC——CC bondbond distancesdistances == 140140 pmpm 140140 pmpm 140140 pmpm 146146 pmpm 140140 pmpm 140140 pmpm 140140 pmpm 140140 pmpm 134134 pmpm 140140 pmpm isis thethe averageaverage betweenbetween thethe CC——CC singlesingle bondbond distancedistance andand thethe doubdoublele bondbond distancedistance inin 1,31,3-- butadiene.butadiene. AA ResonanceResonance PicturePicture ofof BondingBonding inin BenzeneBenzene KekuléKekulé FormulationFormulation ofof BenzeneBenzene InsteadInstead ofof Kekulé'sKekulé's suggestionsuggestion ofof aa rapidrapid equilibriumequilibrium betweenbetween twotwo structures:structures: H H H H H H H H H H H H ResonanceResonance FormulationFormulation ofof BenzeneBenzene PaulingPauling describeddescribed thethe structurestructure ofof benzenebenzene asas aa resonanceresonance hybridhybrid ofof thethe twotwo LewisLewis structures.structures. ElectronsElectrons areare notnot localizedlocalized inin alternatingalternating singlesingle andand doubledouble bonds,bonds, butbut areare delocalizeddelocalized overover allall sixsix ringring carbons.carbons. HH HH HH HH HH HH HH HH HH HH HH HH ResonanceResonance FormulationFormulation ofof BenzeneBenzene CircleCircle--inin--aa--ringring notationnotation standsstands forfor resonanceresonance descriptiondescription ofof benzenebenzene (hybrid(hybrid ofof twotwo KekuléKekulé structures)structures) UnusualUnusual StabilityStability ofof BenzeneBenzene benzenebenzene isis thethe bestbest andand mostmost familiarfamiliar exampleexample ofof aa substancesubstance thatthat possessespossesses "special"special stability"stability" oror ""aromaticityaromaticity"" AromaticAromatic moleculesmolecules havehave stabilitystability thatthat isis substantiallysubstantially greatergreater forfor aa moleculemolecule thanthan wouldwould bebe expectedexpected onon thethe basisbasis ofof anyany ofof thethe LewisLewis structuresstructures writtenwritten forfor itit ThermochemicalThermochemical MeasuresMeasures ofof StabilityStability heatheat ofof hydrogenation:hydrogenation: comparecompare experimentalexperimental valuevalue withwith "expected""expected" valuevalue forfor hypotheticalhypothetical ""cyclohexatrienecyclohexatriene"" PtPt ++ 3H3H2 ∆∆H°=H°= –– 208208 kJkJ [ 4.186 kJ / kcal ] 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol 120 kJ/mol 3 x cyclohexene "expected""expected" heatheat ofof hydrogenationhydrogenation ofof benzenebenzene isis 33 xx heatheat ofof 360 kJ/mol hydrogenationhydrogenation ofof cyclohexenecyclohexene 120 kJ/mol observedobserved heatheat ofof 3 x cyclohexene hydrogenationhydrogenation isis 152152 kJ/molkJ/mol lessless thanthan "expected""expected" benzenebenzene isis 152152 kJ/molkJ/mol moremore stablestable thanthan 360 kJ/mol expectedexpected 152152 kJ/molkJ/mol isis thethe resonanceresonance energyenergy ofof 208 kJ/mol ?! benzenebenzene hydrogenationhydrogenation ofof 1,31,3--cyclohexadienecyclohexadiene (2H(2H2)) givesgives offoff moremore heatheat thanthan hydrogenationhydrogenation ofof benzenebenzene (3H(3H )!!!)!!! 231 kJ/mol 2 208 kJ/mol CyclicCyclic conjugationconjugation versusversus noncyclicnoncyclic conjugationconjugation 3H3H2 PtPt heatheat ofof hydrogenationhydrogenation == 208208 kJ/molkJ/mol 3H3H2 PtPt heatheat ofof hydrogenationhydrogenation == 337337 kJ/molkJ/mol ResonanceResonance EnergyEnergy ofof BenzeneBenzene comparedcompared toto localizedlocalized 1,3,51,3,5--cyclohexatrienecyclohexatriene 152152 kJ/molkJ/mol comparedcompared toto 1,3,51,3,5--hexatrienehexatriene 129129 kJ/molkJ/mol exactexact valuevalue ofof resonanceresonance energyenergy ofof benzenebenzene dependsdepends onon whatwhat itit isis comparedcompared to,to, butbut regardlessregardless ofof model,model, benzenebenzene isis moremore stablestable thanthan expectedexpected byby aa substantialsubstantial amountamount MolecularMolecular OrbitalOrbital ViewView ofof BondingBonding inin BenzeneBenzene Dr.Dr. AnslynAnslyn willwill covercover thisthis inin detaildetail MolecularMolecular OrbitalOrbital ModelModel ofof BondingBonding inin BenzeneBenzene HighHigh electronelectron densitydensity aboveabove andand belowbelow planeplane ofof ringring AnotherAnother DepictionDepiction ofof BenzeneBenzene ElectronElectron DensityDensity MapMap ofof BenzeneBenzene BenzeneBenzene MOsMOs TheThe ThreeThree BondingBonding ππ MOsMOs ofof BenzeneBenzene BenzeneBenzene MOsMOs AntibondingAntibonding orbitalsorbitals EnergyEnergy BondingBonding orbitalsorbitals 66 pp AOsAOs combinecombine toto givegive 66 ππ MOsMOs 33 MOsMOs areare bonding;bonding; 33 areare antibondingantibonding BenzeneBenzene MOsMOs AntibondingAntibonding orbitalsorbitals EnergyEnergy BondingBonding orbitalsorbitals AllAll bondingbonding MOsMOs areare filledfilled NoNo electronselectrons inin antibondingantibonding orbitalsorbitals The special stability of benzene results from the fact that these three bonding MOs are much
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