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 lower in energy than the six uncombined 2p atomic orbitals….and the bonding orbitals are “full”… FrostFrost Circles…aCircles…a GreatGreat TrickTrick z Inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring z The relative energies of the MOs in the ring are given by where the vertices touch the circle z The MOs – below the horizontal line through the center of the ring are bonding MOs – on the horizontal line are nonbonding MOs – above the horizontal line are antibonding MOs

Bonding Bonding Antibonding Antibonding orbitals orbitals Cyclobutadiene Cyclobutadiene (square planar) (square planar) MOs of MOs of - - π π - - electrons; bonding orbital is filled; other 2 electrons; bonding orbital is filled; other 2 electrons singly occupy two nonbonding π π electrons singly occupy two nonbonding Cyclo Cyclo 4 4 π π butadiene butadiene StructureStructure ofof CyclobutadieneCyclobutadiene structurestructure ofof aa stabilizedstabilized derivativederivative isis characterizedcharacterized byby alternatingalternating shortshort bondsbonds andand longlong bondsbonds

(CH(CH3))3CC C(CHC(CH3))3

138 pm

151 pm

(CH(CH3))3CC COCO2CHCH3 , and 2 , and 2 Bonding Bonding Antibonding Antibonding antibonding antibonding orbitals orbitals

π π Cyclooctatetraene Cyclooctatetraene give 8 give 8 are bonding, 3 are bonding, 3 (square planar) (square planar) MOs of MOs of - - orbitals orbitals π π orbitals p orbitals p 3 8 are nonbonding 3 8 are nonbonding - - Cyclo Cyclo octatetraene octatetraene ππ--MOsMOs ofof CyclooctatetraeneCyclooctatetraene (square(square planar)planar)

AntibondingAntibonding

CycloCyclo-- octatetraeneoctatetraene BondingBonding 88 ππ electrons;electrons; 33 bondingbonding orbitalsorbitals areare filled;filled; 22 nonbondingnonbonding orbitalsorbitals areare eacheach halfhalf--filledfilled StructureStructure ofof CyclooctatetraeneCyclooctatetraene

cyclooctatetraenecyclooctatetraene isis notnot planarplanar hashas alternatingalternating longlong (146(146 pm)pm) andand shortshort (133(133 pm)pm) bondsbonds HeatsHeats ofof HydrogenationHydrogenation

toto givegive cyclohexanecyclohexane (kJ/mol)(kJ/mol)

120120 231231 208208 heatheat ofof hydrogenationhydrogenation ofof benzenebenzene isis 152152 kJ/molkJ/mol lessless thanthan 33 timestimes heatheat ofof hydrogenationhydrogenation ofof cyclohexenecyclohexene HeatsHeats ofof HydrogenationHydrogenation

toto givegive cyclooctanecyclooctane (kJ/mol)(kJ/mol)

9797 205205 303303 410410 heatheat ofof hydrogenationhydrogenation ofof cyclooctatetraenecyclooctatetraene isis moremore thanthan 44 timestimes thethe heatheat ofof hydrogenationhydrogenation ofof cyclooctenecyclooctene….no….no specialspecial stabilitystability here!here! RequirementsRequirements forfor AromaticityAromaticity cycliccyclic conjugationconjugation isis necessary,necessary, butbut notnot sufficientsufficient

notnot notnot aromaticaromatic aromaticaromatic aromaticaromatic ConclusionConclusion

therethere mustmust bebe somesome factorfactor inin additionaddition toto cycliccyclic conjugationconjugation thatthat determinesdetermines whetherwhether aa moleculemolecule isis aromaticaromatic oror notnot Hückel'sHückel's RuleRule

thethe additionaladditional factorfactor thatthat influencesinfluences aromaticityaromaticity isis thethe numbernumber ofof ππ electronselectrons Hückel'sHückel's RuleRule amongamong planar,planar, monocyclic,monocyclic, completelycompletely conjugatedconjugated polyenespolyenes,, onlyonly thosethose withwith 44nn ++ 22 ππ electronselectrons possesspossess specialspecial stabilitystability (are(are aromatic)aromatic) nn 44nn+2+2 00 22 11 66 benzene!benzene! 22 1010 33 1414 44 1818 Hückel'sHückel's RuleRule forfor AromaticityAromaticity

ToTo bebe AromaticAromatic …a…a compoundcompound mustmust :: 1.1. bebe CyclicCyclic 2.2. havehave oneone PP orbitalorbital onon eacheach atomatom inin thethe ringring 3.3. bebe planarplanar oror nearlynearly soso toto givegive orbitalorbital overlapoverlap 4.4. havehave aa closedclosed looploop ofof 4n+24n+2 pipi electronselectrons inin thethe cycliccyclic arrangementarrangement ofof pp orbitalsorbitals Hückel'sHückel's RuleRule definesdefines aa conditioncondition forfor cycliccyclic moleculesmolecules inin whichwhich thethe bondingbonding molecularmolecular orbitalsorbitals areare filledfilled andand therethere areare nono electronselectrons inin nonnon--bondingbonding oror antibondingantibonding orbitalsorbitals ….roughly….roughly analogousanalogous toto thethe “rare“rare gas”gas” conditioncondition fprfpr atomicatomic orbitalsorbitals…… Bonding Bonding Antibonding Antibonding orbitals orbitals are empty are empty MOs of Benzene MOs of Benzene - - orbitals orbitals π π antibonding antibonding electrons fill all of the bonding electrons fill all of the bonding π π π π

all 6 all 6 Benzene Benzene Bonding Bonding orbitals orbitals Antibonding Antibonding Cyclobutadiene Cyclobutadiene (square planar) (square planar) MOs of MOs of - - π π - - electrons; bonding orbital is filled; other 2 electrons; bonding orbital is filled; other 2 electrons singly occupy two nonbonding π π electrons singly occupy two nonbonding 4 4 π π Cyclo Cyclo butadiene butadiene Bonding Bonding Antibonding Antibonding filled filled are filled; 2 are filled; 2 - - orbitals orbitals Cyclooctatetraene Cyclooctatetraene are each half are each half (square planar) (square planar) orbitals orbitals MOs of MOs of - - π π - - electrons; 3 bonding electrons; 3 bonding

π π nonbonding 8 nonbonding 8 Cyclo Cyclo octatetraene octatetraene not not electrons electrons aromatic π π aromatic 8 8 Aromaticity Aromaticity electrons electrons

π aromatic π aromatic 6 6 Electron Requirement for Electron Requirement for not not - - electrons π π electrons aromatic π π aromatic 4 4 CompletelyCompletely ConjugatedConjugated PolyenesPolyenes

66 ππ electrons;electrons; 6 π electrons; 6 π electrons; notnot completelycompletely completely conjugated completely conjugated conjugatedconjugated

HH HH notnot aromatic aromatic aromaticaromatic AnnulenesAnnulenes

AnnulenesAnnulenes areare planar,planar, monocyclic,monocyclic, completelycompletely conjugatedconjugated polyenespolyenes.. ThatThat is,is, theythey areare thethe kindkind ofof hydrocarbonshydrocarbons treatedtreated byby Hückel'sHückel's rule.rule. [10]Annulene[10]Annulene

predicted to be aromatic by Hückel's rule, but too much angle strain when planar and all double bonds are cis

10-sided regular polygon has angles of 144° [10]Annulene[10]Annulene

strain between these two hydrogens

incorporating two trans double bonds into the ring relieves angle strain but introduces another kind of strain into the structure and causes the ring to be distorted from planarity

HowHow aboutabout [14]Annulene[14]Annulene ????

H H

H H

1414 ππ electronselectrons satisfiessatisfies Hückel'sHückel's rulerule ThereThere isis somesome strainstrain betweenbetween hydrogenshydrogens insideinside thethe ringring [16]Annulene[16]Annulene ????

16 π electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bonds not aromatic [18]Annulene[18]Annulene The protons resonate at -3.0 and at 9.3δ!!!

18 π electrons satisfies Hückel's rule H H H resonance energy = 418 kJ/mol H H bond distances range between 137-143 pm H WhatWhat about??about??