AromaticsAromaticsH

H

H

H

H

H

Some HistorySome History1825 1825 Michael Faraday isolates a new Michael Faraday isolates a new

hydrocarbon from illuminating gas.hydrocarbon from illuminating gas.1834 1834 EilhardtEilhardt MitscherlichMitscherlich isolates same isolates same

substance and determines its empirical substance and determines its empirical formula to be formula to be CCnnHHnn. Compound comes . Compound comes to be called to be called benzenebenzene..

1845 1845 August W. von Hofmann isolates benzene August W. von Hofmann isolates benzene from coal tar.from coal tar.

1866 1866 August August KekuléKekulé proposes structure of proposes structure of benzene.benzene.

GreatestGreatest

ScientistScientist

in in

History??History??

Michael Faraday (1791-1867)

What in the World is What in the World is Benzene??Benzene??

C6H6 discovered by Michael Faraday in 1825– Synthesized in 1834 from benzoic acid– Remarkable chemical stability – Unsaturation number is very high but….

Does not add BromineSubstitutiuon with Br2 / FeBr3Not oxidized by Permanganate or ozoneNo reaction with strong HBr (aq)No reaction with Hydrogen on Pd..??????

Addition vs Substitution…a Mystery

+

Br

Br

AdditionAddition of Bromine to Cyclohexene

Br2

+ Br2

FeBr3

Br

+ HBr

SubstitutionSubstitution of Bromine for Hydrogen in Benzene

C6H6 + Br2 C6H5Br + HBr

∆∆H° of HydrogenationH° of Hydrogenation

Name Structural Formula∆H°

(kcal/mol)

ethylenepropene

1-butenecis-2-butenetrans-2-butene2-methyl-2-butene2,3-dimethyl-2-butene

-32.8-30.1-30.3-28.6-27.6-26.9-26.6

CH2 =CH 2

CH3 CH=CHCH 3

CH3 CH2CH=CH 2

(CH 3 ) 2C=CHCH 3(CH 3 ) 2C=C(CH 3 ) 2

CH3 CH=CHCH 3

CH3CH=CH 2

A quantitative measure of the special stability of benzene

Friedrich August Kekulé (1829-1896)

Friedrich August von Friedrich August von KekuleKekule had a dream of whirling snakes, and the structure of had a dream of whirling snakes, and the structure of benzene. He reported the dream in the following words in 1890, ibenzene. He reported the dream in the following words in 1890, in a speech at a dinner n a speech at a dinner commemorating his discovery that reportedly occurred during a “rcommemorating his discovery that reportedly occurred during a “reverie” on a London everie” on a London bus.bus.

Again the atoms were gamboling before my eyes. This time the smaAgain the atoms were gamboling before my eyes. This time the smaller groups ller groups kept modestly to the background. My mental eye, rendered more ackept modestly to the background. My mental eye, rendered more acute by ute by repeated vision of this kind, could not distinguish larger strucrepeated vision of this kind, could not distinguish larger structures, of manifold tures, of manifold conformation; long rows, sometimes more closely fitted together;conformation; long rows, sometimes more closely fitted together; all twining and all twining and twisting in snakelike motion. But look!twisting in snakelike motion. But look!What was that? One of the snakes had seized hold of its owWhat was that? One of the snakes had seized hold of its own tail, and the form n tail, and the form whirled mockingly before my eyes. As if by a flash of lighting Iwhirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us awoke... Let us learn to dream, gentlemen.learn to dream, gentlemen.

Arthur Arthur KoestlerKoestler (in "The Act of Creation") called this incident "probably the (in "The Act of Creation") called this incident "probably the most most important dream in history since Joseph's seven fat and seven leimportant dream in history since Joseph's seven fat and seven lean cows.an 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 thechains....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.

Contributors to our understanding of Contributors to our understanding of the Structure of Benzenethe Structure of Benzene

1901-19941829-18961791-1867

KekuléKekulé Formulation of BenzeneFormulation of Benzene

Note bond lengthsNote bond lengths

HH

HH

HH

HH

HH

H

H

KekuléKekulé Formulation of BenzeneFormulation of Benzene

Later, Later, KekuléKekulé revised his proposal by suggestingrevised his proposal by suggestinga rapid equilibrium between two equivalenta rapid equilibrium between two equivalentstructures.structures.

HH

HH

HH

HHHH

HH

HH

HH

HH

HHHH

HH

KekuléKekulé Formulation of BenzeneFormulation of Benzene

However, this proposal suggested isomers of theHowever, this proposal suggested isomers of thekind shown were possible. Yet, none were everkind shown were possible. Yet, none were everfound.found.

HH

XX

XX

HHHH

HH

HH

XX

XX

HHHH

HH

All CAll C——C bond distances = 140 pmC bond distances = 140 pm

146 pm146 pm

134 pm134 pm140 pm140 pm 140 pm140 pm

140 pm140 pm 140 pm140 pm

140 pm140 pm140 pm140 pm

140 pm is the average between the C140 pm is the average between the C——C single C single bond distance and the double bond distance in 1,3bond distance and the double bond distance in 1,3--butadiene.butadiene.

A Resonance Picture of A Resonance Picture of Bonding in BenzeneBonding in Benzene

KekuléKekulé Formulation of BenzeneFormulation of Benzene

Instead of Instead of Kekulé'sKekulé's suggestion of a rapidsuggestion of a rapidequilibrium between two structures:equilibrium between two structures:

H

H

H

H

H

H

H

HH

HHH

Resonance Formulation of BenzeneResonance Formulation of Benzene

PaulingPauling described the structure of benzene as a described the structure of benzene as a resonanceresonancehybridhybrid of the two Lewis structures. Electrons areof the two Lewis structures. Electrons arenot localized in alternating single and double bonds,not localized in alternating single and double bonds,but are delocalized over all six ring carbons.but are delocalized over all six ring carbons.

HH

HH

HH

HHHH

HH

HH

HH

HH

HHHH

HH

Resonance Formulation of BenzeneResonance Formulation of Benzene

CircleCircle--inin--aa--ring notation stands for resonance ring notation stands for resonance description of benzene (hybrid of two description of benzene (hybrid of two KekuléKekuléstructures)structures)

Unusual Stability of BenzeneUnusual Stability of Benzene

benzene is the best and most familiar example benzene is the best and most familiar example of a substance that possesses "special stability" of a substance that possesses "special stability" or "or "aromaticityaromaticity""

Aromatic molecules have stability that is substantially Aromatic molecules have stability that is substantially greater for a molecule than would be expected on greater for a molecule than would be expected on the basis of any of the Lewis structures written for it the basis of any of the Lewis structures written for it

ThermochemicalThermochemical Measures of StabilityMeasures of Stability

heat of hydrogenation: heat of hydrogenation: compare experimentalcompare experimentalvalue with "expected" value for hypotheticalvalue with "expected" value for hypothetical""cyclohexatrienecyclohexatriene""

PtPt

∆∆H°= H°= –– 208 kJ208 kJ

++ 3H3H22

[ 4.186[ 4.186 kJ / kcal ]kJ / kcal ]

3 x 3 x cyclohexenecyclohexene

120 kJ/mol120 kJ/mol

360 kJ/mol360 kJ/mol

208 kJ/mol

231 kJ/mol231 kJ/mol

3 x 3 x cyclohexenecyclohexene

120 kJ/mol120 kJ/mol

360 kJ/mol360 kJ/mol

"expected" heat "expected" heat of of

hydrogenation hydrogenation of benzene is 3 x of benzene is 3 x

heat of heat of hydrogenation hydrogenation of of cyclohexenecyclohexene

observed heat of observed heat of hydrogenation is hydrogenation is 152 kJ/mol less 152 kJ/mol less than "expected"than "expected"benzene is 152 benzene is 152 kJ/mol more stable kJ/mol more stable thanthanexpectedexpected152 kJ/mol is the 152 kJ/mol is the resonance energy of resonance energy of benzenebenzene

3 x 3 x cyclohexenecyclohexene

360 kJ/mol360 kJ/mol

208 kJ/mol ?!

hydrogenation of hydrogenation of 1,31,3--cyclohexadiene cyclohexadiene (2H(2H22) gives off ) gives off more heat than more heat than hydrogenation of hydrogenation of benzene (3Hbenzene (3H22)!!!)!!!

208 kJ/mol

231 kJ/mol231 kJ/mol

Cyclic Cyclic conjugation versus conjugation versus noncyclicnoncyclic conjugationconjugation

heat of hydrogenation = 208 kJ/molheat of hydrogenation = 208 kJ/mol

heat of hydrogenation = 337 kJ/molheat of hydrogenation = 337 kJ/mol

3H3H22

PtPt

3H3H22

PtPt

Resonance Energy of BenzeneResonance Energy of Benzene

compared to localized 1,3,5compared to localized 1,3,5--cyclohexatrienecyclohexatriene

152 kJ/mol152 kJ/mol

compared to 1,3,5compared to 1,3,5--hexatrienehexatriene

129 kJ/mol129 kJ/mol

exact value of resonance energy of benzene exact value of resonance energy of benzene depends on what it is compared to, but depends on what it is compared to, but regardless of model, benzene is more stable regardless of model, benzene is more stable than expected by a substantial amount than expected by a substantial amount

Molecular Orbital ViewMolecular Orbital Viewof Bonding in Benzeneof Bonding in Benzene

Dr. Dr. AnslynAnslyn will cover this in detailwill cover this in detail

Molecular Orbital Model of Molecular Orbital Model of Bonding in BenzeneBonding in Benzene

High electron density above and below plane of High electron density above and below plane of ringring

Another Depiction of BenzeneAnother Depiction of Benzene

Electron Density Map of BenzeneElectron Density Map of Benzene

Benzene MOsBenzene MOs

The Three Bonding The Three Bonding ππ MOs of BenzeneMOs of Benzene

Benzene MOsBenzene MOs

AntibondingAntibondingorbitalsorbitals

EnergyEnergy

BondingBondingorbitalsorbitals

6 6 p p AOsAOs combine to give 6 combine to give 6 ππ MOsMOs3 MOs are bonding; 3 are 3 MOs are bonding; 3 are antibondingantibonding

Benzene MOsBenzene MOs

AntibondingAntibondingorbitalsorbitals

EnergyEnergy

BondingBondingorbitalsorbitals

All bonding MOs are filledAll bonding MOs are filledNo electrons in No electrons in 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”…

Frost Circles…a Great TrickFrost Circles…a Great TrickInscribe 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 ringThe relative energies of the MOs in the ring are given by where the vertices touch the circleThe 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

ππ--MOs of MOs of CyclobutadieneCyclobutadiene(square planar)(square planar)

AntibondingAntibonding

CycloCyclo--butadienebutadiene BondingBonding

4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbonding electrons singly occupy two nonbonding orbitalsorbitals

Structure of Structure of CyclobutadieneCyclobutadiene

structure of a stabilized derivative is characterizedstructure of a stabilized derivative is characterizedby alternating short bonds and long bondsby alternating short bonds and long bonds

C(CHC(CH33))33(CH(CH33))33CC

COCO22CHCH33(CH(CH33))33CC151 pm151 pm

138 pm138 pm

ππ--MOs of MOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)

8 8 pp orbitalsorbitals give 8 give 8 ππ orbitalsorbitals3 3 orbitalsorbitals are bonding, 3 are are bonding, 3 are antibondingantibonding, and 2 , and 2 are nonbondingare nonbonding

CycloCyclo--octatetraeneoctatetraene

AntibondingAntibonding

BondingBonding

AntibondingAntibonding

BondingBonding

8 8 ππ electrons; 3 bonding electrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbonding nonbonding orbitalsorbitals are each halfare each half--filledfilled

ππ--MOs of MOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)

CycloCyclo--octatetraeneoctatetraene

Structure of Structure of CyclooctatetraeneCyclooctatetraene

cyclooctatetraenecyclooctatetraene is not planaris not planar

has alternating long (146 pm)has alternating long (146 pm)and short (133 pm) bondsand short (133 pm) bonds

Heats of Hydrogenation

to give to give cyclohexanecyclohexane (kJ/mol)(kJ/mol)

Heats of Hydrogenation

heat of hydrogenation of benzene is 152 kJ/mol heat of hydrogenation of benzene is 152 kJ/mol less than 3 times heat of hydrogenation of less than 3 times heat of hydrogenation of cyclohexenecyclohexene

120120 231231 208208

Heats of HydrogenationHeats of Hydrogenation

to give to give cyclooctanecyclooctane (kJ/mol)(kJ/mol)

9797 205205 303303 410410

heat of hydrogenation of heat of hydrogenation of cyclooctatetraenecyclooctatetraene is is more than 4 times the heat of hydrogenation of more than 4 times the heat of hydrogenation of cyclooctenecyclooctene….no special stability here!….no special stability here!

Requirements for Requirements for Aromaticity

cyclic conjugation is necessary, cyclic conjugation is necessary, but not sufficient but not sufficient

Aromaticity

notnotaromatic

notnotaromaticaromaticaromaticaromatic aromatic

ConclusionConclusion

there must be some factor in additionthere must be some factor in additionto cyclic conjugation that determines to cyclic conjugation that determines whether a molecule is aromatic or notwhether a molecule is aromatic or not

Hückel'sHückel's RuleRulethe additional factor that influences the additional factor that influences

aromaticityaromaticity is the number of is the number of ππ electronselectrons

among planar, monocyclic, completely among planar, monocyclic, completely conjugated conjugated polyenespolyenes, only those with 4, only those with 4nn + 2 + 2 ππ electrons possess special stability (are electrons possess special stability (are aromatic)aromatic)

nn 44nn+2+2

0 0 22

1 1 66 benzene!benzene!

22 1010

33 1414

44 1818

Hückel'sHückel's RuleRule

Hückel'sHückel's Rule for Rule for AromaticityAromaticity

To be Aromatic …a compound must :To be Aromatic …a compound must :

1.1. be Cyclicbe Cyclic

2.2. have one P orbital on each atom in the ringhave one P orbital on each atom in the ring

3.3. be planar or nearly so to give orbital overlapbe planar or nearly so to give orbital overlap

4.4. have a closed loop of 4n+2 pi electrons in the have a closed loop of 4n+2 pi electrons in the

cyclic arrangement of p cyclic arrangement of p orbitalsorbitals

Hückel'sHückel's RuleRule

defines a condition for cyclic molecules in whichdefines a condition for cyclic molecules in which

the bonding molecular the bonding molecular orbitalsorbitals are filled and there are filled and there

are no electrons in nonare no electrons in non--bonding or bonding or antibondingantibonding

orbitalsorbitals ….roughly analogous to the “rare gas” ….roughly analogous to the “rare gas”

condition condition fprfpr atomic atomic orbitalsorbitals……

ππ--MOs of BenzeneMOs of Benzene

AntibondingAntibonding

BenzeneBenzene

BondingBonding6 6 ππ electrons fill all of the bonding electrons fill all of the bonding orbitalsorbitalsall all ππ antibondingantibonding orbitalsorbitals are emptyare empty

ππ--MOs of MOs of CyclobutadieneCyclobutadiene(square planar)(square planar)

AntibondingAntibonding

CycloCyclo--butadienebutadiene BondingBonding

4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbonding electrons singly occupy two nonbonding orbitalsorbitals

ππ--MOs of MOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)

CycloCyclo--octatetraeneoctatetraene

AntibondingAntibonding

BondingBonding

8 8 ππ electrons; 3 bonding electrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbonding nonbonding orbitalsorbitals are each halfare each half--filledfilled

ππ--Electron Requirement for Electron Requirement for AromaticityAromaticity

notnotaromaticaromatic aromaticaromatic notnot

aromaticaromatic

4 4 ππ electronselectrons 6 π electrons6 π electrons 8 8 ππ electronselectrons

Completely Conjugated Completely Conjugated PolyenesPolyenes

6 6 ππ electrons;electrons;not completelynot completely

conjugated

6 π electrons;completely conjugated

6 π electrons;completely conjugated conjugated

notnotaromaticaromatic

HH HH

aromaticaromatic

AnnulenesAnnulenes

AnnulenesAnnulenes are planar, monocyclic, completely are planar, monocyclic, completely conjugated conjugated polyenespolyenes. That is, they are the . That is, they are the kind of hydrocarbons treated by kind of hydrocarbons treated by Hückel'sHückel'srule.rule.

[10]Annulene[10]Annulene

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

1010--sided regular polygon has angles of 144°sided regular polygon has angles of 144°

[10]Annulene[10]Annulene

strain betweenstrain betweenthese two these two hydrogenshydrogens

incorporating two trans double bonds intoincorporating two trans double bonds intothe ring relieves angle strain but introducesthe ring relieves angle strain but introducesanother kind of strain into the structure andanother kind of strain into the structure andcauses the ring to be distorted from planaritycauses the ring to be distorted from planarity

14 14 ππ electrons satisfies electrons satisfies Hückel'sHückel's rulerule

There is some strain between There is some strain between hydrogenshydrogens insideinsidethe ring the ring

How about [14]Annulene ??How about [14]Annulene ??

HH HH

HH HH

16 16 ππ electrons does not satisfy electrons does not satisfy Hückel'sHückel's rulerule

alternating short (134 pm) and long (146 pm) bondsalternating short (134 pm) and long (146 pm) bonds

not aromatic

[16]Annulene ??[16]Annulene ??

not aromatic

[18]Annulene[18]AnnuleneThe protons resonate at-3.0 and at 9.3δ!!!

18 18 ππ electrons satisfies electrons satisfies Hückel'sHückel's rulerule

resonance energy = 418 kJ/molresonance energy = 418 kJ/mol

bond distances range between 137bond distances range between 137--143 pm143 pm

HH HHHH

HHHHHH

What about??What about??