Introduction to AromaticityLecture Supplement page 45

Historical Background

•Street lamps fueled by gas derived from whale and cod oils

•Liquid named benzene

•Vapor density experiment reveals MW = 78; formula = C6H6

•C:H ratio 1:1 implies high reactivity like acetylene (HCCH)•However benzene is fairly inert:

Stable at room temperature

More resistant to catalytic hydrogenation (H2/Pt) than related substances

•Analyzed by Michael Faraday

Conclusion: Benzene is not like related substances

London, 1825

•Unknown liquid condensed in pipes

Benzene Structure?

•Problem remained unsolved for 40 years, until two key ideas formulated:328 C6H6 isomers possible

1857: Fredrich Kekulé suggests carbon can form four bonds

C

1864: Alexander Crum Brown suggests carbon can form multiple bonds

C C

C C

The C6H4Cl2 Isomer Problem

None of these are the correct structure for benzene

Dewar benzene

H H

Fulvene

2,4-HexadiyneH3C C C C C CH3

XPrismane

X X

X

Structure must be consistent with experimental facts

Fact: C6H6 C6H4Cl2 Three isomersCl2

Some benzene structure candidates:

Six C6H4Cl2 isomers Four C6H4Cl2 isomers Nine C6H4Cl2 isomers

Two C6H4Cl2 isomers

Thinkbook p.83 Question 2

Benzene Structure?Kekulé’s Dream

1866: Solution to the benzene problem comes to Kekulé in a dream.

“During my stay in Ghent [Belgium] I resided in elegant bachelor quarters in the main

thoroughfare. My study, however, faced a narrow side alley and no daylight penetrated

it. For a chemist who spends his day in the laboratory this mattered little.”

“I was sitting writing at my textbook but the work did not progress; my thoughts were

elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling

before my eyes. This time the smaller groups kept modestly in the background. My

mental eye, rendered more acute by repeated visions of the kind, could now

distinguish larger structures of manifold conformation: long rows, sometimes more

closely fitted together all twining and twisting in snakelike motion.”

Benzene Structure?Kekulé’s Dream

“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 lightning I awoke; and this

time I also spent the rest of the night in working out the consequences of the

hypothesis.”

Kekulé benzene

CC

CC

C

C

H

HH

H

H

H

The Ouroboros

Problems with Kekulé Benzene StructureThe Isomer Problem

H

H

Cl

ClH

H

Observation: These 1,2-dichlorobenzene isomers never isolated or detected

Conclusion: Kekulé structure cannot be accurate

Cl

HCl

H

H

H

•Bond length: C-C > C=C

•Therefore Kekulé structure suggests two isomers for 1,2-dichlorobenzene:

Longer C-C bondShorter C-C bond

Cl

Cl

H

H

Cl

ClH

H Cl

HCl

H

H

H

Problems with Kekulé Benzene Structure

Kekulé’s solution to the isomer problem

•Separate structures cannot be isolated or detected

•Test: Regardless of equilibrium rate, C=C present

Expect typical C=C reactions

very fast

•Isomers in very fast equilibrium

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Br2

Br

Br

Problems with Kekulé Benzene Structure

Test benzene in typical alkene reaction

Addition reaction: Product has all atoms of reactant plus

new groups

Prediction for Kekulé benzene:

Br2

Br

Br

Similar functional groups = similar reactions

Alkene reaction:

Both have pi bonds

Br2

Br

Problems with Kekulé Benzene Structure

But in fact...

Br2

Br

Br

X No reaction occurs

•Catalyst required: Benzene less reactive than alkene

•Substitution (not addition) occurs

•Substitution reaction: Portion of reactant is replaced

Br2

BrH

FeBr3

Revised Kekulé Benzene Structure

•Very rapid equilibrium reminiscent of resonance

•Kekulé benzene “isomers” = resonance contributors•These "isomers” have no discrete existence•Benzene is not “just three alkenes in a ring”•Resonance hybrid:

or Prediction: All C-C bond lengths equal•Verified by x-ray diffraction (Kathleen Lonsdale; 1928)

How to revise Kekulé structure to be consistent with the actual properties?

Revised Kekulé Benzene Structure

•How can we explain its special stability?

Resonance?

Conjugation?

•How do we measure this special stability?

Catalytic hydrogenation

What is so special about Kekulé benzene?

Vollhardt, Figure 15.3

Calculations:Expected Hhyd, 1,3-cyclohexadiene = -28.6 x 2 = -57.2 kcal/molExpected vs. Actual: 57.2 - 54.9 = 2.3 kcal/mol (stabilization through conjugation)

Expected Hhyd, “1,3,5-cyclohexatriene” = (-28.6 x 3) + 6.9 = -78.9 kcal/molExpected vs. Actual: 78.9 – 49.3 = 29.6 kcal/mol (stabilization through _________ ?????)Benzene is NOT simply a cyclic, conjugated triene! AROMATICITY

Heats of Hydrogenation (Hhyd)

Measuring the Special Stability of Benzene

H2

PtH = -28.6 kcal mol-1

Compare cyclohexene and benzene via catalytic hydrogenation

3 H2

Pt

Observations•Benzene +3 H2 requires more heat and more pressure than cyclohexene + H2

• H (benzene + 3 H2) = - 49.8 kcal mol-1

Prediction: If benzene is “just three alkenes”...H = 3 x (cyclohexene + H2) = 3 x -28.6 = -85.8 kcal mol-1 (~10 kcal mol-1 less if conjugation is included)

Conclusions• H (benzene + 3 H2) < H [3 x (cyclohexene + H2)], so benzene more stable•Extra stability = 85.8 - 49.8 = 36.0 kcal mol-1 = Resonance energy

= Aromaticity

Why Called “Aromatic”?Why is this special stability called “aromaticity”?

Tolueneglue

CH3

Methyl salicylateoil of wintergreen

OH

OCH3O

Vanillinvanilla flavor

OCH3

OH

H O

Benzoic acidodorless

OHO

Thiophenolskunky

SH

•First molecules known to contain benzene ring have pleasant aromas; hence “aromatic”

•But not all benzene-containing molecules have pleasant odors

Is Benzene Ring the Only Aromatic Structure?

Observation: Aromatic stability due to resonance of C=C in ring

Cyclobutadiene C4H4

Conclusion: Other rings with resonance might also be aromatic Examine other cyclic CnHn isomers with alternating pi bonds

•Many synthesis attempts failed•1965: Isolated in matrix at 4 K Cyclobutadiene is very reactive•1991: Isolated in a “carcerand” molecule (Donald Cram, UCLA)•Unstable...not aromatic?•Instability due to ring strain?•Quantum mechanics: Instability due to two unpaired electrons

Conclusion: No special stability; not aromatic

Is Benzene Ring the Only Aromatic Structure?

Cyclooctatetraene C8H8•Synthesis: 1911, Richard Willstätter

Conclusion: No special stability; not aromatic

4 H2

Pt

Hydrogenation: Reacts easily with H2/Pt:

Shape = tub

Is Benzene Ring the Only Aromatic Structure?

Conclusion: Not all cyclic CnHn molecules are aromatic

Aromatic Not aromatic Not aromatic

Conclusion?

How to Predict Aromaticity?Benzene is not the only aromatic molecule

Aromatic:

Benzene Naphthalene Pyridine

N

Furan

O

Cyclopentadienylanion

1,3-Cyclohexadiene Cyclobutadiene Cyclooctatetraene Cyclopentadienylcation

Not aromatic:

All the structures on this page are conjugated, but not all of them are aromatic!:

How to Predict Aromaticity?Studies on many molecules reveal three requirements for aromaticity

Required: Closed loop of p orbitals (loop of sp2 or sp atoms)

Required: Atoms of closed loop must be planar (p orbital overlap)

~30 kcal mol-1 in benzene

To override planarity (and aromaticity) strain must be severe.

2 + 2 + 2 = 6 pi electrons (4n+2 = 6; n = 1)

Required: Closed loop must contain 2, 6, 10, 14... pi electrons (Hückel’s Rule)•Series described by 4n+2 = pi electrons (where n = integer: 0, 1, 2, 3...)•Six pi electrons is most common number (benzene) (n =1)

How to Predict Aromaticity?How much strain is too much?

Stabilization from aromaticity

Aromatic Aromatic Aromatic Not aromatic

Destabilization from strain

Aromaticity: A Definition

Aromaticity: Special stability possessed by a molecule which has 4n+2 (n = an

integer) pi electrons contained within a closed loop of adjacent, parallel,

overlapping p orbitals.

How to Predict Aromaticity?Additional examples

Naphthalene

Closed p orbital loop?

Planar? No significant reason to be nonplanar

Pi electron count?

5 C=C @ 2 e- each = 10(4n+2 = 10 when n = 2)

Conclusion: Naphthalene is predicted to be aromatic

How to Predict Aromaticity?Additional examples

Furan

Closed p orbital loop?

Planar? No significant reason to be nonplanar

Pi electron count?

2 C=C @ 2 e- each = 41 lone pair in oxygen p = 2

Total = 6

O

sp3; no p orbital

Build a model!

sp2; one lone pair in p one lone pair in sp2

Oxygensp3

Oxygensp2

How to Predict Aromaticity?Additional examples and requirements

Example: Tropylium cation

Unoccupied p orbital

Example: Cyclopentadienone

O Oxygen not in closed loop

These pi electrons do not add to aromaticity

Is cyclopentadienone aromatic?

•Atoms must all lie in closed loop for their pi electrons to participate in aromaticity

Is tropylium cation aromatic?

•Closed loop p orbitals do not have to be occupied

• p orbitals in a closed loop • p orbitals aligned (molecule is planar) • Pi electron count:

4n+2 = 6, where n = 1 (an integer) YES

• p orbitals arranged in a closed loop • p orbitals aligned (molecule is planar) • Pi electron count:

4n+2 = 4 , where n = integer X

NO

Consequences of Aromaticity•Aromatic rings are planar•Aromatic rings are more stable than similar nonaromatic structures

Biological example: DNA nucleobases are aromatic

•Planarity: More bases (more genetic information) into smaller space•Chemical stability: Not easily degraded; effectively preserves DNA function•Pi stacking: A noncovalent interaction; adds stability to the molecule

N

N

O

NH2

N

N

O

H3C H

O N

NN

N

NH2

N

NN

N

O

H

NH2

How to Predict Aromaticity?Benzene is not the only aromatic molecule

Aromatic:

Benzene Naphthalene Pyridine

N

Furan

O

Cyclopentadienylanion

1,3-Cyclohexadiene Cyclobutadiene Cyclooctatetraene Cyclopentadienylcation

Not aromatic:

All the structures on this page are conjugated, but not all of them are aromatic!:

Homework: Justify the assignments in each case

Aromatic species

H

H

benzene

cyclopropenyl cation

cyclopentadienyl anion (Cp)

H

tropylium cation

NH

pyrrole

N

pyridine

O

furan

H

HH

H

justify the aromatic, non-aromatic, and anti-aromatic assignments in each case identify the hybridization of the atoms involved (especially those bearing a charge and also heteroatoms)

O

Cyclopropenone

Azulene(intense blue!)

naphthalene (mothballs)

anthracene

NH

N

HN

N Porphyrin (the core of things like hemoglobin and chlorophyll)

Hint: For this molecule (for reasons beyond the

scope of this course) do not include the circled

groups in your pi electron count. The system has

18 e- for the Huckel rule. Porphyrins are aromatic!

[18]Annulene(p. 697, Vollhardt)

NH

NH

O

O

Thymine (one of the DNA bases)

# of electrons can be described by 4n + 2, where n = integer

Homework: Justify the assignments in each case

Non-aromatic species

justify the aromatic, non-aromatic, and anti-aromatic assignments in each case identify the hybridization of the atoms involved (especially those bearing a charge and also heteroatoms)

(things are non-aromatic when they fail to meet any of the aromaticity conditions, such as:- molecule is not planar- molecule doesn't have a closed loop of p orbitals

Do you see why this is non-aromatic instead of anti-aromatic? (Vollhardt, p. 695)

(Hint: Ex 15.15 in Vollhardt)

(Hint: Ex 15.15 in Vollhardt)

Anti-aromatic species

1,3-butadiene Hint: Vollhardt, Ex 15-16

H

cyclopentadienyl cation

(we can draw their structures on paper, but they are VERY difficult to isolate in the lab. They are inherently unstable!)

Hint: Vollhardt, Ex 15-21

# of electrons can be described as 4n, where n = integer

1,3-cyclopentadiene

fulvene

O

O

1,4-benzoquinone(core of the Vitamin K family)