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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)