Introduction to Aromaticity

1
Introduction to Aromaticity

• Lecture supplement Thinkbook page 26

2
Historical Background
London, 1825

• Street lamps fueled by gas derived from whale and
  cod oils
• Unknown liquid condensed in pipes

• Named benzene
• Vapor density experiment reveals MW 78 formula
  C6H6
• CH ratio 11 implies high reactivity like
  acetylene (HC?CH)
• However benzene is fairly inert
• Stable at room temperature
• More resistant to catalytic hydrogenation (H2/Pt)
  than related substances

Conclusion Benzene is not like related
substances.
3
Benzene Structure?
Hundreds of C6H6 isomers possible

• Problem remained unsolved for 40 years, until two
  key ideas formulated

4
Benzene Structure?Some possible structures for
benzene
Explore these structures with models!
X
X
X
X
None of these are correct.
5
Benzene Structure?Kekulés 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.
6
Benzene Structure?Kekulés Dream
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.
7
Benzene Structure?Kekulés 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.
8
Problems with Kekulé Benzene Structure

• The Isomer Problem
• Bond length C-C gt CC
• Therefore Kekulé structure suggests two isomers
  for 1,2-dichlorobenzene

Observation Isomeric 1,2-dichlorobenzenes never
isolated or detected
Conclusion Kekulé structure cannot be accurate
9
Problems with Kekulé Benzene Structure
Kekulés solution to the isomer problem

• Isomers in very fast equilibrium

• Separate structures cannot be isolated or
  detected
• Test Regardless of equilibrium rate, CC present
• ? Expect typical CC reactions

10
Problems with Kekulé Benzene Structure
Test benzene in typical alkene reaction
Alkene reaction
Addition reaction product has all atoms of
reactant plus new groups
Similar functional groups similar reactions
11
Problems with Kekulé Benzene Structure
But in fact...
X
NR (no reaction occurs)

• Catalyst required benzene less reactive than
  alkene

• Substitution (not addition) occurs
• Substitution reaction portion of reactant is
  replaced

12
Revised Kekulé Benzene Structure
How to revise Kekulé structure to be consistent
with the actual properties?

• Very rapid equilibrium reminiscent of resonance

• Kekule benzene isomers resonance contributors
• Isomers have no discreet existence
• Benzene is not just three alkenes in a ring
• Resonance hybrid

• Prediction all C-C bond lengths equal
• Verified by x-ray crystallography (1923)

13
Revised Kekulé Benzene Structure
What is so special about Kekulé benzene?

• How can we explain its special stability?
• Resonance?
• Conjugation?
• How do we measure this special stability?
• Catalytic hydrogenation

14
Measuring the Special Stability of Benzene
Compare cyclohexene and benzene via catalytic
hydrogenation
DH -28.6 kcal mol-1
Prediction If benzene is just three
alkenes... DH (3 x cyclohexene) (3 x -28.6)
-85.6 kcal mol-1 (10 kcal mol-1 less if
conjugation is included)

• Observations
• Benzene hydrogenation requires more heat and
  pressure than cyclohexene
• DH - 49.8 kcal mol-1

• Conclusions
• DH (benzene) lt DH (3 x cyclohexene), so benzene
  more stable
• Extra stability 85.6 - 49.8 36 kcal mol-1
• Extra stability called resonance energy or
  aromaticity

15
Why Aromatic?Why is this special stability
called aromaticity?

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

• Not all benzene-containing molecules have
  pleasant odors

16
Is Benzene Ring the Only Aromatic Structure?
Observation aromatic stability due to resonance
of CC in ring
Conclusion other rings with resonance might also
be aromatic Examine other
cyclic CnHn isomers with alternating pi bonds
Cyclobutadiene C4H4

• Many synthesis attempts
• 1965 isolated in matrix at 4 K
• Unstable...not aromatic?
• Instability due to ring strain?
• Quantum mechanics instability due to two
  unpaired electrons

Conclusion no special stability not aromatic
17
Is Benzene Ring the Only Aromatic Structure?
Cyclooctatetraene C8H8

• 1915 Nobel Prize in Chemistry for studies on
  chlorophyll and other plant pigments
• Reacts easily with H2/Pt

Conclusion no special stability not aromatic
18
Is Benzene Ring the Only Aromatic Structure?
Conclusion?
Conclusion Not all cyclic CnHn molecules are
aromatic
19
How to Predict Aromaticity?
Benzene is not the only aromatic molecule
Aromatic
20
How to Predict Aromaticity?Studies on many
molecules reveal requirements for aromaticity

• Closed loop of p orbitals (loop of sp2 or sp
  atoms)

• Atoms of closed loop must be planar (p orbital
  overlap)
• To override planarity (and aromaticity) strain
  must be severe.

• Hückels Rule closed loop must contain 2, 6, 10,
  14... pi electrons
• Six pi electrons is most common number (benzene)
• Series described by 4n2 pi electrons (n
  integer 0, 1, 2...)
• Atoms that hold pi electrons must all lie in
  closed loop for electrons to participate in
  aromaticity

21
How to Predict Aromaticity?Additional examples
Naphthalene
Closed p orbital loop?
?
Planar?
No significant reason to be nonplanar ?
Pi electron count?
5 CC _at_ 2 e- each 10 (4n2 10 when n 2) ?
22
How to Predict Aromaticity?Additional examples
Furan
Closed p orbital loop?
?
Energy controls geometry geometry controls
hybridization
Planar?
No significant reason to be nonplanar
?
Pi electron count?
2 CC _at_ 2 e- each 4 1 lone pair in p 2
Total 6
?
Build a model!
23
How to Predict Aromaticity?Additional examples
Cyclopentadienyl cation

• Orbital does not have to be occupied to be in
  closed loop
• Is cyclopentadienyl cation aromatic?

Is cyclopentadienone aromatic?
24
Consequences of Aromaticity
Biological example DNA bases

• DNA has four bases. Example cytosine

• Is cytosine aromatic?
• Planarity allows more DNA bases (more genetic
  information) into smaller space
• Chemical stability not easily degraded
  effectively preserves DNA function