1. End
Aromaticity

2. Coal as a Source of Benzene
Bituminous coal is distilled to form tars
illuminating gas
H2 + CH4
light oil
3.2 gal/ton
60% benzene
tar
pitch
asphalt

3. Faraday Isolates Bicarburetted Hydrogen
Michael Faraday
( )
Michael Faraday isolates
“bicarburetted hydrogen” (C12H6)
from liquid residue in a cylinder of
illuminating gas. [C = 6, H = 1]
"One Christmas was so much like another, in those years around the sea-town corner now and out of all sound except the distant speaking of the voices I sometimes hear a moment before sleep, that I can never remember whether it snowed for six days and six nights when I was twelve or whether it snowed for twelve days and twelve nights when I was six."
"A Child's Christmas in Wales" --- Dylan Thomas

4. The Preparation, Naming and Correct Formula of Benzene
Mitscherlich
The Preparation, Naming and Correct Formula of Benzene
Eilhard Mitscherlich
( )
Mitscherlich prepares benzene
(C6H6) from benzoic acid by heating
with CaO. Benzoic acid was prepared
from gum benzoin resin from Styrax benzoin.

5. Kekulé’s First Structure of Benzene
Kekule’s Theory
Kekulé’s First Structure of Benzene
Proposes a cyclic structure
for benzene with alternating single
and double bonds
1890- Benzene Fest - Kekulé’s Dream
August Kekulé
( )

6. Ladenburg’s Objection
Ladenburg’s Riposte
Albert Ladenburg
( )
Ladenburg, a former student of
Kekulé’s, objects to his mentor’s
formula for benzene because only
one o-dibromobenzene is known,
not two.

7. 1872 - Kekulé revises his theory, suggesting that the
Kekule Revised
Kekule’s Response
Kekulé revises his theory, suggesting that the
single bonds are changing places with the double bonds.
not resonance - barrier implied

8. Ladenburg’s Prismane Structure
Ladenburg proposes the prismane structure
for benzene.
ortho
para
meta .

9. Van’t Hoff’s Objection
Ladenburg proposes the prismane structure
for benzene.
Van’t Hoff objects to Ladenburg’s prismane
structure because the ortho isomer is chiral
and should be resolvable.
ortho
para
meta

10. Ortho Isomer
The ortho-Isomer

11. 1973 - Acton and Katz synthesize prismane. .
104 Years Later…
Acton and Katz synthesize prismane. .
1H NMR:  2.28, s
13C NMR: 30.6 ppm
Prismane
Benzene
1H NMR:  7.34, s
13C NMR: ppm

12. Other Benzenes Dewar (1867) Stadeler (1868) Wichelhaus (1869)
“Dewar benzene”
Claus (1867)
Armstrong-Baeyer (1887)
Thiele (1899)

13. Cyclohexatriene vs. Benzene
est. DHho = kcal/mol
(3 x (-28.5))
est. DHfo = kcal/mol
resonance
stabilization
35.7 kcal/mol
DHfo = kcal/mol
DHfo = kcal/mol
DHfo = -1.0 kcal/mol
DHfo = kcal/mol
DHho = kcal/mol
DHho = kcal/mol
DHfo = kcal/mol
DHho = kcal/mol
Std. State

14. Is Cyclooctateraene Aromatic?
Is Cyclooctatetraene Aromatic?
DHho=-98.0 kcal/mol
D(DHho)=25.6 kcal/mol
D(DHho)=22.6 kcal/mol
D(DHho)=25.5 kcal/mol
DHho=-72.4 kcal/mol
DHho=-49.8 kcal/mol
Std. State
DHho=-24.3 kcal/mol

15. 1H NMR: Aromatic vs. Alkenyl Hydrogens
1H NMR Shifts
1H NMR: Aromatic vs. Alkenyl Hydrogens
7.40
5.66 1 2 3 5 7 8 9 6 4 10 
Aromatic H
Alkenyl H
deshielding / downfield
shielding / upfield

16. Magnetic Anisotropy H Bo Magnetic Anisotropy
Downfield shift for aromatic protons ( )

17. Molecular Orbitals of Benzene
MOs of Benzene
Molecular Orbitals of Benzene
p6*
6 nodes
p4*
p5*
4 nodes
bonding
antibonding p2 p3
2 nodes p1
0 nodes
Aromatic - 4n+2
Frost Diagram

18. MOs of Cyclooctatetraene
Molecular Orbitals of Cyclooctatetraene
Anti-aromatic - 4n

19. MOs of Cyclooctatetraene Dianion
Molecular Orbitals of Cyclooctatetraene Dianion
Aromatic - 4n+2

20. MOs of Cyclooctatetraene Dianion
Molecular Orbitals of Cyclobutadiene
Anti-aromatic - 4n

21. 4n + 2 (E,Z,Z,E,Z,Z)-Cyclodecapentaene
4n + 2 (E,Z,Z,E,Z)-Cyclodecapentaene
(E,Z)-Dihedral angle = 48o
(Z,Z)-Dihedral angle = 31o
4n + 2, non-planar; non-aromatic!

22. 1,6-Methano-(E,Z,Z,E,Z,Z)-Cyclodecapentaene
1,6-Methano-(E,Z,Z,E,Z)-Cyclodecapentaene
(E,Z)-Dihedral angle = 24o
(Z,Z)-Dihedral angle = 0.5o
4n + 2, “planar”; aromatic

23. (E,Z,Z,Z,Z,Z)-Cyclodecapentaene
4n + 2, non-planar; non-aromatic

24. Naphthalene and Azulene
4n + 2, planar; aromatic

25. Pyrenes I
Pyrenes ? hn
14e: aromatic

26. Pyrenes II
Pyrenes
14e: aromatic hn
16e: anti-aromatic?
14e: aromatic!

27. [18]-Annulene [18]-Annulene B0 1H NMR: 120OC; d5.45 (18H,s)
-60oC; d9.25 (12H,s), -2.9 (6H,s)

28. Aromatic Cations Aromatic Cations Cyclopropenium cation
Tropylium cation
1H NMR: d9.17, singlet

29. Aromatic Anions
Aromatic Anions
Cyclopentadienyl anion
Cyclononatetraenyl anion

30. Aromatic Dianions Aromatic Dianions I Cyclooctatetraene anti-aromatic
tub-shaped
Cyclooctatetraenyl anion
aromatic
planar
1H NMR: d5.56, singlet
1H NMR: d6.75, singlet

31. Aromatic Dianions
Aromatic Dianions II

32. End
The End