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End. Aromaticity. light oil 3.2 gal/ton 60% benzene. tar pitch asphalt. illuminating gas H 2 + CH 4. Coal as a Source of Benzene. Coal. 1675 - Bituminous coal is distilled to form tars. Michael Faraday (1791-1867). Faraday Isolates Bicarburetted Hydrogen. Faraday.

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slide1
End

Aromaticity

slide2

light oil

3.2 gal/ton

60% benzene

tar

pitch

asphalt

illuminating gas

H2 + CH4

Coal as a Source of Benzene

Coal

1675 - Bituminous coal is distilled to form tars

faraday

Michael Faraday

(1791-1867)

Faraday Isolates Bicarburetted Hydrogen

Faraday

1825 - 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

mitscherlich

Eilhard Mitscherlich

(1794-1863)

The Preparation, Naming and Correct Formula of Benzene

Mitscherlich

1834 - Mitscherlich prepares benzene

(C6H6) from benzoic acid by heating

with CaO. Benzoic acid was prepared

from gum benzoin resin from Styrax benzoin.

kekule s theory

Kekulé’s First Structure of Benzene

Kekule’s Theory

1865 - Proposes a cyclic structure

for benzene with alternating single

and double bonds

1890- Benzene Fest - Kekulé’s Dream

(Snake “Tails” - Oroboros)

August Kekulé

(1829-1896)

ladenburg s objection

Albert Ladenburg

(1842 - 1911)

Ladenburg’s Riposte

Ladenburg’s Objection

Ladenburg, a former student of

Kekulé’s, objects to his mentor’s

formula for benzene because only

one o-dibromobenzene is known,

not two.

kekule revised

not resonance - barrier implied

Kekule’s Response

Kekule Revised

1872 - Kekulé revises his theory, suggesting that the

single bonds are changing places with the double bonds.

ladenburg s prisman

ortho

para

meta

Ladenburg’s Prismane Structure

Ladenburg’s Prisman

1869 - 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.

van t hoff s objection

Van’t Hoff’s Objection

Van’t Hoff’s Objection

1869 - 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

acton and katz

Prismane

Benzene

1H NMR:  2.28, s

13C NMR: 30.6 ppm

1H NMR:  7.34, s

13C NMR: 128.4 ppm

104 Years Later…

Acton and Katz

1973 - Acton and Katz synthesize prismane.

.

other benzenes

“Dewar benzene”

Claus (1867)

Armstrong-Baeyer (1887)

Dewar (1867)

Stadeler (1868)

Wichelhaus (1869)

Thiele (1899)

Other Benzenes

Other Benzenes
cyclohexatriene

cyclohexatriene

resonance

stabilization

35.7 kcal/mol

est. DHho = -84.5 kcal/mol

(3 x 28.5)

DHfo = +24.0 kcal/mol

DHfo = +25.0 kcal/mol

DHho = -26.0 kcal/mol

Std. State

DHfo = +19.8 kcal/mol

DHfo = -1.0 kcal/mol

DHho = -28.5 kcal/mol

DHho = -49.3 kcal/mol

DHfo = 29.5 kcal/mol

Cyclohexatriene vs. Benzene

Cyclohexatriene

est. DHfo = +55.0 kcal/mol

is cyclooctateraene aromatic

D(DHho)=25.6 kcal/mol

D(DHho)=22.6 kcal/mol

DHho=-98.0 kcal/mol

D(DHho)=25.5 kcal/mol

DHho=-72.4 kcal/mol

Std. State

DHho=-49.8 kcal/mol

DHho=-24.3 kcal/mol

Is Cyclooctatetraene Aromatic?

Is Cyclooctateraene Aromatic?
1h nmr shifts

7.40

5.66

10

9

8

7

6

5

4

3

2

1

0

Aromatic H

Alkenyl H

deshielding / downfield

shielding / upfield

1H NMR: Aromatic vs. Alkenyl Hydrogens

1H NMR Shifts
magnetic anisotropy

H

H

H

H

H

H

Downfield shift for aromatic protons ( 6.0-8.5)

Bo

Magnetic Anisotropy

Magnetic Anisotropy
mos of benzene

6 nodes

p6*

p4*

4 nodes

p5*

antibonding

bonding

p2

p3

2 nodes

0 nodes

p1

Molecular Orbitals of Benzene

MOs of Benzene

Aromatic - 4n+2

Frost Diagram

4n 2 e z z e z z cyclodecapentaene

(E,Z)-Dihedral angle = 48o

(Z,Z)-Dihedral angle = 31o

4n + 2 (E,Z,Z,E,Z,Z)-Cyclodecapentaene

4n + 2 (E,Z,Z,E,Z)-Cyclodecapentaene

4n + 2, non-planar; non-aromatic!

1 6 methano e z z e z z cyclodecapentaene

(E,Z)-Dihedral angle = 24o

(Z,Z)-Dihedral angle = 0.5o

1,6-Methano-(E,Z,Z,E,Z,Z)-Cyclodecapentaene

1,6-Methano-(E,Z,Z,E,Z)-Cyclodecapentaene

4n + 2, “planar”; aromatic

e z z z z z cyclodecapentaene
(E,Z,Z,Z,Z,Z)-Cyclodecapentaene

(E,Z,Z,Z,Z,Z)-Cyclodecapentaene

4n + 2, non-planar; non-aromatic

pyrenes i

?

hn

Pyrenes I

Pyrenes

14e: aromatic

pyrenes ii

hn

14e: aromatic

16e: anti-aromatic?

14e: aromatic!

Pyrenes II

Pyrenes

18 annulene

B0

[18]-Annulene

[18]-Annulene

1H NMR:

120OC; d5.45 (18H,s)

-60oC; d9.25 (12H,s), -2.9 (6H,s)

aromatic cations

Cyclopropenium cation

Tropylium cation

1H NMR: d9.17, singlet

Aromatic Cations

Aromatic Cations
aromatic anions

Cyclopentadienyl anion

Cyclononatetraenyl anion

Aromatic Anions

Aromatic Anions
aromatic dianions

Aromatic Dianions I

Aromatic Dianions

Cyclooctatetraene

anti-aromatic

tub-shaped

Cyclooctatetraenyl anion

aromatic

planar

1H NMR: d5.56, singlet

1H NMR: d6.75, singlet

slide32
End

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