Arenes and Aromaticity
Hydrocarbons
Aliphatic
Alkanes
Aromatic
Alkenes
Alkynes
Examples of Aromatic Hydrocarbons
CH3
H
H
H
H
H
H
H
H
H
H
H
Benzene
H
H
Toluene
H
H
H
H
H
H
Naphthalene
Some history
1834 Eilhardt Mitscherlich isolates a new
hydrocarbon and determines its empirical
formula to be CnHn. Compound comes
to be called benzene.
1845 August W. von Hofmann isolates benzene
from coal tar.
1866 August Kekulé proposes structure of
benzene.
Kekulé Formulation of Benzene
Later, Kekulé revised his proposal by suggesting
a rapid equilibrium between two equivalent
structures.
H
H
H
H
H
H
H
H
H
H
H
H
Structure of Benzene
Structural studies of benzene do not support the
Kekulé formulation. Instead of alternating single
and double bonds, all of the C—C bonds are the
same length.
Benzene has the shape of a regular hexagon.
Resonance Formulation of Benzene
Circle-in-a-ring notation stands for resonance
description of benzene (hybrid of two Kekulé
structures)
Orbital Hybridization Model of
Bonding in Benzene
High electron density above and below plane
of ring
Hückel's Rule
among planar, monocyclic, completely
conjugated polyenes, only those with 4n + 2
p electrons possess special stability (are
aromatic)
n
4n+2
0
2
1
6
2
10
3
14
4
18
General Points
1) Benzene is considered as the parent and
comes last in the name.
2) List substituents in alphabetical order
3) Number ring in direction that gives lowest
locant at first point of difference
Example
Cl
Br
F
2-bromo-1-chloro-4-fluorobenzene
Ortho, Meta, and Para
alternative locants for disubstituted
derivatives of benzene
1,2 = ortho
(abbreviated o-)
1,3 = meta
(abbreviated m-)
1,4 = para
(abbreviated p-)
Naphthalene
resonance energy = 255 kJ/mol
most stable Lewis structure;
both rings correspond to
Kekulé benzene
Anthracene and Phenanthrene
Anthracene
Phenanthrene
resonance energy:
347 kJ/mol
381 kJ/mol
Reactions of Arenes:
Electrophilic Aromatic Substitution
H
d+
+E
d–
Y
E
+H
Y
Representative Electrophilic Aromatic
Substitution Reactions of Benzene
H
d+
+E
d–
Y
E
+H
Y
H
d+
+E
d–
E
Y
+H
Electrophilic aromatic substitutions include:
Nitration
Sulfonation
Halogenation
Friedel-Crafts Alkylation
Friedel-Crafts Acylation
Y
Nitration of Benzene
H
+ HONO2
H2SO4
NO2
+ H2O
Nitrobenzene
(95%)
Sulfonation of Benzene
H
heat
+ HOSO2OH
SO2OH
+ H2O
Benzenesulfonic acid
(100%)
Halogenation of Benzene
H
+ Br2
FeBr3
Br
+ HBr
Bromobenzene
(65-75%)
Friedel-Crafts Alkylation of Benzene
H
+ (CH3)3CCl
AlCl3
C(CH3)3
+ HCl
tert-Butylbenzene
(60%)
Friedel-Crafts Acylation of Benzene
O
O
H
AlCl3
+ CH3CH2CCl
CCH2CH3
+ HCl
1-Phenyl-1-propanone
(88%)