Aromatic compounds
1435-1436
2014-2015
Aromatic compounds
Learning Objectives
By the end of chapter four the students will:
 Understand the resonance description of structure of benzene
Understand the hybridization in benzene
 Understand the relation between the stability of benzene and resonance energy
 Know the criteria of aromaticity and Huckel rule
 Understand the nomenclature rules of aromatic compounds and know the Common names of
some aromatic compounds
Understand the reactivity of aromatic compounds, know what are electrophiles and know the
four types of electrophilic aromatic substitution reactions (halogenation, Freidel Crafts
alkylation and acylation, nitration and Sulfonation).
 Know the reactions of alkyl side chains of aromatic compounds (halogenation, oxidation)
 Understand the orientation and reactivity of E.A.S reactions in monosubstituted benzene
derivatives.
Aromatic compounds
Benzene : Resonance Description
Primary analysis revealed benzene had...
a
molecular mass of 78
a
molecular formula of C6H6
Structure:
Kekulé suggested that benzene was...
PLANAR
CYCLIC
Had Alternating Double And Single Bonds
Thus These Double Bonds Are Described
As Conjugated Bonds.
Aromatic compounds
 However, all bond lengths in benzene to be equal and intermediate
between single bond and double bond lengths (1.39 Å) and the ring is
more stable than expected.
 To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID ( average of two structures
that differ only in the placement of the valence electrons).
Contributing Structures
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The energy calculated for a
resonance hybrid is lower
than the energies of the two
alternative structure.
Resonance hybride
Aromatic compounds
6 single bonds
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one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
Aromatic compounds
Characteristics of Aromatic Compounds
*  electron cloud delocalized all over the ring
* The resonance picture this helps to explain lack of reactivity
of benzene (substitution not addition )
Aromatic compounds are compounds that resemble benzene in
chemical behavior thus they tend to react by substitution rather
than by addition and fulfill the aromaticity requirements.
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Aromatic compounds
Characteristics of Aromatic Compounds
To be classified as aromatic, a compound must have :
1) Cyclic structure.
2) Planar structure.
3) Each atom of the ring must have a p orbital to form a delocalized π
system i.e. no atoms in the ring can be sp3 hybridized instead all
atoms must be sp2 hybridized (N.B. carbocation and carbanions are
sp2 hybridized or an unshared pair electrons).
4) Fulfill Huckel rule i.e. the system must have 4n + 2 pi electrons :
thus by calculating n value it will be an integral number i.e. n=0, 1,
2, 3,
Aromatic compounds
Examples of aromatic compounds
O
N
N
H
4n+2=6
n=1
4n+2=6
n=1
4n+2=6
n=1
4n+2=6
n=1
4n+2=6
n=1
N
H
4n+2=2
n=0
4n+2=10
n=2
4n+2=10
n=2
4n+2=6
n=1
Aromatic compounds
Examples of non aromatic compounds


carbon indicted by
n=1/2
n=1/2
 is sp
3
n=1/2
4n+2=8
n=1.5
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Aromatic compounds
Nomenclature of Aromatic Compounds
1. Monosubstituted Benzenes
a. IUPAC Names
They are named as derivatives of benzene. One side group is named
as a prefix in front of the word benzene.
 No number is needed for mono-substituted benzene.
C(CH3)3
tert-Butyl-benzene
CH2CH3
Ethyl-benzene
NO2
Nitro-benzene
Cl
Chloro-benzene
Aromatic compounds
 Benzene ring has priority over :side chains with alkyl, alkoxy groups,
halogens, double and triple bonds
C CH
Vinyl-benzene
Allyl-benzene
Ethynyl-benzene
OCH3
Butyl-benzene
Methoxy-benzene
 In some cases the side chains on aromatic ring contain functional
groups of higher priorities (NH2, OH, CHO,C=O, COOH, COOR) thus
in this case the aromatic ring will be considered as a substituent and the
side chain will be used to give the root name. Two aromatic radials
are known
CH2
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Benzyl group
phenyl group
(C6H5-)
Aromatic compounds
Any aryl group (Ar) is the aromatic group that remains after
removal of hydrogen Atom from an aromatic ring. When the
benzene ring is named as substituent, it is called a phenyl group
(often abbreviated Ph).
A benzyl group (the phenyl methyl group) is the seven carbon
unit consisting of benzene ring and methylene (-CH2-).
b. Common Names Of Monosubstituted Benzenes
CH3
Toluene
CH=CH2
Styrene
OH
Phenol
H
O
Benzaldehyde
HO
O
Benzoic acid
NH2
Aniline
OCH3
Anisol
Aromatic compounds
A hydrocarbon composed of one saturated chain and one benzene
ring is usually named as a derivative of the larger structural unit.
However, if the chain is unsaturated compound may be named as
a derivative of that chain, regardless of ring size. The following
are examples:
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Aromatic compounds
2. Nomenclature of Disubstituted and polysubstituted Benzenes
 All disubstituted benzenes (two groups are attached to benzene), can
give rise to three possible positional isomers.
X
X
X
Y
Y
Y
Common: orthIUPAC:
1,2-
meta
1,3-
para
1,4-
 When the substituents are different, they are of equal priorities they
will should be listed in alphabetical order.
C2H5
NO2
Cl
I
Common:
IUPAC:
o-Chloroethylbenzene
1-Chloro-2-ethylbenzene
Br
m-Bromonitrobenzene
1-Bromo-3-nitrobenzene
F
p-Fluoroiodobenzene
1-Fluoro-4-iodobenzene
Aromatic compounds
CH3
CH3
CH3
CH3
CH3
CH3
Common:
IUPAC:
o-Xylene
1,2-Dimethyl-benzene
m-Xylen
1,3-Dimethyl-benzene
p-Xylene
1,4-Dimethyl-benzene
 If one of the substituents is part of a parent compound, then the
disubstituted or polysubstituted benzene is named as a derivative of that
parent compound i.e. priorities determine the root name and substituents.
OH
COOH
NO2
Cl
CHO
CH3
OCH3
NO2
O 2N
NO2
Br
CH3
Common: o- Chlorophenol
IUPAC: 2-Chlorophenol
m-Bromobenzoic acid
3-Bromobenzoic acid
p-Nitrotoluene
4-Nitrotoluene
o-Methoxybezaldehyde
2-Methoxybezaldehyde
2,4,6- Trinitrotoluene
Aromatic compounds
Reactions
Electrophilic Aromatic Substitution Reactions
COR
RCOCl, AlCl3
Acylation
Aromatic compounds
acetophenone
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Aromatic compounds
Side-Chain Reactions of Aromatic Compounds
1)Halogenation
a. Halogenation of an Alkyl Side Chain
CH3
CH 2Br
Br2
+
UV light
Toluene
HBr
Benzyl Bromide
CHClCH3
CH2CH3
CH2CH2Cl
Cl2/ UV
Major
Minor
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Aromatic compounds
b. Oxidation of an Alkyl Side Chain
CH3
COOH
-
KMnO 4, OH , H2O, heat
H3O
+
Toluene
Benzoic acid
CH3
COOH
CH2CH3
KMnO 4, OH , H2O, heat
H3O
NO 2
COOH
+
+
CO 2
+
H2O
NO 2
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Aromatic compounds
Orientation effects of substituents in electrophilic aromatic
substitution reactions of monosubstituted Benzenes
 Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to ortho, para-positions and speed-up the reaction (i.e. o & p directors and activating groups).
 Halogens direct new groups to ortho-, para- positions but they slow down the
reaction (i.e. halogens are o & p directors and deactivating groups).
 Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new
groups to the meta-position and slow the reaction down (i.e. i.e. m directors and deactivating
groups).
 Thus the order of reactivity of benzene and monosubstituted benzene derivatives in E.Ar.sub.
is as in the following chart
Substituted benzene with o,p directors > Benzene > Halobenzene derivatives > Substituted
benzene with
m- directors
Aromatic compounds
Orientation effects of substituents in electrophilic aromatic
substitution reactions of monosubstituted Benzenes
Ortho , para directors
Meta directors
-OH, -OR
-NH2, -NHR, -NR2
-C6H5
-CH3, -R (alkyl)
-F, -Cl, -Br, -I
-NO2
-SO3H
-COOH, -COOR
-CHO, -COR
-CN
OH
OH
OH
NO2
HNO3 / H2SO4
+
o-Nitrophenol
53 %
NO2
NO2
NO2
p-Nitrophenol
47 %
SO3 / H2SO4
SO3H
m-Nitrobezenesulfonic acid
Aromatic compounds
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Aromatic compounds
Q1: What are the major products of the following reaction?
Q2: What is the empirical formula of the following compound: (pmethyl-Toluene):
a) C8H10
b) C8H12
c) C8H14
d) C6H14
Aromatic compounds
Q3: What is the final product of the following reaction?
a) o-chlorobenzaldehyde b) m-chlorobenzaldehyde c) p-chlorobenzaldehyde d) a,c
Q4:Which one of the following compounds has aromatic
character?
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Aromatic compounds
Thank You for your kind attention !
Questions?
Comments
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