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Title: AROMATIC COMPOUNDS
Description: Well comprehensive notes on Aromatic Compounds
Description: Well comprehensive notes on Aromatic Compounds
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An aromatic hydrocarbon or arene (or sometimes
aryl hydrocarbon) is a hydrocarbon with alternating
double and single bonds between carbon atoms
forming rings
...
The configuration of six carbon atoms in aromatic
compounds is known as a benzene ring, after the
simplest possible such hydrocarbon, benzene
...
Monocyclic aromatic hydrocarbons (MAH include
benzene and its derivatives
...
Toluene is also an important organic chemical
...
CH2CH3
HCl
AlCl3
+ H2C
CH2
Ethene
Ethylbenzene
Benzene
CH2CH3
o
CH
630 C
ZnO (catalyst)
+
CH2
H2
Styrene (90 - 92% yield)
Styrene is one of the most important industrial chemicals
...
As seen above, the starting material for a major synthesis
of styrene is ethylbenzene, produced by the FriedelCraft’s acylation of benzene
...
CH
CH2
CH2 CH
CH2 CH
Catalyst
Styrene
Polystyrene
n
CH2 CH
Arenes are hydrocarbons based on the benzene ring
as a structural unit
...
Arenes are also referred to as aromatic hydrocarbons
...
An aryl group is one derived from an arene by
removal of a hydrogen atom, and its symbol is Ar-
...
The most characteristic reactions of benzenoid arenes
are the substitution reactions that occur when they
react with electrophilic (electron rich) reagents
...
Two of the more active catalysts are rhodium and
platinum, and it is possible to hydrogenate arenes in the
presence of these catalysts at room temperature and
modest pressure
...
+
Benzene
3H2
Hydrogen
2 - 3 atm
Acetic acid
Pt 30o C
Cyclohexane
(100%)
Halogenation
X2, FeX3
(X = Cl, Br)
X
+ HX
Nitration
HONO
H2SO4
NO 2
+ H 2O
Sulfonation
SO3
H2SO4
SO3H
General mechanism for electrophilic aromatic
substitution Ar-H + E-A
Ar-E + H-A
E
+ E-A
+ H-A
Electrophiles attack the p system of benzene to form a
(or sometimes as a s complex non-aromatic
carbocation known as an arenium ion
Arenium ion (s complex)
21
A proton is removed from the carbon atom of the
arenium ion that bears the electrophile
...
A-
+ HA
The electrophiles are either a positive ion (E+) or some
other electron deficient species with a large partial
positive charge
...
a complex
+ Cl
:
Cl
...
The concentration of nitronium
ion in nitric acid alone is too low to nitrate benzene
at a convenient rate, but can be increased by adding
sulfuric acid
...
O
OH
O
O
+
N
O
-
NO 2
O
26
Sulphonation of benzene: Benzene reacts with
fuming sulphuric acid (sulphuric acid with added
sulphur trioxide)to produce benzene sulphonic acid
O
O
+
S
O
...
OH
H
S
O
O
OH
O
O
O
O
...
H2SO4
+
O
rate determining
step
CH
O
O
S
HO
OH
S
O
OH
Benzene sulphonic acid
27
Friedel-Crafts Alkylation
R
RCl, AlCl 3
+ HCl
R can rearrange
H3C
...
:
Cl
+
+
+
-
CH Cl Al Cl
Al
Cl
Cl H C
H3C
3
A Lewis acid-base reaction
CH3
Cl
H3C
+
CH
Cl
H3C
+
CH
-
+ Cl
Al Cl
H3C
Cl
Complex dissiciates
Cl
CH3
CH3
CH3
CH CH3
CH CH3
CH CH3
+
CH
CH3
+
HC
+
CH
CH3
CH3
CH CH3
H
+
Cl
...
CH
Al
CH
-
Cl
CH3
HCl + AlCl 3
+
Cl
Notice that the carbocation can rearrange to a more
+
+
stable one eg from H3C CH2 CH2 to H3C CH CH3
Friedel-Crafts acylation
Acylation is the introduction of the acylO group
O
O
R
eg H3C
acyl group
Ethanoyl group
Benzoyl group
(Acetyl group)
29
O
O
R
R
+ HCl
Cl
AlCl 3
O
C
...
:
Cl
+
Al
Cl
Cl
R
R
A Lewis acid-base reaction
+
R C
O:
Cl
O
C
+
-
Cl Al Cl
Cl
O
+
C
+ Cl
R
Cl
Complex dissiciates
R C
-
Al Cl
Cl
+
O
An acylium ion (a resonance hybrid)
30
O
+
+
R C
O
+
R
Arenium ion
O
+
O
R
+
AlCl4
-
R
+ HCl + AlCl3
Arenium ion
31
The ketone in this step forms a complex
with the Lewis acid (AlCl3)
R
R
...
O
...
O
H3C
O
+
SOCl2
OH
H3C
+ SO 2 + HCl
Cl
Ethanoyl chloride
(acetyl chloride)
O
O
+ PCl5
Benzoic acid
OH
+ POCl 3 + HCl
Cl
Benzoyl chloride
33
Friedel-Crafts acylation can also be carried out
using carboxylic acid anhydrides
...
H
O
...
Wanjohi
36
Nomenclature of Aromatic compounds
➢ When a benzene ring is a substituent, the term
phenyl is used (for C6H5-)
You may also see “Ph” or “f” in place of “C6H5”
➢ “Benzyl” refers to “C6H5CH2-”
37
❑ Benzene is the parent name for some monosubstituted benzenes;
the substituent name is added as a prefix
...
39
❑ When two substituents are present their position may be indicated
by the prefixes ortho, meta, and para (o, m and p) or by the
corresponding numerical positions
...
▪ If one substituent is part of a common root, name the
molecule as a derivative of that monosubstituted benzene
...
▪ Alphabetize the substituent names
...
The
substituent that comprises the common root is located at C1
...
▪ Substitution of a hydrogen keeps the aromatic ring intact
...
46
47
Examples:
48
49
2- Side-Chain Reactions of Aromatic Compounds
A) Halogenation of an Alkyl Side-Chain
50
B) Oxidation of an Alkyl Side-Chain
hot
hot
51
Disubstituted Benzenes: Orientation
Product ratio conclusion:
40% ortho, 40% meta, 20% para
52
Orientation and Reactivity Effects of Substitutions Y
in Electrophilic Aromatic Substitution
53
Examples:
54
Homework
1- Name the following compounds
...
A) p-Nitroethylbenzene
B) 2,2-Dimethyl-1-phenylbutane
C) o-Ehtylaniline
D) 1,5-Dimethylnaphthalene
E) 9-Bromoanthracene
F) 2,4-Dinitroflurorbenzene
G) 2-Benzyl-3-nitro-5-bromophenol
3- Complete the following reactions
Title: AROMATIC COMPOUNDS
Description: Well comprehensive notes on Aromatic Compounds
Description: Well comprehensive notes on Aromatic Compounds