Post on 08-Apr-2018
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Aromatic Compounds
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Arenes or aromatic hydrocarbons have at leastone benzene ring.
Benzene has molecular formula, C6H6 All carbon atoms are sp2 hybridised and
benzene has three bonds. The electrons aredelocalised due to resonance and this electrons delocalised system gives extra stabilityto the benzene ring.
Introduction to Arenes
Kekule structure
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Benzene is thought to be similar toalkenes but a few evidences belowsuggest otherwise (contain electrons
delocalised system). Benzene does not react with bromine in
tetrachloromethane ; and acidified
potassium manganate (VII) solution.Brown colour ofBr2 / CCl4 and purplecolour of KMnO4 / H
+ remains unchanged.
Cont- Introduction to Arenes
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Physical Properties
In the absence of polar substituents, arene
or benzene are typical of hydrocarbons :
low melting and boiling points, lowsolubility in polar solvents.
All 12 atoms, C6H6 in benzene lie in the
same plane. Benzene has a planar, cyclic,
conjugated structure.
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Cont-Physical Properties
Aromatic hydrocarbons (arenes)
resemble
other hydrocarbons. They are: nonpolar insoluble in water
less dense than water
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1)1) Benzene is considered as the parent andBenzene is considered as the parent andcomes last in the name.comes last in the name.
2)2) List substituents in alphabetical order.List substituents in alphabetical order.
3)3) Number ring in direction that gives lowestNumber ring in direction that gives lowest
locant at first point of difference.locant at first point of difference.
4)4) Simple compounds are named as substitutedSimple compounds are named as substituted
benzene derivatives.benzene derivatives.
Naming aromatic compounds
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Nomenclature ofBenzene Derivatives Benzene is the parent name for some
monosubstituted benzenes; the substituent name isadded as a prefix
For other monosubstituted benzenes, the presence of
the substituent results in a new parent name
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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
Dimethyl substituted benzenes are called xylenes
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Reactivity
Benzene (like alkene) has the nucleophilic
character of the system.
Benzenes (unlike alkenes whichundergo addition reactions) typically
undergo electrophilic substitution reactions
in which a group (usually H) is replaced
and the aromatic system is retained.
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Reactions of Arenes: Electrophilic aromatic
substitution reactions
H
E
+ E Y + H Y
Warm,fuming
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H
++ HCl
AlCl3
Cl2
Cl
Chlorobenzene
A) Halogenation of benzene
Chlorine
Reaction of benzene
Aromatic rings are less reactive towards
electrophiles than alkenes.
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Step 1
AlCl3, an electron deficient compound acts as a Lewisacid accepts lone electrons pair from one of the chlorine
atom thus induces polarisation of Cl2 molecule tu form
electrophile, Cl+ ion.
Chemistry for Matriculation 2
(Oriental Academic Publication)
Mechanism
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Step 2
Cl+ the electrophile attacks the benzene ring to form the
resonance stabilised cation intermediate which is not
stable. This is because the electron delocalised system is
destroyed.
Chemistry for Matriculation 2
(Oriental Academic Publication)
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Step 3
The intermediate loses proton, H+ to form chlorobenzene and
catalyst, AlCl3
Chemistry for Matriculation 2
(Oriental Academic Publication)
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H
++ H2O
H2SO4HONO2
NO2
Nitrobenzene
B) Nitration of benzene
Nitric acid
Reaction of benzene
Nitration of aromatic rings is a key step in the
synthesis of explosives such as TNT (trinitrotoluene),
dyes and many pharmaceutical agents.
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Chemistry Matriculation 2(Oriental Academic Publication)
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Chemisty Matriculation 2(Oriental Academic Publication)
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Chemistry Matriculation 2(Oriental Academic Publication)
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H
+ + H2O
heat
HOSO2OH
SO2OH
Benzenesulfonic acid
C) Sulfonation of benzene
Sulfuric acid
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Sulfur trioxide, SO3, in fuming sulfuric acid
is the electrophile.
SO
O OSO
O OS
O
O OS
O
O O
+ + +
_
_ _
S
O
OO
H
SO
O
OH
+
_
S
HOO
O
benzenesulfonic acid=>
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H
+ + HCl
AlCl3
C(CH3)3
tert-Butylbenzene
(CH3)3CCl
D) Friedel-Crafts Alkylation
tert-butyl chloride
(alkyl halide)
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Chemistry for Matriculation 2
(Oriental Academic Publication)
Examples:
Cl
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AlCl3 acts as a catalyst and halogen carrier accepts loneelectron pair from Cl in CH3CH2Cl to generate the
electrophile, CH3-CH2+. It is a lewis acid by accepting lone
electron pair from the chloroethane.
Step 1 (To generate the electrophile, CH3-CH2+)
Chemistry for Matriculation 2
(Oriental Academic Publication)
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Step 2 (Electrophile attacks the benzene ring)
Chemistry for Matriculation 2
(Oriental Academic Publication)
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Step 3 (Loss of proton, H+ to regain stability of the kekule structure)
Chemistry for Matriculation 2
(Oriental Academic Publication)
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Our objective for this topic!!!
Nomenclature
Structures and aromaticity
Physical properties of benzene
Reaction of benzene and toluene Halogenation Nitration
Sulphonation
Friedel-crafts alkylation
F
riedel-crafts acylation Side chain oxidation
Addition under certain conditions
Effect of subtituents
Uses
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Oxidation of alkylbenzene (side chain oxidation)
Alkylbenzene is oxidized to benzoic acid using oxidising
agents such as acidified potassium manganate (VII),warm in and acidified potassium dichromate (VI), warm.
The benzene ring remains intact.
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In order for alkylbenzene to be oxidised, it
must posses 1 benzylic hydrogen ( hydrogen
atom bonded to C atom which is bonded
directly to benzene ring).
CC C
C
[O]No reaction
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Free radical substitution of alkylbenzene
(halogenation of alkylbenzene)
Involving halogens such as Cl2 / Br2 in the presence of
an ultraviolet light or high temperature.
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Addition reaction under certain
condition: a) Catalytic Hydrogenation
Elevated heat and pressure is required.
Possible catalysts: Pt, Pd, Ni, Ru, Rh.
Reduction cannot be stopped at anintermediate stage.
CH3
CH3
Ru, 100C
1000 psi3H2,
CH3
CH3
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b)Chlorination ofBenzene
Addition to the benzene ring may occurwith high heat and pressure (or light).
The first Cl2 addition is difficult, but the
next 2 moles add rapidly.
The product, benzene hexachloride, is
an insecticide sold commercially as
Lindane
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Effect of substituents groups on the
benzene ring
Substituent groups already present in the
benzene ring can influence the rate of
electrophilic substituents and orientation of
the products formed.
There are 2 types of sustituent groups on
the benzene ring; ACTIVATING and
DEACTIVATING.
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Cont
Activating group : result higher reactivity of the
sustituted benzene towards electrophilic attack.
This is because the activating group have
electron donating effect resulting higher electrondensity of the benzene ring thus easier to be
attacked by electrophiles.
Deactivating groups : results lower reactivity of
the subtituted benzene towards electrophilicattack. The reason is opposite to that of
activating groups.
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Ortho-para directing Meta-directing
Activating -NH2, -NHR, -NR2, -OH, -OR C6H5, -R,
-NHCOR, -OCOR
Deactivating (-F, -Cl, -Br, -I) -NO2, -COOH, -CN, -COOR,
-CHO, -COR, -SO3H,-COCl, -CONH2, -CX3
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Uses and effects of aromatic compounds
vanillin(in vanilla flavour)
ibuprofen(painkiller drugs)
Adrenaline(hormone) Aspirin
BHA(Food antioxidant)
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di za t ac
z py
Similar to benzene, benzopyrene and dibenzanthracene are known as
carcinogens-chemical that cause cancer. These compounds are formedfrom heating of organic materials at high temperature. They are present in
tobacco smoke, automobile exhaust and in burned and heavily browned
foods.
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2. Propose a synthesis of each of the for folowing substances from
benzene:
a) o-bromotolueneb) m-chlorobenzoic acid
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