Post on 07-Apr-2018
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Jog your memory !!!
Test 1 week 4 (30th Sept. 2011)
Midterm exam week 9 ( 31stOct. 2011)
Test 2 week 12 (21st Nov. 2011)
Final exam week 16 ( 12th Dec. 2011)
Viel gluck
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Aromatic Compound
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Learning Objectives
1. Determine the nomenclature and the physical propertiesof an aromatic compound
2. Determine the reaction of benzene
3. Propose a mechanism on Chlorination and Bromination,Nitration, Friedel-Crafts alkylation and acylation
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*monosubstituted
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Aniline
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*disubstituted
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*polysubstituted
4-chloro-2-nitrotoluene
2,4,6-tribromophenol
2-bromo-1-ethyl-4nitrobenzene
3,4-dimethoxybenzaldehyde
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Naming the Aromatic Hydrocarbon
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Physical Properties of Benzene
*Colorless liquid with Melting point : 5.5 oC; Boiling point : 80oC
*Benzene burns, and has a toxic vapor
*Immiscible with water but dissolved in organic solution
Sources of Benzene
Benzene is obtained from petroleum oil by fractional distillation followed
by reforming, and also by destructive distillation of coal.
Reactivity of Benzene
The benzene ring is a planar hexagon with a
cloud of delocalised electrons lying aboveand below the ring.
The reactions of benzene usually involve the
attack of an electrophile on the cloud of
electrons
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Reaction of alkenes
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b)Addition Reactions of Benzene
a)Oxidation Reactions
Benzene is unaffected by strong oxidizing agents (H2
CrO4
& KMnO4
)However, side chain methyl group is oxidized to a carboxyl group to give
carboxylic acid
Eg; C6H5CH3 C6H5COOH + H2O
Hydrogenation and Chlorination are undergo the addition reaction of
benzene
Eg; C6H6 + 3H2 C6H12
Eg; ; C6H6 + 3Cl2 C6H5Cl 6
KMnO4, OH-
H3O+,
Ni, 200C
h
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1. Halogenation of benzene and toluene
- Cl2
and Br2
can be substituted in the ring in the presence of a Friedel-Crafts
catalyst which is called a halogen-carrier
C6H6 + X2 C6H5X + HX
Eg; Benzene + Br2 Bromobenzene + HBr
Eg; Toluene + Cl2 o-chlorotoluene + p-chlorotoluene + HCl
! Mechanism
c) Substitution Reactions
Fe/FeX3
Undergoes electrophilic substitution where, electrophile attack electron (at
the aromatic ring) caused substitution of proton
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Mechanism
1. Lewis acid (eg. FeBr3) accepts a pair of electrons from Br atom the
polarised Br2 molecule and enables Br-Br bond to split.
2. A bromonium ion and FeBr4- complex ion are formed.
3. The bromonium ion rapidly loses a proton to form bromobenzene with
a regeneration of the catalyst, FeBr3
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Effects of a Substituent Group on further
substitution
Affect the orientation of new group.
~classified as ortho-para directing or
~classified as meta directing
Affect the rate of further substitution
~classified as activating (caused the rate of second substitution
to be faster than that for benzene itself)or
~classified as deactivating (caused the rate of second
substitution to be slower than that for benzene itself)
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Ortho-Para directing Meta directing
Activating
-NH2. NHR, -NR2, -OH,
-OR,-C6H5, -R
___
Deactivating Halogen (-F, -Cl, -Br, -I)-NO2, -COOH, -CN, -COOR,
-SO3H, -CHO, -COR,
-CONH2, -NH3+, -CF3, -CCl3
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2. Nitration of benzene and toluene
- Substitution of a H atom by a NO2 group
Eg; Benzene + HNO3 Nitrobenzene + H2O
Eg; Toluene + HNO3 o-chlorotoluene + H2O +
p-chlorotoluene
! Mechanism-Formation of nitryl cation (or nitronium ion), NO2
+
H2SO4
H2SO4
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- NO2+ is an electrophile attacks the ( electron) benzene ring to give resonance
stabilized cation intermediate
-This intermediate rapidly loses a proton to form the product
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i. Alkylation
-An alkyl group can be introduced into benzene ring by the reaction of ahalogenoalkane with benzene:
3. Friedel-Craft reaction
An electrophilic aromatic substitution in which a hydrogen of an aromatic
ring is replaced by an alkylor acylgroup
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Mechanism
1. Formation of an ion complex:
2. A carbocation complex attacks benzene to form an intermediate
which loses a proton to form product:
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ii. Acylation
-Introduction of an acyl group :
-Acylation needs acyl halide (RCOCl) + Friedel-Craft catalyst
C
O
R
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Mechanism
1. F-C catalyst + RCO form a complex
2. The elecrophilic complex attacks benzene to form a reactive
intermediate which rapidly converted into the product
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Thank you