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PPT 102 ORGANIC CHEMISTRY 1SEM 1 (2012/2013)
Aromatic Compounds
Dr. Hayder Kh. Q. [email protected]
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Course Outline
• Aromatic Compound• Aromatic Heterocyclic Compound• Antiaromatic• Nomenclature• Electrophilic aromatic substitution• Nitration of benzene• Friedel Craft Acylation• Friedel Craft Alkylation
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Criteria for Aromaticity
1. A compound must have an uninterrupted cyclic cloud of p electrons above and below the plane of the molecule:
- For the p cloud to be cyclic, the molecule must be cyclic
- For the p cloud to be uninterrupted, every atom in the ring must have a p orbital
- For the p cloud to form, each p orbital must overlap with the p orbitals on either side of it, therefore the molecule must be planar
2. The p cloud must contain an odd number of pairs of p electrons, or 4n + 2 (n = 0, 1, 2 …) total electrons.
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Criteria for Aromaticity
a) Each carbon of benzene has a p orbitalsb) The overlap of the p orbitals form a cloud of π electrons above and
below the planec) The eleectrostatic potential map of benzene
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Hückel’s Rule
For a planar, cyclic compound in order to be aromatic, its
uninterrupted p cloud must contain (4n + 2) p electrons,
where n is any whole number.
Needs 4n + 2 (n = 0, 1, 2, 3…) electrons to fill orbitals
According to Huckel’s rule the aromatic compounds
must have 2 (n=0), 6(n=1), 10(n=2), 14(n=3) and so on π
electrons.
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Aromatic Heterocyclic Compounds
A heterocyclic compound is a cyclic compounds in which one or more of the ring
atom is an atom other than carbon.
The atom that is not carbon is called a heteroatom.
The most common hetereatoms are N, O, and S
The heteroatom donates either one or two electrons to the system
Heteroatom donates two electronsHeteroatom donates one electron
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Examples of Heterocyclic Aromatic Compounds
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A compound is classified as being antiaromatic if it fulfills the
first criterion for aromatic but does not fulfill the second.
A compound is antiaromatic if it is a planar, cyclic, continuous
loop of p orbitals with an even number of pairs of p electrons:
Antiaromaticity
Antiaromatic compounds are highly unstable, but the nonplanar
versions are stable
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[1] Aromatic: A cyclic, planar, completely conjugated compound
with 4n + 2 π electrons.
[2] Antiaromatic: A cyclic, planar, completely conjugated
compound with 4n π electrons.
[3] Not aromatic: A compound that lacks one (or more) of the four
requirements (nonaromatic) to be aromatic or
antiaromatic.
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Nomenclature of Monosubstituted Benzenes
Some are named by attaching “benzene” after the name of the substituent:
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Some have to be memorized:
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A benzene substituent is called phenyl.
A benzene substitutuent with a methylene group is called benzyl.
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Electrophilic Aromatic
Substitution Reactions
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INTRODUCTION• Although aromatic compounds have multiple double bonds,
these compounds do not undergo addition reactions.
• Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete π electron delocalization (resonance).
• Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved.
• For example, benzene reacts with bromine to form bromobenzene.
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REACTIONS ;
• NITRATION OF BENZENE
• FRIEDEL – CRAFTS ACYLATION
• FRIEDEL – CRAFTS ALKYLATION
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• An electrophile, E+, is an electron poor species that will react with an electron rich species.
• Aromatic because the reaction is characteristic of aromatic systems.
• A substitution implies that a group is replaced (usually H).
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Electrophilic Aromatic Substitution
H
H
H
H
H
H
E
H
H
H
H
HE X+ H X
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Benzene reacts with the electrophile (E+ ) forming a
carbocation intermediate.
A base in the reaction mixture (Y-) pulls off a proton
from the carbocation intermediate, and the electrons that held the
proton move into the ring to reestablish its aromaticity.
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NITRATION OF BENZENE
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EQUATION OF NITRATION OF BENZENE
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FORMATION OF ELECTROPHILE
• Nitronium ion is formed by reaction between the nitric acid and
sulphuric acid.
• Sulphuric acid protonates nitric acid.
• Protonated nitric acid loses water to form nitronium ion.
• Nitronium ion is the electrophile required for nitration.
Nitronium ion
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MECHANISM OF BENZENE NITRATION
• The electrophile, nitronium ion attaches to the ring.
• As the NO2+ ion approaches the delocalised electrons in the benzene, those electrons
are strongly attracted towards the positive charge.
• Two electrons from the benzene ring are used to form a new bond with the NO2+ ion.
• The delocalisation is partly broken, and in the process the ring gains a positive charge.
•A base, HSO4 - in the reaction mixture remove the proton from the carbon that form the
bond with the electrophile.
• The remove hydrogen from the ring form back sulphuric acid.
• The catalyst has therefore been regenerated.
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EXAMPLE:The nitration of methylbenzene (toluene)
2-nitromethylbenzene
4-nitromethylbenzene
And
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NO2
CH3
NO2
NO2
CH3
+ +3HNO33H2O
o If the reaction temperature is above 55oC, further nitration will occur which lead to the formation of 2,4,6-trinitromethylbenzene.
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question
•Why toluene is more reactive than benzene?
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ANSWERo Methylbenzene (toluene) is formed when a
methyl group is attached to the benzene ring.
o The methyl group is an electron donating group, thus, it destabilizes the benzene ring by increasing the electron density of the benzene ring.
o This allow the electrophilic aromatic substitution to take place easier.
o The electrophile can attached to the benzene ring more readily.
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THE FRIEDEL-CRAFTS ACYLATION OF BENZENE
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Friedel-Crafts Acylation• Friedel-Crafts acylation forms a new C-C bond
between a benzene ring and an acyl group.
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Friedel-Crafts Acylation
• The electrophile is an acylium ion.
R-C ClO
Cl
ClAl-Cl
OR-C Cl Al Cl
Cl
Cl
O
R-C+ AlCl4-
Aluminumchloride
An acyl chloride
A molecular complexwith a positive charge
charge on chlorine
An ion pair containing an acylium ion
+ -
••
•• •• +(1)
(2)
••••
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Friedel-Crafts Acylation– An acylium ion is represented as a resonance hybrid
of two major contributing structures.
• Friedel-Crafts acylations are free of major limitation of Friedel-Crafts alkylations; acylium ions do not rearrange, do not polyacylate (why?), do not rearrange.
:+ +
complete valence shells
The more importantcontributing structure
O OR-C R-C::
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FRIEDEL-CRAFTS ALKYLATION
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friedel-crafts alkylation
• Reaction develop by
Charles Friedel and James Mason Crafts
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introduction
Friedel- Crafts alkylation involves alkylation of an aromatic ring with an alkyl
halide using a strong Lewis acid catalyst,
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• Friedel-Crafts alkylation substitutes an alkyl group for a hydrogen
R = alkyl group
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GENERATION OF THE ELECTROPHILE
Formation of carbocation from the reaction of an alkyl halide (R-Cl) with (AlCl3).
Alkyl chloride, alkyl bromide and alkyl iodide can also be used
Vinyl halide and aryl halide cannot be use, because their carbocation are too unstable to be formed.
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Rearrangment of carbocation
• The reactive electrophile, the carbocation is prone to rearrangement to a more stable carbocation which will then undergo the alkylation reaction.
1) 1,2-hydride shift
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Continue2) 1,2-methyl shift
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THE MECHANISM:
1) The attack of nucleophile :
• The π electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+.
• This step destroys the aromaticity giving the cyclohexadienyl cation intermediate
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Continue…
2) Protonation and regeneration :
• Removal of the proton from the sp3 C bearing the alkyl- group.
• Reforms the C=C and the aromatic system,
• Generating HCl and regenerating the active catalyst.
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Biological application
A Friedel-Crafts alkylation reaction involve in one of the step to in the
biosynthesis of Vitamin KH2, the coenzyme required to form
blood clots.
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questions
1)The strength of the polarization of the activated complex depends on the alkyl residue as well as on the ...
Please make a choice.
a. Lewis acid.
b. Lewis base.
c. Conjugate base
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Continue…
?2) What is the major product from the reaction ?
a. b.
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THE END