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PPT 102 ORGANIC CHEMISTRY 1SEM 1 (2012/2013)

Aromatic Compounds

Dr. Hayder Kh. Q. Alihayderali@unimap.edu.my

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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