(8) Aromatic compounds

8/7/2019 (8) Aromatic compounds

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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.




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Step 3

The intermediate loses proton, H+ to form chlorobenzene and

catalyst, AlCl3




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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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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+)




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Step 2 (Electrophile attacks the benzene ring)




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Step 3 (Loss of proton, H+ to regain stability of the kekule structure)




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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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(8) Aromatic compounds

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