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AROMATIC COMPOUNDS

“LET US LEARN TO DREAM

GENTELMAN, AND THEN

PERHAPS, SHALL LEARN THE

TRUTH”.

(KEKULE)

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AROMATIC COMPOUNDS

KEKULE fall asleep while sitting in front of fire,

dreamed about chains of atoms in form of twisting

snakes. one of snake caught hold of its own tail ,

forming a whirling ring.

KEKULE awoke , freshly inspired , spent remainder

night working on his now-famous hypothesis.

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AROMATIC COMPOUNDS• The term aromatic compounds was 1st used by

Kekule to classify benzene & its derivatives.

• Benzene and its derivatives, many of which possess

fragrance/ aroma.

• According to new fact, aromatic is associated with

“chemical stability” rather than aroma.

• So aromatic is used for series containing benzene

ring.

• May be called as “ Arenes and derivatives.”

• Include H-carbons, ketones, aldehydes, amines,

ethers etc.

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Benzene and itsderivatives

• Constitute the most important class of Arenes

(Aromatic hydrocarbons).

• Aryl group. By dropping H from ring.

• Aryl alkyl group. By dropping H from side chain.

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AROMATIC HALOGEN COMPOUNDS &

AROMATIC SULPHONIC ACID

• Halogen atom bonded direct to benzene ring (aryl

halide) or bonded to side chain (aryl alkyl

halide)are aromatic halogen compounds.

• One –OH group of sulfuric acid is replaced by aryl

group, compounds are called aromatic sulphonic

acids.

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AROMATIC NITRO

COMPOUNDS & PHENOLS• Derivative of aromatic hydrocarbon, H atom is

replaced by –NO2 group are called

aromatic nitro compounds.

• Compounds containing –OH group directly attach

to ring, are called phenols.

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AROMATIC AMINES &

AROMATIC CARBOXYLIC ACIDS• H atom is replaced by an amino group.

• Aniline give reactions same as

aliphatic amines.

• Compounds containing –COOH group directly

attached to ring, are called aromatic carboxylic

acids.

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AROMATIC ALDEHYDES & KETONES

• Aromatic aldehyde: -CHO group attached to

benzene ring directly.

• Aromatic ketones: -Carbonyl group attached to

two aryl groups or one aryl and one alkyl group.

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BENZENE• Molecular formula: C6H6

• No straight chain structure is possible.

• All C-atoms are sp2 hybridized.

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IDENTIFICATION TEST FOR BENZENE

• Gas chromatography : used to identify benzene,

xylene, toluene etc. (compounds that can be

vaporized without decomposition).

• Reaction with KMNO4: benzene will not show

reaction as it doesn’t oxidize while methyl benzene

will discharge colour by oxidizing to acid.

• Introducing a glowing splint will burn with smoky

flame.

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IDENTIFICATION TEST FOR BENZENE

• Add conc.H2SO4 and conc.HNO3 to benzene at

55oC,

yellow oil with bitter almond smell produced.

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PREPARATION OF BENZENE

• Benzene and other aromatic

compounds are readily obtained in

large quantities from coal and

petroleum . Benzene and some other

hydrocarbons can also be obtained

from petroleum by special cracking

methods .some of the methods

generally used for the preparation of

benzene are as following :

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GENERAL METHODS1 : Dehydration of Cyclohexane .

2 : From Acetylene .

3 : From Alkanes .

4 : Preparation in the Laboratory .

5 : Wurtz-Fittig Reaction .

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1- Dehydrogenation of Cyclohexane :

• When cyclohexane or its

derivative is dehydrogenated we

get benzene . The reaction is

carried out by the use of a

catalyst , pt or pd ,at elevated

temperature e.g, 2500c .

Cyclohexane Benzene

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2- From Acetylene :• Benzene is formed by passing

acetylene under pressure over an

organo-nickle catalyst at 700c .

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3- From Alkanes :• Benzene and toluene are also

prepared by passing the

vapours of n-hexane or n-

heptane over a mixture of

catalysts Cr2O3 +Al2O3+SiO2 at

5000C .

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4- Preparation in the Laboratory :

• Benzene can be prepared by

any of the following methods :

• * By heating sodium salt of

benzoic acid with soda lime :

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• *By distilling phenol with

zinc dust .

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• *By the hydrolysis of

benzene sulphonic acid

with superheated steam

or by boiling with dilute

HCl .

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5- Wurtz-Fittig Reaction :• The Wurtz reaction for the synthesis of

alkanes was extended by Fittig in 1864

,to the synthesis of alkyl aromatic

hydrocarbons.

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PHYSICAL PROPERTIES OF

BENZENE

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• Benzene is a colorless liquid.

• Benzene is soluble in organic solvents but immiscible in

water.

• It is an aromatic compound so it has a typical

aromatic odor. (Aroma in Greek means pleasant

smelling).

• Benzene is highly inflammable and burns with sooty

flame as compared to alkanes & alkenes which

usually burn with a bluish flame.

• Its vapors are highly toxic which on inhalation

produce loss of consciousness.

• Benzene poisoning in the longer run can prove fatal,

destroying red & white blood corpuscles.

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• Benzene is lighter than water ,specific gravity is0.87g/cm3.

• It has high melting point (5.50c)

• It has moderate boiling point(80.1)

• For homologous series m.p increases with increasingmolecular mass due to vander waal’s forces.

• All c-c bonds are equal in length of 140 pm.

• Bond angel is 120 degree.

• Each carbon is sp2 hybridized.

• Benzene also shows resonance that is it can exist indifferent forms based on the positioning of doublebond and this property of benzene makes it stable.That is why Benzene does not undergo additionreactions readily but it undergoes substitutionreactions.

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CHEMICAL PROPERTIES OF BENZENE

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REACTIONS OF BENZENE

• Electrophilic Substitution Reactions

• Addition reactions

• Oxidation Reaction

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Electrophilic Substitution reactions

Benzene undergoes electrophilic substitution reaction. The

benzene ring with its delocalized pi electrons is an

electron rich system . It is attacked by electrophiles,

giving substitution products.

These reactions can be represented as :

Where E+ is any electrophile . Such reactions in

which hydrogen atom of aromatic ring is

replaced by an electrophile are called

electrophilic aromatic substitution reactions.

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Halogenation

Benzene react with bromine in the presence ofAlBr3 or FeBr3 at room temperature to form bromo-benzene . Iron powder can be used in place offerric chloride.

Benzene Bromobenzene

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Mechanism of Halogenation

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Nitration

Benzene react with concentrated nitric acid in

The presence of concentrated sulphuric acid at

600C to form nitrobenzene.

Benzene Nitrobenzene

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Mechanism of Nitration

• The Nitro group can be reduced to an Amino group

if needed

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Sulphonation

• Benzene react with conc. sulphuric acid at 1200C

or fuming sulphuric acid at room temperature to

give benzene sulfonic acid .

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Mechanism of Sulphonation

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Friedel-Crafts Alkylation

• Benzene reacts with alkyl halides in the presence of

aluminum chloride ( AlCl3) to form alkyl benzenes.

Benzene Toluene

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Drawbacks Of Friedel-crafts AlkylationIt is useful in the synthesis of certain alkyl benzenes ,

the reaction has two serious drawbacks:

It is difficult to stop the reaction when one alkyl

group has entered the ring . Di- and tri- alkyl

benzenes are also formed.

The alkyl group often tends to rearrange .

For Example:

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Friedel-Crafts Acylation • Benzene reacts with acid chlorides in the presence

of aluminum chloride to give aromatic ketones .

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ADDITION REACTIONS

• Addition of hydrogen:

Benzene reacts with hydrogen in the presence of

nickel (or platinum) catalyst at 1500C under pressure

to form cyclohexane .

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Addition of halogens

• Benzene reacts with chlorine (or Br) in the presence

of ultraviolet light to form benzene hexachloride .

• Benzene hexachloride is a powerful insecticide.

Benzene Benzene hexachloride

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OXIDATION REACTIONS

• Ozonolysis:

Benzene reacts with ozone to give a triozonide which

on treatment with Zn/H2O yields glyoxal.

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Vapour phase oxidation

• Benzene undergoes oxidation with air/oxygen in the

presence of vanadium (V2O5) at 4500C to form maleic

anhydride.

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EFFECT OF SUBSTITUENTS

ON FURTHER SUBSTITUTION

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CONCEPT…

Some students get confused over which group

does the directing, the incoming electrophile, E+,or

the initial substituent,-X.

Try thinking about it in terms of an aircraft (the E+)

coming into land at an airport (the Ar-X)…

It is the control tower at the airport on the ground (-

X) that does the directing of which runway and

which ramp the aircraft should go to.

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SUBSTITUTION IN MONO

SUBSTITUTED BENZENES

All hydrogen atoms of the benzene ring are

equivalent. Therefore, only one mono-substitution

product (C6H5- S) is possible.

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A second substituent, E, can occupy any of the

remaining five positions

The positions 2 and 6 are equivalent, and would

give the ortho product.

The positions 3 and 5 are equivalent, and would

give the meta product.

The positions 4 is unique and would give the para

product. 46

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TWO TYPES OF INFLUENCE OF

SUBSTITUENTS

Directive or Orientation Effect.

Activity Effects.

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DIRECTIVE OR ORIENTATION EFFECT.

The first substituent (S) may direct the

next incoming substituent (E) to ortho,

meta, or para position, depending on

the nature of the first substituent.

This is called the Directive or the

Orientation (Orient: to arrange) Effect.

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ACTIVITY EFFECTS.

The substituent already present

may activate or deactivate the

benzene ring toward further

Substitution, these effects are

called the activity effects.

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Ortho = 2/5 of the total, or 40%

meta = 2/5 of the total, or 40%

para = 1/5 of the total, or 20%

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ORTHO-PARA DIRECTING EFFECT.

Certain substituents direct the second

substituent to the ortho and para positions

simultaneously. These are called Ortho-Para

Directors.

For example, when phenol is nitrated, the only

products obtained are 0-nitrophenol and p-

nitrophenol.

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The substituent –OH is said to have directed the –

NO2 group to ortho and para positions on

the ring. Therefore,-OH is designated as

ortho –para Director.

Some common ortho- para directing groups

are –Cl,-Br, -I ,-OH ,-NH2, -CH3 , -C2H5

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META DIRECTING EFFECT

The substituents which direct the

second incoming substituent primarily

to the meta position are referred to as

meta-directors.

For example, nitration of nitro benzene

gives 94% of m –dinitrobenzene, and

only 5% of ortho and 1% of p-

dinitrobenzene 54

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EFFECT OF SUBSTITUENTS ON REACTIVITY

A Substituent which activates the aromatic ring to

further substitution , is called an activating

substituent or ring activator.

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A substituent which deactivates the aromatic ring to

further substitution , is called a deactivating

substituent or ring deactivator.

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Ortho –para directors activate a ring toward

electrophilic substitution, whereas meta

directing groups deactivate a ring toward

electrophilic substitution.

Although F , Cl , Br and I are ortho para

directors ,these substituents deactivate an

aromatic ring in electrophilic substituents

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APPLICATIONS OF AROMATIC COMPOUNDS

Different aromatic compounds

are used in different industries

and for different purposes.

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USES

They are extracted from complex

mixture obtained by the refining of oil or

by distillation of coal tar. they are also

used to produce chemicals, polymers,

including nylon. Phenol and styrene.

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OTHER AROMATIC

COMPOUNDS.

BENZENE: its most widely

produce derivative including

styrene . Which is used to

make polymer and plastics.

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• Small amount of benzene are used

to make some type of rubber ,

lubricant , dyes detergent , drugs

and pesticides .

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TOLUENE

• A common solvents able to dissolvepaints, resins & rubber.

• In the preparation of saccharin &Tolbutamide (hypoglycemic agent).

• For blending petrol.

• As a starting material for benzylderivatives, Benzaldehyde, benzoicacid.

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Aromatic nitro compounds• Useful in lab reagents for the identification of

organic compounds e.g.

• 2,4- DNPH used for the identification of carbonyl

compounds.

• 2,4- dinitrofluoro benzene is widely used in protein

chemistry, commonly known as Sangers reagent.

• In the preparation of TNT which is widely used as

explosive.

• In the preparation of Nitrazepam

(used in the management of

seizures & as sedative/hypnotic)

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PHENOL

• Phenol is also used in preparation of

cosmetic including sunscreen and hair

dyes. In cosmetic surgery it serve as

exfoliator.

• In surgical procedure used to treat an

ungrown nail which is applied to nail

bed to prevent regrowth of nail.

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NAPHTHALENE

• Act as raw material in the production

of dyes and insecticides for preventing

moths in clothes.

• Also used in production of some

pharmaceutical products.

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ANTHRACENE• Anthracene is converted mainly to

anthraquinone, a precursor to dyes &

has antitumor & laxative activity.

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PHENANTHRENE• An important skeletal nucleus in bile acids,

sex hormones & cardiac glycosides.

• Codeine is used as analgesic & cough

suppressant.

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PYRENE

• Pyrene and its derivatives are used

commercially to make dyes.

• DI-BENZONE (A , L) PYRENE

• Most potent carcinogen.

• XYLENE: A mixture of xylene/

xylol is used as solvent &

diluent for lacquers.

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Benzene sulphonic acid

• In the synthesis of saccharin & azo-

dyes.

• Sulfa drugs such as sulfanilamide.

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