B.Sc. Semester VI - ChemZone

Prepared By: Dipen Shah B.Sc. / MATERIAL / SEM-VI / Chemistry - 602 / Unit-1 Page: 1 of 16

B.Sc. Semester – VI

Subject: - CHE - 602: Polynuclear Aromatic Hydrocarbons

Prepared By: - Dipen Shah

Contents:

Introduction

Synthesis and Chemical Properties of:

A. Biphenyl

B. Diphenyl methane

C. Naphthalene

D. Anthracene

Introduction

Two or more aromatic (benzene or non-benzene) rings are fused together its called

polynuclear aromatic hydrocarbon (PAH).

Polynuclear aromatic hydrocarbon potential health risk due to their inner chemical

stability, high reactivity to different types of degradation and high toxicity to living

organisms.

Physical and Chemical Properties of Polynuclear Aromatic Hydrocarbon

PAHs as pure chemicals exist as colorless, white, or pale yellow-green solids.

They are non-polar, hydrophobic compounds, which do not ionize.

They have a faint odor.

Enter the environment (Air, Water and Soil)

PAHs are introduced into the environment mainly via natural and anthropogenic burning

processes.

PAHs enter air as releases from forest fires, residential wood burning and exhausts from

vehicle.

Some PAH particles can readily evaporate into the air from soil or surface waters.

They can also enter surface water through discharges from industrial plants and

wastewater treatment plants.

Most of PAHs don‘t dissolve easily in water.

They stick to solid particles and settle to the bottoms of lakes or rivers.

PAHs in soils also contaminate underground water.

We are most likely to be exposed to PAH vapors or PAHs that are attached to dust and

other particles in the air.

Sources include cigarette smoke, vehicle exhausts, asphalt roads, coal, etc.

PAHs effect on human and animal body

Breathing or touching mixtures of PAHs and other chemicals for long periods of time have

developed cancer in human body.

Some PAHs have caused cancer in laboratory animals when they breathed air containing

them (lung cancer), ingested them in food (stomach cancer), or had them applied to their

skin (skin cancer).

In the body, PAHs are changed into chemicals that can attach to substances within the

body. Special tests that can detect PAHs in body tissues or blood.

Analytical determination of PAHs

Samples of PAHs are mostly analysed by HPLC using fluorence detection, or by gas

chromatography method with flame ionizazion detection, or mass spectrometry.


Classification of Polynuclear Hydrocarbon

Polynuclear Hydrocarbons

Benzenoid Non-Benzenoid

Isolated Rings Fused Rings

Linear Angular

Isolated Polynuclear Hydrocarbon

One or more benzene ring which are either isolated from each other or attached to each

other (via one carbon) it’s called Isolated PAHs.

They have independent benzene ring.

They are also known as polyphenyl compounds.

Examples: Biphenyl, Terphenyl, Diphenylmethane, Triphenylmethane etc.

Fused (Condensed) Polynuclear Hydrocarbon

Two benzene ring are fused with each other at two common points (Ortho to each other)

it’s called fused PAHs.

The two benzene rings two carbon atoms have common.

A large number of fused polynuclear hydrocarbon are carcinogenic.

Examples: Naphthalene, Anthracene, Phenanthrene etc.

A. Biphenyl

Introduction

Biphenyl is an organic compound that forms colorless crystals. It has a distinctively

pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula C12H10.

It is notable as a starting material for the production of polychlorinated biphenyls (PCBs),

which were once widely used as dielectric fluids and heat transfer agents.


Biphenyl is also an intermediate for the production of a host of other organic compounds

such as emulsifiers, optical brighteners, crop protection products, and plastics. Biphenyl

is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule

consists of two connected phenyl rings.

Synthesis

Many reactions have been reported for the biphenyl. Among them some reactions are given below.

By Fitting reaction (extension of Wurtz reaction)

Diphenyl can be synthesized by heating an ethereal solution bromobenzene with metallic sodium.

By the reaction between bromobenzene and hydrazine

Diphenyl is obtained by refluxing an alkaline solution of ethanolic bromobenzene and hydrazine in presence of palladium catalyst supported over CaCO3.

From aryl magnesium halide

Aryl magnesium halides on reaction with aryl halides in presence of a small amount of CoCl2, NiCl2, or FeCl3 give polynuclear hydrocarbon. For example, phenyl magnesium bromide on reaction with bromobenzene in presence of CoCl2 gives diphenyl.

Ullmann reaction

The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions.

By Suzuki reaction

The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions.

Physical properties of diphenyl

Colourless crystalline solid compound

Molecular formula C12H10

Molecular Weight 154 g/mole

Melting Point 69-71°C

Insoluble in water but soluble in organic solvent


Chemical properties of diphenyl or Chemical reaction of diphenyl

Diphenyl can be regarded as phenyl substituted benzene, it gives almost the same

reaction as observed with benzene. The phenyl group cause the substitution to take place

mainly at the para position and a small proportion of ortho substituted product is

obtained. Here one phenyl group act as an electron releasing and the other as an electron

accepting group. The second substitution generally goes to the un-substitution phenyl

group.

Biphenyl shows aromatic character with remarkable stability. It can be represented in

several contributing structures as shown below:

Nitration

Nitration of diphenyl gives mainly 4-nitrodiphenyl and negligible proportion of 2-

nitrodiphenyl. Further nitration gives 4,4’-dinitrodiphenyl main product and negligible

amount of 2,2’-dinitrodiphenyl and 2,4’-dinitrodiphenyl.

Halogenation or Chlorination of Diphenyl

Diphenyl is chlorinated in presences of Fe gives 4-chlorodiphenyl as main product and 2-

nitrodiphenyl as negligible proportion. Further chlorination gives 4,4’-dichlorodiphenyl

and negligible amount of 2,2’-dichlorodiphenyl and 2,4’-dichlorodiphenyl.


Sulphonation

Sulphonation of diphenyl using concentrated H2SO4 as a sulphonating agent gives

diphenyl-4-sulphonic acid main product (by product diphenyl-2-sulphonic acid is

obtained in negligible amount) which on further sulphonation gives diphenyl-4,4’-

disulphonic acid (by product diphenyl-2,2’-disulphonic acid diphenyl-2,4’-disulphonic

acid are obtained in negligible amount).

Oxidation

Biphenyl is oxidized with chromic acid undergoes oxidation to give mainly CO2 and H2O

along with small amount of benzoic acid. However, ozonolysis of biphenyl at about -20 °C

gives mainly benzoic acid (80 %).

Uses of Diphenyl

Use in production of polychlorinated biphenyls (use as plasticizers).

Widely used as dielectric fluids and heat transfer agents.

Intermediate for the production of a host of other organic compounds such as emulsifiers,

optical brighteners, crop protection products, and plastics.

B. Diphenylmethane

Introduction

Diphenylmethane is an organic compound with the formula (C13H12). The compound

consists of methane wherein two hydrogen atoms are replaced by two phenyl groups.

Diphenylmethane forms a common skeleton in organic chemistry; the diphenylmethyl

group is also known as benzhydryl.

Synthesis


By Friedel Crafts Reaction

Friedel Crafts alkylation of benzene on with benzyl chloride in presence of anhydrous AlCl3 yields diphenylmethane.

By the reaction with benzene with formaldehyde

Diphenylmethane can be also synthesized by the reaction between benzene and formaldehyde in presence of concentrated H2SO4.


From benzyl alcohol and benzene

Diphenylmethane can be obtained by heating a mixture of benzyl alcohol and benzene with H2SO4 and CH3COOH.

From Grignard reagent

Reaction of benzyl chloride with phenylmagnesium bromide gives diphenylmethane.

Physical properties of diphenylmethane

Colourless oil



Melting Point 22-24 °C


Chemical properties of diphenylmethane or Chemical reaction of diphenylmethane

The chemical reactions of diphenylmethane are similar to that of biphenyl.

Diphenylmethane may be considered as benzyl substituted benzene. The benzyl group is

an ortho-para (o-p) directing group. The mono-substituted product is mainly 4-

substituted and the second substituent mainly enters p’ or 4’ position.

In diphenylmethane both the benzene rings are electron attracting groups making the –

CH2 group highly reactive. Hence in some reactions, like bromination, instead of

substitution at the phenyl group the hydrogen of the methylene group is substituted.

Nitration

Diphenylmethane on reaction with conc. HNO3 and conc. H2SO4 undergoes nitration. The

first –NO2 group enter the 4 or p position and giving p-nitro diphenylmethane. The second

–NO2 group enter the 4’ or p’ position of the second ring and giving p,p’ dinitro

diphenylmethane.

Bromination

In diphenylmethane both the benzene rings act as negative groups, hence on reaction

with Br2 the –H of the –CH2 group being highly reactive undergoes substitution giving

diphenyl methyl bromide.


Chlorination

Chlorination of diphenylmethane in presence of Fe gives 4,4’-dichloro diphenylmethane.

But chlorination of diphenylmethane in presence of sunlight gives diphenyl

dichloromethane.

Oxidation

Oxidation of diphenylmethane with chromic acid gives benzophenone. In this reaction

methylene group is converted to a carbonyl group (>C=O).

Cyclization

On passing vapours of diphenylmethane through red hot iron pipe cyclization take place

and converting diphenylmethane to flurorene.

Uses of Diphenylmethane

It has pleasant odour (like orange) and hence is used in making soaps, shampoo etc.

C. Naphthalene

Introduction

Naphthalene is an organic compound with molecular formula C10H8. It has simplest

polycyclic aromatic hydrocarbon, and is a white crystalline solid with characteristic odor.

As an aromatic hydrocarbon, naphthalene’s structure consists of a fused pair of benzene

ring.

Synthesis


From 4-phenylbut-1-ene

Naphthalene can be obtained by passing vapours of 4-phenyl-1-ene over red hot calcium oxide.


From 4-phenylbut-3-enoic acid

On heating with concentrated H2SO4 4-phenylbut-3-enoic acid undergoes cyclization to from 1-naphthol, which on heating with Zn dust yields naphthalene.

By Haworth reaction

This method consists of condensing benzene with succinic anhydride in presence of

anhydrous AlCl3 (friedel craft reaction) to from keto acid which is subsequently

reduced, heated with concentrated H2SO4, again reduced and finally dehydrogenated with

selenium to give naphthalene.

Physical properties of naphthalene

White crystalline solid compound




Insoluble in water but soluble in ether, benzene, and hot alcohol

Characteristic tar like odour and volatile

Consists of two benzene rings fused at ortho position in which like benzene each carbon

atom is sp2 hybridize and forms 3σ bonds in the same plane.

Each carbon atom has an electron in the p-orbital that is not involved in hybridization.

Due to overlapping of p-orbitals a π-electron cloud is formed that is spread equally over

both the rings.

The resonance energy of naphthalene is 61.0 kcal/mole while that the benzene is 36

kcal/mole. Naphthalene fused with two benzene ring so theoretically resonance energy is

72.0 kcal/mole. The deference in the resonance energy is 11.0 kcal/mole. Thus

naphthalene undergoes addition, oxidation and reduction reaction more rapidly than

benzene.

Chemical properties of naphthalene or Chemical reaction of naphthalene

Reactivity of Naphthalene

Naphthalene undergoes electrophilic substitution reaction to form theoretically two

different product α and β. Generally, the α-isomer predominates except in case of

sulphonation and friedeal craft reaction.

Electrophilic substitution reaction depends on two steps:

1. Formation of carbocadions – rate determination step

2. Joining of the electrophile to the carbocation.


When the electrophile attacks α-position, the carbocation formed is a resonance structure

I and II. In both this structures the positive charged remains on the ring which is

attacked while the other ring maintains its aromatic character. Hence both the structures

are equally stable.

However, when the electrophile attacks the β-position the carbocation formed is a

resonance structure III and IV. In structure III the positive charge remains on the ring

that is attacked and the other ring maintain its aromatic character (hence more stable)

while in structure IV the aromatic character of both the rings is disrupted (hence less

stable).

Orientation

Whether the second substituent would enter the mono-substituted naphthalene ring and

if so at what position depends on the nature and position of the first substituent.

Generally, the first substituent is at the α-position (exception sulphonation and friedeal

craft reaction). On the basis of the nature of the first substituent (electro releasing or

electro withdrawing) the second substituent would occupy certain specific position.

Group present Position of 1st Substitution

Position of 2nd Substitution

Type

-CH3, -OH, -NH2, -Cl, -Br, -NHR, -NHCOCH3

α (1-position) 4th – Major

2nd – Minor Homonuclear

-CH3, -OH, -NHCOCH3 β (2-position)

1-position Homonuclear

If incoming group is –SO3H

than at 6-position Heteronuclear

-NO2, -SO3H, -COOH

α (1-position)

or

β (2-position)

5th and 8th position Heteronuclear

-X, -NH2 β (2-position) 5th and 8th position Heteronuclear

Substitution reaction

Nitration

Nitration of naphthalene at 50-60 °C gives 1 or α-nitronaphthalene. Now since the –NO2

group deactivates the ring further reaction take place in the second ring giving mainly

1,8-dinitronaphthalene along with some 1,5-dinitronaphthalene.


Halogenation (Bromination and Chlorination)

Naphthalene undergoes bromination on reaction of Br2 in boiling CCl4 or CH3COOH as

solvent to give 1-bromonaphthalene which on further bromination gives 1,2-

dibromonaphthalene (minor product) and 1,4-dibromonaphthalene (major product).

Reaction naphthalene with thionyl chloride (SOCl2) and sulfurly chloride (SO2Cl2) in

equimolar proportion at 25 °C in presence of AlCl3 gives 1-chloronaphthalene. However at

100-140 °C in the ration of 1:2 further chlorination gives 1,2-dichloronaphthalene (minor

product) and 1,4-dichloronaphthalene (major product).

Sulphonation

Sulphonation of naphthalene by concentrated H2SO4 at 80 °C gives mainly naphthalene-

1-sulphonic acid. This reaction is reversible. When naphthalene-1-sulphonic acid is

heated its converted to naphthalene-2-sulphonic acid. However, sulphonation at a higher

temperature of 160 °C gives mainly naphthalene-2-sulphonic acid.

Friedel Craft Acylation

Acetylation of naphthalene gives a mixture of 1 and 2 isomers. When the reaction is

carried out in carbon disulfide (CS2) solvent 75 % 1-actyl naphthalene and 25 % 2-actyl

naphthalene is obtained. When in nitrobenzene solvent it gives 10 % 1-acetyl naphthalene

and 90 % 2-acetyle naphthalene is obtained.


Oxidation reaction

With HgSO4 and H2SO4

Oxidation of naphthalene with HgSO4 and Concentrated H2SO4 or V2O5 in presence of air

gives phthalic acid.

By alkaline KMnO4

Oxidation of naphthalene by alkaline KMnO4 gives phthalonic acid.

By acidic KMnO4

Oxidation of naphthalene by acidic KMnO4 gives phthalic acid.

By chromic acid

Oxidation of naphthalene with chromic acid gives 1,4-naphthaquinone.

With ozone

On reaction with ozone naphthalene forms diozonide, which on further hydrolysis forms

phthalyldehyde.

Addition reaction

Reaction with Na and ethanol

Naphthalene on reaction with sodium and ethanol undergoes reduction (addition of H2)

giving 1,4-dihydronaphthalene (1,4-dialine) which is unstable and rapidly undergoes

dehydrogenation giving back naphthalene.


Reaction with Na and isopentanol

Reduction of naphthalene with sodium and isopentanol gives 1,2,3,4-tetrahydronaphthalene

called tetralin.

Reaction with H2/Ni

On reaction with naphthalene in presence of nickel as a catalyst. First tetralin and finally

dehydronaphthalene (decalin) is obtained.

Reaction with dry Cl2

Naphthalene undergoes addition reaction with dry Cl2 giving 1,2-dichloro-1,2-

dihydronaphthalene and 1,2,3,4-tetrachloro-1,2,3,4-tetrahydronaphthalene.

Reaction with Sodium

Addition of naphthalene with sodium 1,4-disodium-1,4-dihydronaphthalene, which react

with CO2 giving sodium salt of 1,4-dihydronaphthalene-1,4-dicarboxylic acid.

Uses of Naphthalene

Important source of phthalic acid and anthranilic acid which constitute the intermediates

of important dyes such as indigo, triphenylmethane and azodyes.Use in production of

polychlorinated biphenyls (use as plasticizers).

Used as moth repellent and insecticide.

The reduced forms naphthalene i.e. decalin and tetralin are used in motor fuel and as

lubricants and also as solvents.


D. Anthracene

Introduction

Anthracene is solid PAH of formula C14H10 consisting of three benzene rings. It is a

component of coal tar. Anthracene is used in the production of the red dye alizarin and

other dyes. Anthracene is colorless but exhibits a blue fluorescence under ultraviolet

light.

Synthesis


By Friedel Crafts Reaction from benzyl chloride

Friedel Crafts reaction of two moles of benzyl chloride in presence of anhydrous AlCl3 gives 9,10-dihydroanthraquinone which readily undergoes dehydrogenation to give anthracene.

By Friedel Crafts Reaction from benzene and methylene dibromide

Anthracene can be also obtained by the reaction of benzene with methylene dibromide in presence of anhydrous AlCl3.

By Fitting reaction

In this reaction o-bromobenzylbromide is heated with Na in presence of ether to give dihydroanthracene which on further oxidation gives anthracene. By this reaction phenanthrene may be obtained as by product.

From phthalic anhydride and benzene

The reaction between phthalic anhydride and benzene gives o-benzyl benzoic acid. On

heating with H2SO4 o-benzyl benzoic acid undergoes cyclization to give anthraquinone which on further distillation with Zn dust gives anthracene.

From phthaloyl chloride and benzene

Reaction of phthaloyl chloride and benzene in presence of anhydrous AlCl3 gives anthraquinone which on distillation with Zn dust gives anthracene.


Physical properties of anthracene

Colourless solid





More reactive than benzene and naphthalene

Exception reactivity of 9,10 position (points of maximum electron density)

Chemical properties of anthracene or Chemical reaction of anthracene

Isomerism and anthracene derivatives

Anthracene forms three monosubstituted isomers 1 or α, 2 or β, and 9 or γ.

In this disubstituted anthracene, if both the substituents are identical there are 15

isomers but more isomer if the substituents are different.

Addition reaction

Reduction in presence of Na and ethanol

Reduction of anthracene with sodium and ethanol gives 9,10-dihydroanthracene which

on reaction with concentrated H2SO4 give back anthracene.

Reduction with H2/Ni

On reduction with Ni at 200-250 °C anthracene gives gradually tetra, hexa, octa and

finally deca or perhydroanthracene (C14H24).

Addition with 1 mole of O2

Anthracene undergoes addition reaction with one mole of oxygen in presence of sunlight

to give anthracene peroxide.

Dimerization of anthracene

Anthracene in its saturated solution in xylene undergoes dimerization in sunlight giving a

dimer of anthracene.


Substitution reaction

Halogenation

Chlorination of anthracene: On passing chlorine gas through a cold solution of

anthracene in CS2 anthracene first undergoes an addition reaction forming 1,2-dichloro-

1,2-dihydroanthracene which on heating or on reaction with alkali gives 9-

chloroanthracene. 9-chloroanthracene can be also obtained by the reaction of anthracene

with chlorine at 100 °C.

Anthracene can also undergo bromination on reaction with Br2 the reaction take place in

boiling CCl4

Nitration

Anthracene undergoes nitration on reaction with HNO3 in acetic anhydride at 15-20 °C

giving 9-nitroanthracene and 9,10-dinitroanthracene.

Sulphonation

Anthracene undergoes sulphonation easily giving a mixture of anthracene-1-sulphonic

acid. If the reaction is carried out at mild conditions the 1and 2 isomer obtained in

equimolar proportion but at high temperature the major product is anthracene-2-

sulphonic acid.

If the reaction is carried out using excess H2SO4 at low temperature anthracene-1,8-

disulphonic acid is obtained and at high temperature anthracene-2,7-disulphonic acid is

obtained.


Acylation

Friedel craft reaction of anthracene with acetyl chloride in benzene or nitrobenzene gives

a complex mixture. However, the main product in nitrobenzene solvent is 1-acetyl

anthracene while in ethylene dichloride solvent the product is 9-acetyl anthracene.

Oxidation reaction

Oxidation with HNO3 and chromic acid

Oxidation of anthracene by concentrated HNO3 or chromic acid or vanadium pentoxide in

air gives anthraquinone.

Uses of Anthracene

Anthracene is used in the synthesis of dyes like anthraquinone and alizarin.

Best of Luck

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B.Sc. Semester VI - ChemZone