Chapter 22 Aromatic Hydrocarbons Part 2 by TEACHING CARE Online coaching and tuition classes

Chapter 22 Aromatic Hydrocarbons Part 2 by TEACHING CARE Online coaching and tuition classes

File name : Chapter-22-Aromatic-Hydrocarbons-Part-2.pdf

 

 

(2)  Directive effect in disubstituted benzene

  • If the directive effects of two substituents reinforce, then a single product is

 

 

Example :

CH3

 

;

 

NO2 (m)

OH

 

 

 

NO2

 

 

Nitration

+NO2

OH

 

 

 

NO2

NO2

 

Thus, both (CH3, NO2) direct further substitution to the same position (Orth).

  • If the directing effect of two groups oppose each other strongly activating groups win over deactivating or weakly activating The sequence of directing power is

 

  • NH 2 > –OH > –OCH3 – > NHCOCH3  > –C6 H5  > CH3  >

meta directors

 

 

 

Example :

OH

Directs

OH         OH

Directs (Powerful activator)

;

OH                                OH

Br

Br2 FeBr3

 

CH3

Directs

CH3     CH3

Directs

CH3

CH3

 

  • There is normally little substitution when the two groups are meta to each other. Aromatic rings with three adjacent substituents are generally prepared by same other

CH3       Too hindered position

 

 

 

Toluene is the simplest homolouge of benzene. It was first obtained by dry distillation of tolubalsam and hence named toluene. It is commercially known as tolual.

 

(1)  Methods of preparation

  • From benzene [Friedel-craft’s reaction] :

 

Alkyl halide employed may undergo an isomeric change

Note : ®

3

Benzene

+  CH Cl          AlCl3

CH3

Toluene

+        HCl

 

 

C H + ClCH CH CH

¾¾AlC¾l3  ®  C  H  CH           CH3  + HCl

 

6     6                  2        2        3

n

6    5              CH3

 

Propyl chloride

Isopropyl benzene (65 -70%)

 

  • Catalysts can be used in place of anhydrous

AlCl3 are,

 

AlCl3 > SbCl3  > SnCl4  > BF3  > ZnCl2  > HgCl2

 

  • Wurtz fitting reaction :

Br  +  2Na +

BrCH3

Ether                          CH

+ 2NaBr

 

 

3

Bromobenzene                          Methyl bromide                Toluene

 

 

  • Decarboxylation :

C6 H4

CH3

COONa

  • NaOH ¾¾Sod¾a lim¾e ® C6

H5 CH3

  • Na

2CO3

 

 

(o-,m- or p-) Sodium toluate

Toluene

 

 

 

 

  • From cresol :

CH3

OH

+   Zn        heat

CH3

 

+   ZnO

 

 

 

o-Cresol                                               Toluene

 

CH3

CH3

 

 

 

  • From toluene sulphonic acid :

SO3H

+   HOH                                       + H2SO4

Toluene

 

 

CH3

p-Toluene sulphonic acid

 

CH3

CH3

 

  • From toluidine :

NH2

NaNO2

HCl

N2Cl

C2H5OH

Toluene

+ N2  + CH3CHO + HCl

 

p-Toluidine       p-Toluene diazonium chloride

MgBr

CH3

 

  • From grignard reagent :

+ CH3Br

Phenyl magnesium bromide

Toluene

+ MgBr2

 

  • Commercial preparation

From coal tar : The main source of commercial production of toluene is the light oil fraction of coal-tar. The

 

light oil fraction is washed with conc.

HSO4

to remove the bases, then with NaOH to remove acidic substances

 

and finally with water. It is subjected to fractional distillation. The vapours collected between

80 – 110°C

is 90%

 

benzol which contains

70 – 80%

benzene and

14 – 24%

toluene. 90% benzol is again distilled and the portion

 

distilling between 108 – 110°C

is collected. It is toluene.

CH3

 

  • From n- heptane and methyl cyclohexane

 

H2C H2C

|

CH2

 

 

 

CH2

 

CH3 CH2

 

 

Cr2O3 / Al2O3

 

500-550°C

150 atms

CH3

Toluene

 

(2)  Physical properties

n-Heptane

 

  • It is a colourless mobile liquid having characteristic aromatic odour.
  • It is lighter than water ( gr. 0.867 at 20°C).
  • It is insoluble in water but miscible with alcohol and ether in all
  • Its vapours are It boils at 110°C and freezes at –96°C.
  • It is a good solvent for many organic
  • It is a weak polar compound having dipole moment 4D.
  • Chemical properties : Toluene shows the behavior of both

CH3                        Side chain (Aliphatic)

 

Benzene ring (Aromatic)

 

 

  • Electrophilic substitution reactions : Aromatic character (More reactive than benzene) due to electron

 

releasing nature of methyl group.

CH3

CH3                 CH3

 

+   E Å

Electrophile

E       +

o-Derivative                        E

 

Note : ® E+ may be chlorine,

HNO3 , H2SO4 ,CH3Cl  .

p-Derivative

 

  • Reactions of side chain

CH3                CH2Cl                             CHCl2

CCl3

 

  • Side chain halogenation :

Cl2 UV

Cl2 UV

Cl2 UV

 

Toluene                    Benzyl chloride                      Benzal chloride                    Benzo trichloride

 

Note : ® Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol.

 

C6 H5CH2Cl

(Phenyl methyl chloride)

  • NaOH ¾¾® C6 H5CH2OH + NaCl

 

  • Benzal chloride on hydrolysis forms

 

C6 H5 CHCl2 + 2NaOH ¾¾®

(Benzylide chloride)

C6 H5 CH(OH)2 + 2NaCl

¯

C6 H5CHO+H2O

 

  • Benzo trichloride on hydrolysis forms benzoic

 

C6 H5CCl3

(Benzylidyne chloride)

 

  • Oxidation :
  • 3NaOH ¾¾® C6 H5C(OH)3 + 3NaCl

¯

C6 H5COOH+ H2O

CH3                          COOH

 

  • With hot acidic KMnO4 :

 

 

Toluene

KMnO4 / H+

3[O]

 

 

Benzoic acid

+ H2O CH3

CHO

 

  • With acidic manganese or chromyl chloride (Etards reaction) :

+ 2[O]

CrO2C2

+  H2O

 

 

 

 

Note : ® All alkyl benzenes on oxidation with hot acidic length of the side chain does not matter.

R

Toluene

 

KMnO4  or

 

R

Na2Cr2O7

Benzaldehyde

 

form benzoic acid. The

 

 

  • Hydrogenation :

+    3H2

Na / liquid NH3 – C2H5OH

Birch reduction

 

 

 

Alkyl benzene

CH3

|

C

HC               CH

HC               CH

 

 

 

+    3H2

 

 

 

 

 

Ni

200°C

Alkyl cyclohexane

CH3

|

CH

H2C               CH2

H C               CH

 

2                         2

CH                                                                               CH2

Methyl benzene                                                            Methylcyclohexane

 

 

  • Combustion : C6 H5CH3 + 9O2 ¾¾®7CO2  + 4 H2O

 

  • Ozonolysis :

H

C  CH

 

 

+3O

O       O

O

O          CH

C            CH                     Zn

3

®                                         O

 

CH–C=O

 

CHO

+ 3H O

 

C            CH

HOH

+2                  2   2

H – C=O          CHO

 

HC            CH C

H

O          C                 O O      O

Methyl glyoxal

Glyoxal

 

(4)         Uses

Toluene

Triozonide

 

  • In the manufacture of benzyl chloride, benzal chloride, benzyl alcohol, benzaldehyde, benzoic acid, saccharin,
  • In the manufacture of trinitrotoluene (TNT), a highly explosive
  • As an industrial solvent and in
  • As a petrol
  • In the manufacture of certain dyes and

T.N.T. (Tri-nitro toluene)

 

 

Preparation :

CH3

 

 

Toluene

 

+  3HNO3

Fuming

 

 

H2SO4

CH3

O2N                   NO2

 

 

NO2

 

+     3H2O

 

Properties : It is pale yellow crystalline solid (M.P. = 81°C).

Uses : · It is used as an explosive in shells, bombs and torpedoes under the name trotyl.

  • When mixed with 80% ammonium nitrate it forms the explosive amatol.
  • TNT is also used as a mixture of aluminium nitrate, alumina and charcoal under the name ammonal.

T.N.B. (Tri-nitro benzene)

 

 

 

Preparation :

CH3

 

 

Toluene

 

H2SO4 HNO3

CH3

O2N                   NO2

 

 

NO2

 

K2Cr2O7 H2SO4

COOH

O2N                   NO2

 

 

NO2

 

 

Soda

lime

O2N

 

 

 

NO2

T.N.B.

NO2

 

Properties and uses: It is colourless solid (M.P. = 122°C). It is more explosive than T.N.T. and used for making explosive.

 

The molecular formula, C8 H10 represents four isomers.

 

CH3

CH3

 

o-Xylene

CH3

 

CH3

m-Xylene

CH3

 

CH3

p-Xylene

C2H5

 

 

Ethyl benzene

 

 

 

These are produced along with benzene, toluene and ethylbenzene when aromatisation of C6C8

fraction of

 

petroleum naphtha is done. The xylenes are isolated from the resulting mixtrue (BTX) by fractional distillation.

These can be prepared by Wurtz – Fittig reaction. A mixture of bromotoluene and methylbromide is treated with sodium in dry ethereal solution to form the desired xylene.

 

CH3

Br

+ 2Na + BrCH3

CH3

CH3

CH3

 

+ 2Na + BrCH

CH3

 

+ 2NaBr

 

 

 

o-Bromotoluene

 

o-Xylene

+ 2NaBr ;                                                       3

Br

CH3

 

 

CH3

+ 2Na + BrCH3

Br

  • romotoluene

CH3

 

CH3

 

 

+ 2NaBr

m-Xylene

 

p-Bromotoluene

p-Xylene

 

 

  • These can also be obtained by Friedel – craft’s synthesis,

 

CH3

3

+   CH Cl          AlCl3

CH3

CH3

+

 

  • Xylene

CH3

 

 

CH3

 

  • m-Xylene can be obtained from

CH3                                     CH3

[O]

  • Xylene

 

 

Soda lime

CH3

 

+  CO2

 

H3C

CH3

HOOC

CH3

heat

CH3

 

Mesitylene                                                 Mesitylenic acid                                    m-Xylene

Xylenes are colourless liquids having characteristic odour. The boiling points of three isomers are,

o-Xylene = 144°C; m-Xylene = 139°C; p-Xylene = 138°C.

Xylenes undergo electrophilic substitution reactions in the same manner as toluene. Upon oxidation with

 

KMnO4

or KCrO7 , Xylenes form corresponding dicarboxylic acids.

COOH

COOH

 

COOH COOH

Phthalic acid

,                                         ,

COOH

Isophthalic acid

 

COOH

Terephthalic acid

 

Xylenes are used in the manufacture of lacquers and as solvent for rubber. o-Xylene is used for the manufacture of phthalic anhydride.

It can be prepared by the following reactions,

  • By Wurtz-Fittig reaction : C6 H5 Br + 2Na + BrC2 H5 ¾¾® C6 H5C2 H5 + 2NaBr
  • By Friedel-craft’s reaction : C6 H5 H + BrC2 H5  ¾¾AlC¾l3  ® C6 H5 C2 H5  + HBr

 

 

  • By catalytic reduction of styrene : C6 H5CH = CH2 + H2 ¾¾® C6 H5CH2CH3

 

(4)  By alkyl benzene synthesis :

C6 H5 H + H2C = CH2 ¾¾AlC¾l3 , H¾Cl ® C6 H5CH2CH3

95°C,Pressure

 

It undergoes electrophilic substitution reactions in the same way as toluene. When oxidised with dil.

HNO3  or

 

alkaline

KMnO4

or chromic acid it forms benzoic acid.

 

C6 H5C2 H5 ¾¾[¾O] ® C6 H5COOH

 

It is present in storax balsam and in coal-tar traces.

(1)  Preparation

  • Dehydrogenation of side chain of ethylbenzene : Dehydrogenation of side chain is affected by heating ethylbenzene to high temperature in presence of a

 

 

+  CH  = CH

 

AlCl3

CH2CH3

600°C

CH = CH2

 

2             2

 

Benzene

Cr2O3 / Al2O3

 

Ethylbenzene

 

Styrene

 

  • Decarboxylation of cinnamic acid : This is the laboratory It involves heating of cinnamic acid with a small amount of quinol.

C6 H5CH  = CHCOOH ¾¾Qui¾n¾ol ® C6 H5CH  = CH2  + CO2

 

  • Dehydration of 1-phenyl ethanol with H2SO4 :
  • Dehydration of 2-phenyl ethanol with ZnCl2 :

C6 H5CHOHCH3 ¾¾H2 S¾O¾4  ® C6 H5CH  = CH2

H 2 O

C6 H5CH2CH2OH ¾¾ZnC¾l2 , h¾e¾at ® C6 H5CH  = CH2

H 2 O

 

  • Dehydrohalogenation of 1-phenyl-1-chloro ethane : On heating with alcoholic potassium hydroxide, a molecule of hydrogen chloride is eliminated by the

C6 H5CHClCH3 ¾¾Alc.¾KO¾H ® C6 H5CH  = CH2

Heat

  • Properties : It is a colourless liquid, boiling point 145°C. On keeping, it gradually changes into a solid polymer called metastyrene. The polymerisation is rapid in sunlight or when treated with sodium. It shows properties of benzene ring (Electrophilic substitution) and unsaturated side chain (Electrophilic addition). However, the side chain double bond is more susceptible to electrophilic attack as compared to benzene ring.

At lower temperature and pressure, it reacts with hydrogen to produce ethylbenzene and at higher temperature and pressure, it is converted into ethyl cyclohexane.

 

CH = CH2

 

H2 / Ni

CH2CH3

 

H2 / Ni

CH2CH3

 

 

 

Styrene

20°C, 3 atm

 

Ethyl benzene

125°C, 110 atm

 

Ethyl cyclohexane

 

 

 

With bromine, it gives the dibromide.

CH = CH2

+    Br2

CHBr.CH2Br

 

 

Styrene                                       Styrene dibromide

 

 

 

 

Halogen acids add to the side chain. CH5CH = CH2 + HX ¾¾® C6 H5CHXCH3

Preparation of ring substituted styrenes is not done by direct halogenation but through indirect route.

 

CH2CH3

+   Cl2

 

FeCl3

CH2CH3

Cl

 

Cl2 hv

CHClCH3

Alc.. KOH

Heat

Cl

CH = CH2

Cl

 

When oxidised under drastic conditions, the side chain is completely oxidised to a carboxyl group.

 

CH = CH2

 

[O]

KMnO4

COOH

 

 

Styrene

In presence of peroxides, styrene undergoes free radical polymerisation resulting in the formation of polystyrene – an industrially important plastic.

é                       ù

ê                       ú

nC6 H5 CH  = CH 2  ¾¾Pero¾xi¾de ®ê- CHCH 2  -ú

 

ê    |

ëê  CH5

ú

úû n

 

Co-polymers of styrene with butadiene and other substances are also important since many of them are industrially useful products such as SBR ( A rubber substitute).

It occurs in coal-tar. It is the simplest example of an aromatic hydrocarbon in which two benzene rings are directly linked to each other.

(1)  Methods of formation

  • Fittig reaction : It consists heating of an ethereal solution of bromobenzene with metallic

Br  +    2Na     +     Br                                                                            +   2NaBr

  • Ullmann biaryl synthesis : Iodobenzene, on heating with copper in a sealed tube, forms biphenyl. The reaction is facilitated if a strong electron wihtdrawing groups is present in ortho or para

I  +  2Cu      +     I                                                                             +  2CuI

 

  • Grignard reaction : Phenyl magnesium bromide reacts with bromo benzene in presence of CoCl2 .

 

 

MgBr     +  Br

CoCl2

+ MgBr2

 

 

  • Properties : It is a colourless solid, melting point 71°C. It undergoes usual electrophilic substitution Since aryl groups are electron withdrawing , they should have deactivating and m-orientating effect. But, it has been experimentally shown that presence of one benzene ring activates the other for electrophilic substitution and directs the incoming group to o- and p- positions. It has been shown that monosubstitution in the bi-phenyl results in the formation of para isomer as the major product.

 

 

Another special feature of the biphenyl is the behaviour towards second substitution in a monosubstituted biphenyl. The second substituent invariably enters the unsubstituted ring in the ortho and para position no matter what is the nature of substituent already present.

 

 

2

HNO3 / H2SO4                                                NO       HNO3 / H2SO4

O2N

NO2

 

 

 

 

(1)  Methods of preparation

  • Friedel-craft’s reaction :

C6 H5 CH 2 Cl+ C6 H6  ¾¾AlC¾l3  ® C6 H5 CH 2 C6 H5 + HCl  or

 

Benzyl chloride

Benzene

Diphenyl methane

 

2C6 H6 +

CH2Cl2

Dichloromethane

¾¾AlC¾l3  ® C6 H5CH2C6 H5  + 2HCl

 

  • By action of formaldehyde on benzene in presence of sulphuric acid

2C6 H6  + O = CH2 ¾¾Con¾c. H¾2 SO¾4  ® C6 H5CH2C6 H5  + H2O

  • By Grignard reaction : Phenyl magnesium bromide reacts with benzyl bromide to from diphenyl

 

methane.

C6 H5 MgBr + BrCH2C6 H5 ¾¾® C6 H5CH2C6 H5 + MgBr2

 

  • By reduction of benzophenone : Reduction can be done with

C6 H5COC6 H5 ¾¾4[¾H] ® C6 H5CH2C6 H5  + H2O

LiAlH4 or P and HI.

 

  • Properties : It is a colourless solid, melting point 26°Like biphenyl, it also easily undergoes electrophilic substitution reactions.

 

CH2

HNO3 H2SO4

CH2

NO2

HNO3 H2SO4

O2N

CH2

NO2

 

The methylene hydrogens of diphenylmethane are situated on carbon atom linked by two electron attracting benzene rings. Thus, these are somewhat acidic in nature.

C6 H5 CH 2 C6 H5 + Br2  ¾¾® C6 H5 CHBrC6 H5  + HBr

 

When oxidised with

K 2 Cr2 O7  / H 2 SO4

mixture, it forms benzophenone.

 

C6 H5 CH 2 C6 H5  ¾¾[¾O] ® C6 H5 CC6 H5

||

O

It forms fluorene when its vapours are passed through a red hot tube.

CH2

 

Heat

 

H H

 

Fluorene

 

 

Compounds having two or more benzene rings fused together in ortho positions are termed as fused polynuclear hydrocarbons. These hydrocarbons also called fused ring hydrocarbons.

 

 

 

 

 

Naphthalene                    Anthracene                      Phenanthren

 

 

(1) Naphthalene

Naphthalene is the largest single constituent of coal-tar (6-10%). It is obtained in the middle oil fraction of coal-tar distillation. It is recovered as crude product when the middle oil fraction is cooled. The crude crystalline

 

product is separated by centrifugation and purified by washing successively with dilute

H 2SO4 (to remove basic

 

impurities), sodium hydroxide solution (to remove acidic impurities) and water. Finally, the solid is sublimed to get pure naphthalene.

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