Chapter 20 Aliphatic Hydrocarbons Part 2 by TEACHING CARE Online coaching and tuition classes

 

 

(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

¾¾^AlC¾^l3  ®  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 ¾¾^Sod¾^{a lim}¾^e ® 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.

H 2 SO4

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 C₆ – C₈

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 K2 Cr2 O7 , 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  ¾¾^AlC¾^l3  ® 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 ¾¾^AlC¾^l3 , H¾^Cl ® 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^[¾^O] ® 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 ¾¾^Qui¾ⁿ¾^ol ® C6 H5CH  = CH2  + CO2

 

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

C6 H5CHOHCH3 ¾¾^H2 S¾^O¾4  ® C6 H5CH  = CH2

– H 2 O

C6 H5CH2CH2OH ¾¾^ZnC¾^l2 , h¾^e¾^at ® 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 ¾¾^Alc.¾^KO¾^H ® 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. C6 H5CH = 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  ¾¾^Pero¾^xi¾^de ®ê- CH – CH 2  -ú

 

ê    |

ëê  C6 H5

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  ¾¾^AlC¾^l3  ® C6 H5 CH 2 C6 H5 + HCl  or

 

Benzyl chloride

Benzene

Diphenyl methane

 

2C6 H6 +

CH2Cl2

Dichloromethane

¾¾^AlC¾^l3  ® C6 H5CH2C6 H5  + 2HCl

 

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

2C6 H6  + O = CH2 ¾¾^Con¾^{c. H}¾2 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 ¾¾^4[¾^H] ® 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^[¾^O] ® 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.