Chapter 22 Aromatic 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
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 >
Directs (Powerful activator)
- 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] :
Note : ®
+ CH Cl AlCl3
C H + ClCH CH CH
¾¾AlC¾l3 ® C H CH CH3 + HCl
6 6 2 2 3
6 5 CH3
Isopropyl benzene (65 -70%)
- Catalysts can be used in place of anhydrous
AlCl3 > SbCl3 > SnCl4 > BF3 > ZnCl2 > HgCl2
- Wurtz fitting reaction :
Br + 2Na +
Bromobenzene Methyl bromide Toluene
- Decarboxylation :
- NaOH ¾¾Sod¾a lim¾e ® C6
(o-,m- or p-) Sodium toluate
- From cresol :
+ Zn heat
- From toluene sulphonic acid :
+ HOH + H2SO4
p-Toluene sulphonic acid
- From toluidine :
+ N2 + CH3CHO + HCl
p-Toluidine p-Toluene diazonium chloride
- From grignard reagent :
Phenyl magnesium bromide
- 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
benzol which contains
70 – 80%
14 – 24%
toluene. 90% benzol is again distilled and the portion
distilling between 108 – 110°C
is collected. It is toluene.
- From n- heptane and methyl cyclohexane
Cr2O3 / Al2O3
(2) Physical properties
- 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.
+ E Å
Note : ® E+ may be chlorine,
HNO3 , H2SO4 ,CH3Cl .
- Reactions of side chain
CH3 CH2Cl CHCl2
- Side chain halogenation :
Toluene Benzyl chloride Benzal chloride Benzo trichloride
Note : ® Benzyl chloride on hydrolysis with aqueous caustic soda forms benzyl alcohol.
(Phenyl methyl chloride)
- NaOH ¾¾® C6 H5CH2OH + NaCl
- Benzal chloride on hydrolysis forms
C6 H5 CHCl2 + 2NaOH ¾¾®
C6 H5 CH(OH)2 + 2NaCl
- Benzo trichloride on hydrolysis forms benzoic
- Oxidation :
- 3NaOH ¾¾® C6 H5C(OH)3 + 3NaCl
C6 H5COOH+ H2O
- With hot acidic KMnO4 :
KMnO4 / H+
+ H2O CH3
- With acidic manganese or chromyl chloride (Etards reaction) :
Note : ® All alkyl benzenes on oxidation with hot acidic length of the side chain does not matter.
form benzoic acid. The
- Hydrogenation :
Na / liquid NH3 – C2H5OH
H C CH
Methyl benzene Methylcyclohexane
- Combustion : C6 H5CH3 + 9O2 ¾¾®7CO2 + 4 H2O
- Ozonolysis :
C CH Zn
+ 3H O
+2 2 2
H – C=O CHO
HC CH C
O C O O O
- 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)
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)
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.
These are produced along with benzene, toluene and ethylbenzene when aromatisation of C6 – C8
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.
+ 2Na + BrCH3
+ 2Na + BrCH
+ 2NaBr ; 3
+ 2Na + BrCH3
- These can also be obtained by Friedel – craft’s synthesis,
+ CH Cl AlCl3
- m-Xylene can be obtained from
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
or K2 Cr2 O7 , Xylenes form corresponding dicarboxylic acids.
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
It undergoes electrophilic substitution reactions in the same way as toluene. When oxidised with dil.
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.
- Dehydrogenation of side chain of ethylbenzene : Dehydrogenation of side chain is affected by heating ethylbenzene to high temperature in presence of a
+ CH = CH
CH = CH2
Cr2O3 / Al2O3
- 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
- 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
H2 / Ni
20°C, 3 atm
125°C, 110 atm
With bromine, it gives the dibromide.
CH = CH2
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.
CH = CH2
When oxidised under drastic conditions, the side chain is completely oxidised to a carboxyl group.
CH = CH2
In presence of peroxides, styrene undergoes free radical polymerisation resulting in the formation of polystyrene – an industrially important plastic.
nC6 H5 CH = CH 2 ¾¾Pero¾xi¾de ®ê- CH – CH 2 -ú
ëê C6 H5
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
- 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.
HNO3 / H2SO4 NO HNO3 / H2SO4
(1) Methods of preparation
- Friedel-craft’s reaction :
C6 H5 CH 2 Cl+ C6 H6 ¾¾AlC¾l3 ® C6 H5 CH 2 C6 H5 + HCl or
2C6 H6 +
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
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.
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
It forms fluorene when its vapours are passed through a red hot tube.
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
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.