Chapter 21 Aromatic Hydrocarbons Part 1 by TEACHING CARE Online coaching and tuition classes

 

 

Word aromatic is now reserved for benzene and for the carbocyclic derivatives which resembles with benzene in chemical behaviour. These are also known as benzenoid compounds. All aromatic hydrocarbons (benzene, naphthalene, anthracene etc.) have been given a new name “Arenes”.

 

Source of arenes is coal. It contains benzene, xylene, naphthalene etc. Arenes are obtained by destructive distillation of coal.

(1)  Distillation of coal

 

 

 

 

 

 

 

COKE (Solid residue nearly 70%). It is used as a fuel and as reducing agent in metallurgy.

Cooled and passed through water

 

Allowed to settle,

COAL GAS (Mixture of uncondensed gases nearly 17%). It is used as a fuel.

two layers are separated

 

Upper layer                                                          Lower layer

 

• Coal tar is a mixture of large numbers of
• Distillation of coal tar : Arenes are isolated by fractional distillation of coal tar,

 

Name of the fraction	Temperature range (K)	Main constituents
Light oil (or crude oil) fraction	Upto 443	Benzene, toluene, xylene
Middle oil fraction (Carbolic oil)	443-503	Phenol, naphthalene, pyridine
Heavy oil fraction (Creosote oil)	503-543	Naphthalene, naphthol and cresol
Green oil (Anthracene oil)	543-633	Anthracene, phenanthrene
Pitch (left as residue)	Non-volatile	Carbon

Note : ® The residue left after fractional distillation of coal-tar is called pitch.

(3)  Isolation of benzene

Light oil ¾¾^cold¾^H2¾^SO¾4  ®Basic impurities removed  ¾¾^NaO¾^H ®Phenols removed ¾¾^disti¾^llati¾^on ®Benzene (255 – 257K)

 

[Like pyridene]

[Acidic impurities]

Toluene (383 K)

 

 

 

• All arenes have general formula [CnH ₂n – 6y] . Where y is number of benzene rings and n is not less than
• Arenes are cyclic and They undergo substitution rather than addition reactions.
• Aromaticity or aromatic character : The characteristic behaviour of aromatic compounds is called Aromaticity is due to extensive delocalisation of p-electrons in planar ring system. Huckel (1931) explained aromaticity on the basis of following rule.

Huckel rule : For aromaticity the molecule must be planar, cyclic system having delocalised

(4n + 2)p electrons where n is an integer equal to 0, 1, 2, 3, –.

Thus, the aromatic compounds have delocalised electron cloud of 2,6,10 or 14 p electrons.

 

Example :

4n + 2 = 6 ; 4n = 4

; n = 4 = 1

4

 

 

Naphthalene 10p electrons n = 2

Anthracene 14p electrons n = 3

 

Similarly cyclolpentadienyl anion or tropylium ion are also aromatic because of containing 6p electrons (n=1).

H                  H                                                           H

H                  H                                                  Å

Å

H                  H                                              H                    H

H                                                            H             H

Cyclopentadienyl anion 6p electrons                              Tropyllium ion 6 p electrons                          Cyclopropenyl cation (n = 0)

Hetrocyclic compounds also have 6p electrons (n = 1).

 

 

 

 

Furan

Thiophene

Pyridine

 

 

 

Molecules do not satisfy huckel rule are not aromatic.

 

 

Cyclopentadiene 4p electrons

Cyclopentadienyl cation 4p electrons

Cyclooctatetraene 8p electrons

Cyclopropenyl anion 4p electrons

 

 

• Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated

species are called antiaromatic. Molecular orbital calculations have shown that such compounds have 4np

 

electrons. In fact such cyclic compounds which have characteristic is called antiaromaticity.

4np

electrons are called antiaromatic compounds and this

 

Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has

(n = 1).

4np

electrons

 

4n = 4

; n = 4 = 1

4

 

 

 

Thus, cyclobutanediene shows two equivalent contributing structures and it has n = 1 .

 

In terms of Huckel rule antiaromatic compounds have cyclic, planar structure with destabilised by resonance. Some other examples are,

                                                Å                                                                     

4np

electrons. They are

 

 

 

Cyclopropenyl anion 4p electrons

Cyclopentadienyl 4p electrons

Cycloheptatrienyl anion 8p electrons

Cycloctatetraene 8p electrons

 

 

Comparison of aromatic and aliphatic hydrocarbons

Characteristic	Benzene and its homologous	Aliphatic hydrocarbons
Composition	These     are  closed     ring     compounds. These                    are                represented                by     general formula CnH2n-6 .	These are open chain compounds. These    are    represented    by    general formulae;    Cn H2n+ 2    (Alkanes),    Cn H2n
		(Alkenes) and Cn H2n– 2  (Alkynes).
Carbon percentage	These contain high percentage of carbon. In benzene C6 H6 , the carbon percentage is 92.3.	These have low percentage of carbon in             comparison          to         aromatic hydrocarbons. In hexane, C6 H14 , the
		carbon percentage is 83.7.
Combustion	These burn with smoky flame.	These burn with non smoky flame.
Nature	These    have    high    unsaturation.  For example, benzene molecule consists three double bonds.	These     are     saturated     as     well     as unsaturated.
Physical state	These are colourless liquids or solids. They have characteristic odour (Aromatic).	A few lower members are colourless gases while higher members are liquids or solids. Generally no characteristic odour exists.
Addition reactions	Inspite of the fact that these are unsaturated, generally resist addition reactions. These do not react with  HCl, HBr, HI or HClO .	The unsaturated hydrocarbons show addition reactions.
Substitution reactions	Generally            exhibit            substitution (Electrophilic) reactions such as halogenation, nitration, sulphonation, Friedel-craft’s reaction etc.	The saturated hydrocarbons show substitution reactions such as halogenation.           The          unsaturated hydrocarbons resist substitution reactions. Nitration and sulphonation occur with difficulty in higher alkanes. Friedel-craft’s reaction is not shown by aliphatic hydrocarbons.
Stability	Highly stable.	The unsaturated hydrocarbons are less stable.
(4n + 2) rule	Follow     (4n + 2)    rule,     i.e.,      contain	(4n + 2)    rule     does     not     apply     to
	(4n + 2)p                electrons                where	aliphatic unsaturated hydrocarbons.
	n = 0,1,2,3,……….
Oxidation	Except benzene, all oxidise easily.	Alkanes do not oxidise easily while unsaturated hydrocarbons oxidise easily.

 

 

 

Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich

(1833) obtained it by distillating benzoic acid with lime. Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene.

• Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated
• Kekule’s structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate
• Evidence in favour of Kekule’s structure
• Benzene combines with 3 molecules of hydrogen or three molecules of It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds.
• Studies on magnetic rotation spectroscopy show the presence of three double
• The synthesis of benzene from three molecule of acetylene also favour’s Kekule’s 3CH º CH ¾¾^D ®
• Benzene gives cyclohexane by reduction by hydrogen. C6 H6 + 3H 2 O ¾¾^Ni ®

 

• Objections against Kekule’s formula
• Unusual stability of

Cyclohexane

 

• According to Kekule, two ortho disubstituted products are But in practice only are ortho disubstituted product is known.
• Heat of hydrogenation of benzene is 8 kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole.
• C – C bond length in benzene are equal, although it contains 3 double bonds and 3 single

Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions.

(ii)  Some other structures of benzene

• Ladenberg’s prism formula : This formula shows benzene three dimensional structure where X-ray studies of benzene molecule indicate a
• Claus diagonal formula :

 

• Dewar’s parallel formula :

 

• Armstrong and Baeyer’s centric formula :

 

• Thiele’s formula :

 

 

• Valence bond theory [Resonance theory] : According to this theory, benzene can not be represented by only one structural formula but as a hybrid of

 

 

 

 

 

⇌

• (b)

º

 

(c)

Resonance hybrid

 

 

The resonance hybrid structure of benzene explain all the properties of benzene. The resonance structure of benzene is supported by the following facts,

• The C – C bond length in benzene is 139 pm which is intermediate between bond lengths for C – C bond

(154 pm) and C = C (134 pm).

• Due to resonance the p electron charge in benzene get distributed over greater area. As a result of delocalisation the energy of resonance hybrid decrease as compared to contributing structure by about 50 kJ/ The decrease in energy is called resonance energy.
• O.T. [Modern concept] : According to the orbital concept each

2

carbon atom in benzene is sp       hybridised and one orbital remains

unhybridised. Out of three hybrid orbitals two overlap with neighbouring carbon atoms and third hybrid orbital overlap with hydrogen atom for s bonds. Thus benzene has a planar structure with bond angle of 120° each.

(2)  Methods of preparation of benzene

 

 

(i)  Laboratory method :

COONa

+

NaOH

CaO heat

+   Na₂CO₃

 

Sodium benzoate

(ii)  From benzene derivatives

OH

Benzene

 

• From phenol :

 

 

Phenol

+ Zn Cl

distill

 

 

Benzene

ZnO

 

• From chlorobenzene : +

Ni-Al alloy

2H

NaOH

+ HCl

 

Chlorobenzene                                            Benzene

• By preparing grignard reagent of chlorobenzene and then hydrolysed

 

C6 H

₅Cl

¾¾^M¾^g ® C6 H5

dry ether

MgCl

¾¾^H2¾^O ® C6 H6

• Mg OH

Cl

 

Chlorobenzene

Phenyl magnesium chloride

Benzene

 

 

• From benzene sulphonic acid :

SO₃H

+

HOH

Steam

 

150°-200°C

HCl,pressure

+H₂SO₄

 

Benzene sulphonic acid

N₂Cl

• From benzene diazonium chloride : +

 

 

2H

SnCl₂ NaOH

Benzene

 

Benzene

 

+N₂

 

+HCl

 

 

 

 

• From acetylene :

+ HC

HC                 CH

+

HC                 CH

 

 

 

 

red hot tube 1500-2000°C

 

 

Benzene

 

+ HC

Three molecules of acetylene

 

Note : ® Cyclic polymerisation takes place in this reaction.

• Aromatisation : C6 H14 ¾¾^Cr2O¾3 /¾^Al2¾^O¾3  ® C6 H6 + 4 H 2

 

n– Hexane

 

(3)  Properties of benzene

(i)  Physical properties

500°C

at high pressure

Benzene

 

• Benzene is a colourless, mobile and volatile It’s boiling point is 80°C and freezing point is 5.5°C. It has characteristic odour.
• It is highly inflammable and burns with sooty
• It is lighter than It’s specific gravity at 20°C is 0.8788.
• It is immiscible with water but miscible with organic solvents such as alcohol and
• Benzene itself is a good Fats, resins, rubber, etc. dissolve in it.
• It is a non-polar compound and its dipole moment is
• It is an extremely poisonous Inhalation of vapours or absorption through skin has a toxic effect.
  • Chemical properties : Due to the presence of p electron clouds above and below the plane benzene ring, the ring serves as a source of electrons and is easily attacked by electrophiles (Electron loving reagents). Hence electrophilic substitution reaction are the characteristic reactions of aromatic

Substitution reactions in benzene rather than addition are due to the fact that in the former reactions resonance stabilised benzene ring system is retained while the addition reactions lead to the destruction of benzene ring. Principal reactions of benzene can be studied under three heads,

(a) Addition reactions  (b) Substitution reactions   (c) Oxidation reactions

• Addition reactions : In which benzene behaves like unsaturated
  • Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) catalyst at 150°C under pressure to form

 

+  3H                                                 or C H

 

 

Benzene

2

 

H           Cyclohexane

C

HC                CH

6   12

Cl H C

H      C                 C       Cl

 

• Addition of halogen :

+

HC                CH

C H

Benzene

hv

3Cl₂

Cl                                    H

C

C

H                                     Cl

Cl                                    H

C

H     Cl

 

Benzene hexachloride (BHC)

 

 

 

 

• Addition of ozone :

HC

H

C     CH

 

 

+3O  ®

 

CHO

3

 

 

+ 3H O

 

3

HC            CH C

H

2    2

CHO

Glyoxal

 

• Substitution reactionsBenzene Benzene triozonide
• Nucleophilic mechanism
• Unimolecular : Mostly uncommon in aromatic substitution, there is only one example which obtain in

 

benzene diazonium dichloride.

HOH

ArOH

 

2

Ar – N ⁺ Arenediazonium cation

• Bimolecular

Z

¾¾(Slo¾w) ® N 2

 

 

Z

. .

• Ar ⁺

Phenol

ArX

Aryl halide

 

Y      Z           Y      Z

 

Y^ +

Y^ +

+ Z^

 

 

 

 

 

Example :

 

OH^    +

Cl

 

(Slow)

OH     Cl

 

 

 

(Fast)

(Resonating structure of the

OH           hexadienyl anion)

 

+ Cl^

 

 

 

 

• Elimination-addition mechanism (Benzyne mechanism)

Phenol

 

 

Å

 

 

– HCl

(Benzene)

NH₃

* NH₃

+



*

Å

NH₃

 

(47%)

*NH₂

H       +

H₂

(53%)

 

• Electrophilic substitution reaction : Benzene undergoes this reaction because it is an electron rich system due to delocalized p–

Å      H                                                                                       H

+      EÅ         (Slow)                          E  ;                                                  E

Å                                 Å

 

 

Å      H

E

+   Nu:

Carbonium ion (s – complex)

 

 

(Fast)

Resonance forms of carbonium ion (Arenium ion)

E

 

+       H – Nu

 

Substitution product

 

 

 

Electrophile (EÅ)	Name	Source	Name of substitution reaction
Cl +  Br +  NO+ 2    SO3  R+   +  R – C = O	Chloronium	Cl2 + AlCl3  or   FeCl3	Chlorination
	Bromonium	Br2 + AlBr3 or FeBr3	Bromination
	Nitronium	HNO3 + H2SO4	Nitration
	Sulphur trioxide	Conc. H2SO4 , Fuming sulphuric acid	Sulphonation
	Alkyl carbonium	RX + AlX3 (X = Cl or Br), ROH + H+	Friedel-Craft’s
			(Alkylation)
	Acyl carbonium	RCOCl + AlCl3	Friedel-Craft’s (Acylation)

• Free radical aromatic substitution : The aromatic substitution reactions which follow free radical mechanisms are very few and have limited synthetic But some typical example of these reactions are:

 

×               ×

(CH3 )3 COOC – (CH3 )3  ¾¾^he¾^at ® 2(CH3 )3 CO ¾¾® 2CH3 + 2CH3 COCH3

 

X

.

+ CH₃

X                                        X                         X

H

CH3  +                           +                          H

 

o–                                                         m-IntermediateCH3

 

 

X                                      X                             X CH₃

+                         +

O                                                                                                          m

CH₃

Products

.

+        H CH₃

 

The mechanism of chlorination of benzene at high temperature is similar to that of the free radical aliphatic substitution

.     .

Cl₂ ¾¾® Cl + Cl (Chain initiation)

.           .

C6 H6 + Cl ¾¾® C6 H5 + HCl (H- abstraction)

.                                   .

 

C6 H5 + Cl2 ¾¾® C6 H5 Cl + Cl

(Chain propagation)

 

• Oxidation : 2C6 H6 + 15O2 ¾¾®12CO2  + 6H2O

DH  = 6530 kJ/mole

 

When vapours of benzene and air are passed over vanadium pentoxide at 450 – 500°C, maleic anhydride is obtained.

V O             CHCO

 

C6 H6  + 9[O] ¾¾2 ¾5  ® ||

450 – 500°C  CHCO

O+ 2CO₂ + 2H ₂O

 

Maleic anhydride

Note : ® Strong oxidising agents converts benzene slowly into CO₂

and water on heating.

 

 

 

• Reduction : 2

+ 12HI

+                CH3  +     6I

 

2

Benzene

Cyclohexane

Methylcyclopentane

 

• Uses : (a) In dry cleaning (b) As a motor fuel when mixed with petrol. (c) As a solvent. (d) In the manufacture of gammexane (As insecticide). (e) In the preparation of nitrobenzene, chlorobenzene, benzene sulphonic acid, aniline, styrene, Many of these are employed for making dyes, drugs, plastics, insecticides, etc.

 

• Directive effect in mono substituted benzene derivatives : The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is called directive or the orientation effect.

The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.

There are two types of substituents which produce directive effect are,

• Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).
• Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).

 

Theory of ortho – para directing group

ÅS              ÅS              ÅS

 

 

..

:S

 

,i.e.,

 

 

 

S                           S

E

+

 

 

 

 

Para attack

Ortho product

E

Para product

 

The above mechanism is followed when S is – OH, – NH ₂ ,-Cl, – Br,-I,-OR,- NR₂ ,- NHCOR etc.

CH₃            CH₃            CH₃            CH₃

..

 

 

 

In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect. Thus, methyl or alkyl group directs all electrophiles to ortho and para positions.

Theory of meta directing group : The substituent, S withdraws electrons from ortho and para positions. Thus, m-position becomes a point of relatively high electron density and further substitution by electrophile occurs

 

at meta position. For example, explained as :

• NO2

group is a meta directing (Electron withdrawing). Its mechanism can be

 

O^ Å    O N

O^ Å    O^ N

O^ Å    O^ N

O^ Å    O^ N

O^ Å    O^ N

 

Å                                                      .Å.

 

Å

All meta-directing groups have either a partial positive charge or a full positive charge on the atom directly attached to the ring.