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
Cooled and passed through water
Allowed to settle,
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 ¾¾cold¾H2¾SO¾4 ®Basic impurities removed ¾¾NaO¾H ®Phenols removed ¾¾disti¾llati¾on ®Benzene (255 – 257K)
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.
4n + 2 = 6 ; 4n = 4
; n = 4 = 1
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
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).
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.
electrons are called antiaromatic compounds and this
Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has
(n = 1).
4n = 4
; n = 4 = 1
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,
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
|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 ¾¾D ®
- Benzene gives cyclohexane by reduction by hydrogen. C6 H6 + 3H 2 O ¾¾Ni ®
- Objections against Kekule’s formula
- Unusual stability of
- 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
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
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 :
(ii) From benzene derivatives
- From phenol :
+ Zn Cl
- From chlorobenzene : +
- By preparing grignard reagent of chlorobenzene and then hydrolysed
¾¾M¾g ® C6 H5
¾¾H2¾O ® C6 H6
- Mg OH
Phenyl magnesium chloride
- From benzene sulphonic acid :
Benzene sulphonic acid
- From benzene diazonium chloride : +
- From acetylene :
red hot tube 1500-2000°C
Three molecules of acetylene
Note : ® Cyclic polymerisation takes place in this reaction.
- Aromatisation : C6 H14 ¾¾Cr2O¾3 /¾Al2¾O¾3 ® C6 H6 + 4 H 2
(3) Properties of benzene
(i) Physical properties
at high pressure
- 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
Cl H C
H C C Cl
- Addition of halogen :
Benzene hexachloride (BHC)
- Addition of ozone :
+ 3H O
HC CH C
- Substitution reactionsBenzene Benzene triozonide
- Nucleophilic mechanism
- Unimolecular : Mostly uncommon in aromatic substitution, there is only one example which obtain in
benzene diazonium dichloride.
Ar – N + Arenediazonium cation
¾¾(Slo¾w) ® N 2
- Ar +
Y Z Y Z
(Resonating structure of the
OH hexadienyl anion)
- Elimination-addition mechanism (Benzyne mechanism)
- 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
Carbonium ion (s – complex)
Resonance forms of carbonium ion (Arenium ion)
+ H – Nu
|Electrophile (EÅ)||Name||Source||Name of substitution reaction|
| Cl +
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|
|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 ¾¾he¾at ® 2(CH3 )3 CO ¾¾® 2CH3 + 2CH3 COCH3
X X X
CH3 + + H
X X X CH3
+ 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
- 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
Note : ® Strong oxidising agents converts benzene slowly into CO2
and water on heating.
- Reduction : 2
+ CH3 + 6I
- 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
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 :
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
All meta-directing groups have either a partial positive charge or a full positive charge on the atom directly attached to the ring.