Aromatic Hydrocarbon
The word Aromatic is derived from Greek word Aroma which
means sweet smell or pleasant odour . Therefore, according
to ancient definition, Aromatic hydrocarbon is defined as the
hydrocarbon having pleasant odour and which is obtained
from plants .
Later on, by structure analysis of aromatic compounds, it was
found that they contain benzene ring or Benzenoid ring in
their structure. Therefore, Benzenoid are called aromatic
compound .
On further studies, it was found that some aromatic
compound does not contain benzene ring . Therefore,
according to the latest definition, Aromatic compound is
those hydrocarbons which follows Huckel’s rule .
Huckel’s Rule for Aromaticity :- It states that , cyclic planar
compound containing (4n+2) ꙥ electron clouds above and
below the plane of ring . In other words , the ꙥ-electron cloud
present in a cyclic planar molecule or ion is equal to (4n+2)ꙥ
where n is positive integer such as 0, 1, 2, 3, - - - - etc. Thus
cyclic planar compound containing 2, 6, 10, 14 etc.
delocalized ꙥ electron can show aromaticity . Carbon atoms
which forms rings are in sp2 hybridized state . For eg;
3. Cyclopentadienyl anion
no. of ꙥ electron = 6
Here,
(4n + 2)ꙥ = 6ꙥ
Or 4n = 4
჻n=1
Since, the value of n is +ve integer , cyclopentadienyl anion is
aromatic compound.
1. Benzene 2. Naphthalene
no. of ꙥ electron = 6 no. of ꙥ electron = 10
Here, Here,
(4n+2) ꙥ = 6ꙥ (4n+2)ꙥ = 10ꙥ
or 4n = 4 or 4n = 8
჻n=1 ჻n=2
Since, the value of n is +ve integer i.e. 1 and 2, the
compounds are aromatic hydrocarbon
4. Cyclopropenyl cation
Here, no. of ꙥ electron= 2
so, (4n+2)ꙥ = 2ꙥ
or, 4n = 0
჻ n = 0 (since, the value of n is +ve integer, the compound is
aromatic hydrocarbon)
5. Cyclopropenyl anion
Here, no. of ꙥ electron = 4
So, (4n+2)ꙥ = 4ꙥ
or, n = 1/2
(since, the value of n is not +ve integer, the compound is not
aromatic hydrocarbon)
6. Pyridine :- Pyridine is cyclic planar and has a p- orbital on
each ring .( All carbons and the nitrogen atom are sp2
hybridized and thus must have a p- orbital ). There are three
double bonds and 6ꙥ electron which is consistent with the
Huckel’s rule . The lone pair of electron on the nitrogen is in
sp2 hybrid orbital and is not considered in determining
aromaticity .
Here, no. of ꙥ electron = 6
so, (4n+2)ꙥ = 6ꙥ
჻n=1
Therefore, pyridine is aromatic.
It will show Aromaticity
7. Pyrrole :- It is cyclic planar and has a p- orbital on each ring
atom. ( Each ring atom is sp2 hybridized and thus must have a
p-orbital). The nitrogen atom has a lone pair of electron in a
p- orbital parallel with the p-orbitals composing the double
bonds . Thus it has a 6ꙥ electrons which is consistent with the
Huckel’s rule .
Here, no. of ꙥ electron = 6
so, (4n+2)ꙥ = 6ꙥ
჻ n= 1
Therefore, pyrrole is aromatic .
It will show aromaticity .
7. Thiophene :-
Here, no. of ꙥ electron = 6
so, (4n+2)ꙥ = 6ꙥ
჻ n=1
Therefore, thiophene is aromatic . It will show aromaticity .
8. Furan :-
Here, no. of ꙥ electron = 6
so, (4n+2)ꙥ = 6ꙥ
჻n=1
Therefore, Furan is aromatic. It will show aromaticity .
Exception of Huckel’s rule
• Cyclohepta-1,3,5-triene
According to Huckel’s rule
Here, no. of ꙥ electron = 6
so, (4n+2)ꙥ = 6ꙥ
჻n=1
It contain 6ꙥ electron and follows (4n+2)ꙥ electron rule but
one of the carbon of the ring is in sp3 hybridized. So, ꙥ
electron are not delocalized . Because of this cyclohepta-1,
3,5-triene is not aromatic compound .
Cyclobuta-1,3-diene
According to Huckel’s rule
Here, no. of ꙥ electron = 4
so, (4n+2)ꙥ = 4ꙥ
჻ n = 1/2
It contain delocalized ꙥ electron and carbon forming ring is
in sp2 hybridized state but it does not show aromatic
properties. Since , it does not follows (4n+2)ꙥ electron rule .
Kekule’s Structure of Benzene
Benzene (also called Phene) was first isolated by Faraday’s in
1825 from cylinders of compressed illuminating gas obtained
from pyrolysis of whale oil. Hofmann found benzene in coal
tar in 1845. August Kekule proposed the first resonable cyclic
structure for benzene in 1865. Benzene was first synthesized
by Berthelot in 1870 by passing acetylene through a red hot
tube.
According to Kekule, benzene consists of a cyclic planar
structure of six carbons with alternate double and single
bonds and each carbon atom is bonded to a hydrogen atom.
This arrangement allows all the carbon atoms to have four
bonds as required by structural theory.
Evidences in support of Kekule’s cyclic structure of benzene:
Benzene gives only one monosubstituted product.
This indicates that all the six hydrogen atoms are equivalent
which permits the cyclic structure.
Benzene added three molecules of hydrogen in the
presence of nickel catalyst to give cyclohexane. Similarly, it
gives benzenehexachloride by the consumption of three
molecules of hydrogen.
This confirmed the cyclic structure of benzene and also
showed the presence of three carbon-carbon double bond.
Objection of Kekule structure of benzene
• Although benzene contains three double bond it cannot
decolourize Baeyer’s reagent and bromine water and
behaves like saturated compound.
• It gives electrophilic substitution reactions rather than
addition reaction.
• Heats of hydrogenation and combustion of benzene are
lower than hypothetical 1, 3, 5-cyclohexatriene.
• All carbon to carbon bond length in benzene are equal and
they have intermediate length of C to C single and double
bonds.
Resonance structure of benzene
The phenomenon in which two or more structures can be
written for a compound which involve identical positions of
atoms but different arrangement of electrons is called
resonance. The actual structure of the molecule is said to be a
resonance hybrid of various possible alternative structures .
For eg ;
Resonance structure Resonance Hybrid
The resonance hybrid explains the following features of benzene:
• Contributing structures: In two contributing structure of benzene, the
positions of single bond and double bond are interchanged.
• Bond length: The bond length of C – C single bond is 1.54Ǻ and that
of C = C double bond is 1.33Ǻ. But, all the bond length in benzene are
found to be equivalent. The bond length measured in benzene is
1.39Ǻ. Which in the intermediate value between single and double
bond.
• Resonance energy: The difference between calculated energy for
resonance structure and the actual energy of the hybrid structure is
called resonance energy. The greater the resonance energy of
compound, the more stable the compound. A resonance hybrid
structure of benzene is more stable than the resonating structure by
36kcal.
• Stability towards addition reaction: Since I and II are exactly
equivalent, stabilization due to resonance is high. Hence, benzene is
quite stable towards addition reactions.
Benzenoid and non benzenoid aromatic
compounds
Aromatic compounds having benzene ring are called
benzenoid aromatic compound . For eg;
Aromatic compounds having no benzene ring are called non-
benzenoid aromatic compounds. For eg;
Thiophene Furan Pyrrole
Characteristics of aromatic compound
• Most of the aromatic compound has sweet smell .
• They are cyclic compound having planar ring and cyclic
cloud of pi electron .
• The percentage of carbon is higher in aromatic compound
as compared to aliphatic compound . Hence, it produces
sooty flame on burning .
• They are insoluble in water and are used as solvent for
other compound .
• They give electrophilic substitution reaction regardless of
functional group .
• Although unsaturated, they do not give addition reaction
easily .
• They follows Huckel’s 4n+2 rule .
Nomenclature of Benzene Derivatives
Nomenclature of mono-substituted derivatives of benzene
Nomenclature of di-substituted derivatives of benzene
If two substituted are present , three positional isomer are
possible .
1-chloro-3-nitrobenzene 1-bromo-4-iodobenzene
(m- Chloronitrobenzene) (p- Bromoiodobenzene)
1-Bromo-3-chlorobenzene
(m- Bromochlorobenzene)
Nomenclature of polysubstituted derivative of benzene
2,4,6-Trinitrotoluene 2-Chloro-4-nitroaniline 2,4,6- Tribromophenol
Some polynuclear hydrocarbon
Preparation of benzene
1. From acetylene :- When ethyne is passed through red hot
Cu-tube at 5000c. It gives benzene by polymerization .
2. From phenol :- When phenol is heated with Zn dust ,it gives
benzene .
3. From sodium benzoate :- When sodium benzoate is heated
with sodalime , benzene is formed . The reaction is called
sodalime decarboxylation reaction .
4. From Chlorobenzene :- When chlorobenzene is reduced in
presence of different reducing agents such as LiAlH4, Ni/H2,
Ni-Al/ NaOH etc. It gives benzene .
+ HCl
5. From Grignard reagent :-
When phenyl magnesium chloride is warmed with water, it
gives benzene .
+ Mg(OH)Br
Basic magnesium
hydroxide
Questions (2 Marks)
1. Why is benzene called aromatic compound according to
Huckel’s rule ?
2. State Huckel’s rule for aromaticity .
3. What are aromatic compounds according to Huckel’s rule?
4. Name the monomer which on polymerization can give
benzene . What type of polymerization is this ?
5. How would you prepare benzene from ethyne ?
6. Will cyclo - octatetraene show aromaticity ? Explain
7. What happen when
i. Sodium benzoate is heated with sodalime .
ii. Phenol is heated with zinc dust .
Reactivity and orientation of benzene derivatives
Reactivity and orientation of benzene derivatives is
determined by the substituent which is attached to the
benzene ring.
Reactivity of benzene derivatives
a. Ring activating group :- Atom or group of atom which is
attached to the benzene ring that increases the rate of
reaction of benzene derivative than that of benzene is called
ring activating group. Ring activating group contains unshared
pair of electrons (lone pairs). They always donate electron to
benzene ring . For eg; -NH2, -OH, -OR etc.
Nitration of toluene is about 25 times faster than that of
nitration of benzene .
Note:-
Activating Group :- A substituent that makes benzene ring more reactive towards
electrophilic substitution than benzene itself is called activating group.
Deactivating Group:- A substituent that makes benzene ring less reactive towards
electrophilic substitution than benzene itself is called deactivating group.
b. Ring deactivating group :-
Atom or group of atom which is attached to the benzene ring
that decreases the rate of reaction of benzene derivatives
than that of benzene is called ring deactivating group . They
lack electron and withdraw the electron from benzene ring by
resonance . It decreases electron density of the ring and
makes it less attractive to an incoming electrophile . For eg;
-NO2, -COOH, -CHO, -COOR , -Cl, -Br, -I etc.
Chlorination of nitrobenzene is about 30 times slower than
that of chlorination of benzene.
Orientation of benzene derivative
In monosubstituated benzene, substituent can direct an
incoming electrophile at ortho, para or meta position .
Depending on directive nature of substituent, substituent are
following two types.
1. Ortho para director
2. Meta director
Ortho para director
A substituent that directs an incoming group i.e. electrophile
to the ortho and para position is called ortho para director. All
the ring activating groups are ortho para director. For eg; -
NH2, -OH, -CH3, -OCH3 etc. Even though halogens ( -Cl, -Br, -I )
are ring deactivating group , they are ortho para director .
Ortho para director release electron towards benzene ring .
For eg; aniline
≡
resonance
hybrid structure
Fig: Resonating structure of Aniline
Phenol
≡
Fig : Resonating structure of phenol Resonance
hybrid
Chlorobenzene
Fig :- Resonating structure of chlorobenzene
Toluene
Fig :- Resonating structure of Toluene
Anisole
OCH3
≡
Fig :- Resonating structure of Anisole
From above resonating structure, it is clear that ortho and
para position are electron rich centre . So, they are good
centre or site for electrophile to attack. Thus , they are ortho
para director towards electrophilic substitution reaction .
Meta director
A substituent that directs an incoming group i.e. electrophile
to the meta position is called meta director. All the ring
deactivating groups except halogen are meta director . They
withdraw electron from benzene ring . For eg; Nitrobenzene
≡
Benzaldehyde
H-C - - Oծ-
≡
Fig :- Resonating structure of benzaldehyde
Benzoic acid
HO-C- -Oծ-
Fig :- Resonating structure of benzoic acid
From above resonating structure, it is clear that ortho and
para position are electron deficient centre that means meta
position is relatively electron rich centre than that of ortho
and para position . Thus , they are meta director towards
electrophilic substitution reaction .
Physical properties
• Benzene is a colourless liquid with aromatic odour .
• It is insoluble in water but soluble in organic solvent .
• The b.pt of benzene is 80.40c and m.pt is 5.50c .
• The vapour of benzene is highly inflammable, toxic and
burn with sooty flame .
• It is very good solvent for fats , resin, sulphur, iodine etc.
Chemical properties
1. Substitution reaction
a. Halogenation :- This involves the treatment of benzene
with halogen in presence of FeCl3, FeBr3or AlCl3 to give
corresponding halobenzene .
1. convert phenol into chlorobenzene .
2. covert acetylene into bromobenzene .
Br2/ FeBr3
dark
b. Nitration reaction
HNO3 + H2SO4 NO2+ + HSO4- + H2O
c. Sulphonation reaction
+ H2O
d. Friedel Crafts Alkylation reaction (5 Marks)
When benzene reacts with alkyl halide (R-X) in presence of
anhydrous AlCl3 or FeCl3 etc. alkyl group replace the hydrogen
of benzene to give alkyl benzene . The reaction is called
Friedel-Crafts alkylation .
5. Friedel Crafts acylation reaction (5 Marks)
When benzene reacts with acyl halide (RCOCl) or acetic
anhydride in presence of anhydrous AlCl3 or FeCl3etc. Acyl
group replace the hydrogen of benzene to give acyl benzene
(aromatic ketone). The reaction is called Friedel-Crafts
acylation .
Acetophenone (acyl benzene)
2. Addition reaction
a. Addition of Hydrogen :-
b. Addition of Halogen :-
c. Addition of Ozone (Ozonolysis) :-
3. Combustion
C6H6 + 15O2 12CO2 + 6H2O + energy
Uses :-
• It is used as solvent for extraction of oils and fat from seeds
• It is used as drying –cleaning agent for woollen clothes .
• It is used to make styrene, aniline and nitrobenzene. These
organic compounds are then used to make polystyrene,
dyes and nylon .
• It is also used to make types of rubber, lubricants ,
detergent, drugs, pesticides and explosives .
8. Identify A and B in the following reactions and give their
names . i.
NaOH + CaO (A) CH3Cl/AlCl3 (B)
NaOH + CaO Cl2/ light
ii. Sodium benzoate (A) (B)
LiAlH4 CH3Cl/AlCl3
iii. Chlorobenzene (A) (B)
Questions
1. Write resonance hybrid structure of arene containing meta director
and ortho para director substituents of each .
2. Why is –NO2 group meta director ?
3. Why – OH group is ortho para director ?
4. Why phenol undergoes nitration faster than nitrobenzene?
5. Give an example of Friedel Craft’s alkylation .
6. Starting from phenol how would obtain cyclohexane .
7. What happen when
i. benzene is heated with acetic anhydride in presence of anhydrous
AlCl3 .
ii. Benzene is heated with hydrogen in presence of nickel powder .
8. Why is –Cl atom deactivating group ?
