Chapter 23 Aromatic Hydrocarbons Part 3 by TEACHING CARE Online coaching and tuition classes
- Methods of preparation
- Haworth synthesis
C – CH C
+ O |
Conc. H2SO4 2
C – CH2
HO – C
- By 4-phenyl-1-butene :
- In naphthalene all carbon atoms are
sp 2 – hybridized.
sp 2 – hybrid orbital overlap with s- orbital of
hydrogen atoms forming C – C and C – H sigma bond.
- All carbon and hydrogen atoms lie in one plane in
H H C-C
H H s-bond
H H C-H
- According to resonance It is a resonance hybrid.
- (II) (III) Hybrid
(d) Position : 1,4,5,8 = a ; 2,3,6,7 = b
a a 8 1
b b 7 2
b b 6 3
- Resonance energy of naphthalene is 61 kcal/mol. Which is less than of benzene. So that naphthalene is less aromatic e. more reactive than benzene.
- In naphthalene C1 – C2 bond length is shorter (1.36Å) e. C = C and C2 – C3 is 1.40Å i.e. single bond.
- Physical properties : It is a colourless crystalline compound. It melts at 80.2° It is very volatile and sublimes slowly even at room temperature. It has strong characteristic odour. It is insoluble in water but very soluble in ether, benzene and hot alcohol. It burns with smoky flame.
- Chemical properties : It undergoes usual aromatic electrophilic substitution The product of monosubstitution is either a or b -depending on conditions, but the a -product always predominates.
Substitution at b -position occurs only when the reaction is carried at high temperatures or when bulkier
solvents are used.
AlCl3 in CS2 at
O /V O ,
NO NO NO
2 2 5 O
or conc. H2SO4,
(Dialine) 1, 4-Dihydro
(Tetralin) (1, 2, 3, 4-
- As moth It is, however, now being replaced by more powerful insecticides such as p-
dichlorobenzene and DDT.
- For commercial production of phthalic anhydride, a –naphthol, b -naphthol,
- For manufacture of dyes, explosives and synthetic
- For increasing the illuminating power of coal
The hydrocarbon derives its name from the world anthrax (Greek = coal) as coal is the chief source from which it is manufactured. It is present in coal-tar (Less than 0.5%) and is obtained from the anthracene oil or green oil fraction (because of its dark green fluorescence) formed during coal-tar distillation. This fraction is collected between 270 – 360°C.
The anthracene oil fraction is cooled when crude anthracene crystallizes out. The crude product consists phenanthrene and carbazole as impurities. The crude product is successively washed with solvent naphtha as to remove phenanthrene and pyridine to remove carbazol. Finally, the solid is sublimed to get pure anthracene.
- Methods of preparation
|| o-Benzoyl benzoic acid
- Anthracine is tricyclic aromatic
- All carbon atoms in anthracene are
- resonance hybrid are as follows
sp 2 hybridized.
- (II) (III) (IV) Hybrid form
- It gives both addition and electrophilic substitution
- In anthracene the numbering of carbon atom is
8 9 1
5 10 4
a g a
- Physical properties : Anthracene is a colourless It melts at 217°C. It is insoluble in water but soluble in alcohol and ether in small amounts. It is comparatively more soluble in hot benzene. With picric acid, it
forms a red colour picrate.
(4) Chemical properties
(v) Uses : Anthracene is used
- For manufacture of
- For making dyes (Alizarin).
- In smoke
Na2Cr2O7 + H2SO4
These are cyclic compounds in which the ring includes in addition to carbon atoms at least one atom of another element (Hetero = other, different). The common hetero atoms present in the carbon rings are O,N and S.
It is found in the benzene fraction of coal-tar and petroleum. The benzene fraction is shaken with cold concentrated sulphuric acid. Thiophene present in the fraction combines with sulphuric acid more readily than benzene to form thiophene sulphonic acid which is separated with water being soluble. Thiophene sulphonic acid is treated with super-heated steam to recover thiophene.
Properties : It is a colourless liquid. Its boiling point is 84°C. Its odour is similar to that of benzene. It is insoluble in water but soluble in organic solvents. It is flammable and toxic in nature.
Its resonance energy is 31k cal mol -1 . Hence, it is more stable and resembles benzene more closely than
(23 k cal mol -1 )
(25 k cal mol -1 ). It does not show basic properties and does not undergo
Thiophene as a resonance hybrid (Resonance energy is 31 k cal /mol
2 + 400°C
CH2 – CH2
+ 4S 650°C
2-Thiophene sulphonic acid
HCHO + HCl
Furan derives its name from furfur meaning bran in Greek which is the source of its aldehyde, furfural. It is present in pine-wood tar and may be 4
extracted from it. 5
3 H –s– C
2 H –s– C ..
C –s– H
C –s– H
Furan shows aromatic behaviour because resulting p-molecular orbital 1
satisfies the Huckel rule ((4n + 2)p where, n = 1 .
sp2 sp sp2
Furan is also considered as a resonance hybrid of the canonical forms. Out of which the first three are the
main contributing structures.
Furan has resonance energy about
23 k cal mol -1
which is less than benzene. However it
is less aromatic and more reactive than benzene.
Properties : Furan is a colourless liquid. Its
I II III IV V
Canonical forms of furan
boiling point is 32°C. It is insoluble in water but soluble in orgainic solvents. It is a reactive compound. It is a weak base. In Furan electrophilic substitution reactions take place preferably at 2 and 5 position where electron density is high. If these positions are occupied, substitution occurs at 3 and 4 positions. It undergo Diels-Alder reaction.
CH3 – C – ONO2 or HNO3
2-Furan sulphonic acid
– C – CH3
H C –– CH
CH2 – CHO
CH = CHOH P O
2 2 H2C –––– CH2
| | | |
| ⇌ | 2 5
H C – Cl CH – OH
CH – CHO CH = CHOH D 2 2
Tetrahydro furan (THF)
Tetra methylene chloroform
CHOH –– CHOH
H – C – OH H – C – OH
Mucic acid or Saccharic acid or Glucaric acid
CH – CH(OCH )
CH3OH + HCl
CH2 – CH(OCH3)2
Diacetal of succinic dialdehyde
- HCN + HCl, AlCl3
– C – H
(2-Formyl furan) Furfural
C H Li
Pyridine is a six membered aromatic heterocycle with one nitrogen atom in the ring. It may be supposed to
have been derived by replacement of
= CH –
group of benzene by
= N – . Hence, it is isoster of benzene. Its
systematic name is azabenzene. (Prefix aza stands for nitrogen). The hybrid structure of pyridine is represented as:
(C–C, s bond) H H
(C–C, s bond)
(C–N, s bond)
- Properties : It is a colourless liquid having an unpleasant It boils at 115°C. It is miscible with water and is hygroscopic. It is a good solvent for many organic compounds and inorganic salts.
- Aromatic character : Each carbon atom and nitrogen atom in the ring have
hybridized and one
unhybrid p-orbital containing one electron. These orbitals overlap to form p molecular orbital consisting six
electrons. The p molecular orbital satisfies Huckel’s rule pyridine.
(4n + 2)
and thus aromatic properties are observed in
The resonance energy is 43 k cal/mol and bond length of C – C bond is 1.40 Å and
p electron cloud
C – N
4n + 2 = 6 ; n = 1
- Resonance structure : + +
Canonical form of pyridine
- Basic nature : Pyridine is basic in nature due to presence of lone pair nitrogen atom. It is more and more basic than pyrrole and less basic than aliphatic
- Chemical properties :
CH CH NH HCl N NO
2 2 2
H SO /300°C
Piperidine 2 4
Penta methylene diamine dihydrochloride
3-Pyridine sulphonic acid substitution
2C2H2 + HCN
2CH CHO + HCHO + NH
= CH. CHO
HI CH CH CH CH CH
2 D [O]
3 2 2 2 3 3
- To denature
- As a basic solvent in organic
- For preparing sulpha-pyridine and vitamin B6 .
- As a catalyst in many reactions, g., in the formation of Grignard reagent, in Perkin and Knoevenagel reactions.
It occurs in coal-tar and bone oil and is found in many natural products including chlorophyll, haemoglobin and alkaloids.
2 2 |
+ + .–.
| | | |
(C-N, s-bond) H
H H H H H
- Preparation :
C H NK + H O
4 4 2
2C2H2 + NH3
N C=O H
Note : ® Furan needed for the process is obtained from agricultural waste materials which are rich in pentosans. The pantosans on acid hydrolysis yields furfural which is decarbonylated.
- Properties : Pyrrole is a colourless liquid. Its boiling point is 131°C. It is slightly soluble in water but highly soluble in alcohol and Its odour is similar to chloroform. It rapidly becomes brown when exposed to air. Vapours of pyrrole turn a pine splint moistened with HCl red. Pyrrole derives its name from this property.
Chemical properties :
KOH C H N–K+
Br Br 4 4
(CH CO) O, 200°C
Tetraiodo pyrrole (Iodole)
2-Pyrrole sulphonic acid
2, 5-Dihydro pyrrole
diazotisation in acid solution
OC N CO H
2-Formyl pyrrole (Pyrrole-2-aldehyde)
- Uses : It is used as a commercial solvent, as an intermediate in the production of nylon and for making