Module 2 Condensed system-Naphthalene - Synthesis - Haworth synthesis -Reactions - Reduction (or) addition reactions -Oxidation reaction -Substitution reaction - Structural elucidation of Napthalene - Fries rule and Uses

1. 1 | P a g e
Condensed system
Compounds in which two or more aromatic rings are fused together in ortho
positions are known as condensed systems.
Example: Naphthalene, Anthracene, Phenanthrene, etc.
1. Naphthalene
Naphthalene is the simplest condensed polynuclear hydrocarbon. It contains two
benzene rings fused in ortho positions.
Synthesis of Naphthalene
i) Synthesis from 4-phenyl-1-butene:
If 4-phenyl -1- butene is passed over red-hot calcium oxide, naphthalene is
obtained.
ii) Fittig’s synthesis:
Cyclization of 4-phenyl-3-butenoic acid gives -naphthol which on distillation
with zinc dust gives naphthalene.

2. 2 | P a g e
iii) Haworth synthesis:
Benzene reacts with succinic anhydride in the presence of AlCl3 to give a ketonic
acid which is reduced to -phenyl butyric acid. This on cyclization gives  -tetralone
which on reduction gives tetralin. Dehydrogenation of tetralin gives naphthalene.
Reactions of Naphthalene
a) Reduction (or) addition reactions:
i) Naphthalene on reduction with sodium and ethanol gives 1,2 -dialin
ii) On reduction with sodium and isoamyl alcohol it forms tetralin.

3. 3 | P a g e
iii) When reduced with H2 in the presence of Ni catalyst it gives tetralin and then
decalin (decahydro naphthalene)
b) Oxidation reaction:
i) On oxidation with acidic KMnO4 it gives phthalic acid.
ii) On oxidation with oxygen in the presence of vanadium pentoxide catalyst, it gives
phthalic anhydride.
iii) Oxidation of naphthalene with chromic acid gives 1-4 - naphthaquinone.
iv) On oxidation with alkaline KMnO4 it gives phthalonic acid.
v) On treatment with ozone it forms a diozonide and this on reaction with water give
phthalaldehyde.

4. 4 | P a g e
c) Substitution reaction:
Naphthalene undergoes electrophilic substitution more readily than benzene. The
preferred site of electrophilic substitution is (1). This is because of the fact that only
-attack gives more stable carbonium ion intermediate.
1) Naphthalene on chlorination with S2Cl2/ AlCl3 gives 1-chloro naphthalene and
then 1, 3-dichloro naphthalene.
2) Naphthalene on bromination gives 1-bromo naphthalene and then 1, 4 - dibromo
naphthalene.
3) Naphthalene on nitration with conc.H2SO4 and conc. HNO3 gives 1-
nitronaphthalene. But at higher temperature 1, 4 and 1, 8-dinitro naphthalenes are
obtained.
4) On sulphonation, naphthalene gives naphthalene-1- sulphonic acid at 40o. But at
160oC it gives naphthalene-2- sulphonic acid.
5) On Friedel-Crafts (FC) alkylation, naphthalene gives both 1 and 2-alkyl
naphthalenes.

5. 5 | P a g e
Structure of Napthalene
1) On the basis of analytical data, the molecular formula of naphthalene is found to
be C10H8.
2) Like benzene it is resistant to addition reaction.
3) It undergoes electrophilic substitution reactions more readily than benzene.
4) Like benzene its nuclear hydroxy compounds are phenolic (give violet colouration
with neutral FeCl3) and the amino compounds undergo diazotization and coupling
reactions.

6. 6 | P a g e
5) Naphthalene shows unusual stability. This is proved by its heat of combustion
data. Its heat of combustion is 255kJ less than the calculated value.
6) On vigorous oxidation naphthalene gives phthalic acid. This proves the presence
of atleast one aromatic ring and two side chains in ortho position to each other.
Therefore the formula of naphthalene may be given as
7) Using the side chain formula C4H4 , the possible open chain structure is
8) However this open chain structure cannot account for the aromatic character of
naphthalene.
9) Naphthalene on nitration gives nitro naphthalene which on oxidation gives 3-nitro
phthalic acid.
10) The nitro naphthalene on reduction gives amino naphthalene which on oxidation
gives phthalic acid.

7. 7 | P a g e
11) On the basis of the above two reactions (9 & 10), Erlenmeyer proposed that
naphthalene [C10H8] contains two benzene rings fused to each other in ortho
position as follows.
Nitro group deactivates the aromatic ring and makes it resistant to oxidation.
Therefore ring B gets oxidised and the product is 3-nitro phthalic acid. Amino group
activates the aromatic ring towards oxidative cleavage. Thus ring A gets oxidised and
the product is phthalic acid.
12) Evidences in favour of Erlenmeyer’s structure:
i) It shows the presence of five double bonds in the molecule and naphthalene is
known to add with five molecules of hydrogen to form decalin. ii) The formation of
two mono substituted products ( & ) and ten disubstituted products can be
explained using Erlenmeyer’s structure.

8. 8 | P a g e
13) Resonance concept:
Naphthalene is considered to be the resonance hybrid of the following three
contributing structures.
The existence of resonance in naphthalene is proved by the following facts.
i) The resonance energy of naphthalene is 255 kJ per mole.
ii) Naphthalene is a flat molecule.
iii) X-ray diffraction studies show that all the (C-C) bonds in naphthalene are not
equivalent. C1 - C2 has more (2/3) double bond character (1.36Ao) and C2 - C3 has
more (2/3) single bond character (1.40Ao).
14) Molecular orbital concept:
In naphthalene molecule each carbon atom is sp2 hybridised and is attached to 3
other atoms (two carbon atoms and 1 oxygen atom) by  bonds. Thus the whole
molecule lies in one plane and each carbon atom contains one unhybridised p-orbital
which lies perpendicular to the plane. The overlapping of these p-orbitals can lead to
the formation of -electron cloud above and below this plane. This cloud has a shape
of “8” . Naphthalene molecule has ten  electrons and obeys Huckel’s rule of
aromaticity (4n+2).
Molecular orbital picture of naphthalene

9. 9 | P a g e
Therefore naphthalene is a true aromatic compound with two benzene rings fused
in ortho positions.
15) Finally the structure of naphthalene is proved by Haworth synthesis:
Benzene reacts with succinic anhydride in the presence of AlCl3 to give a ketonic
acid which is reduced to -phenyl butyric acid. This on cyclization gives  -tetralone
which on reduction gives tetralin. Dehydrogenation of tetraline gives naphthalene.
Fries rule:
According to this rule, the most stable arrangement of a polynuclear compound is
that form which has the maximum number of rings in the benzenoid form. For
example, naphthalene has three resonating structures.

10. 10 | P a g e
Structure I contains two benzenoid rings; but structures II and III each contain one
benzenoid form. Thus according to Fries rule naphthalene tends to behave like
structure I rather than structures II and III.
Uses of naphthalene
It is used,
i) as an insecticide and moth repellant in the form of moth balls.
ii) in the manufacture industrially important compounds such as phthalic anhydride,
phthalic acid, etc.
iii) as a starting material in the preparation of various dyes.
Prepared by
Dr. A. Syed Mohamed
HoD and Senior Assistant Professor, Research Dept. of Chemistry
Sadakathullah Appa College (Autonomous)
Tirunelveli, Tamilnadu, India. asm2032@gmail.com
The author ackowledges Prof. H. Kassali Rahmathullah, Dr. T. Syed Ismail, Dr. M.
Kamalutheen, Former Professor, Dept. of Chemistry, Sadakathullah Appa College for their
valuable contribution.