Polynuclear hydrocarbons module9

Module 9 - Phenanthrene- Synthesis - Haworth synthesis -Reactions- Structural elucidation of phenanthrene

1. 1 | P a g e

PHENANTHRENE
Phenanthrene is an angular polynuclear aromatic hydrocarbon isomeric with
anthracene.

Synthesis of phenanthrene
a) Phenanthrene is obtained by passing di-benzyl through a red-hot tube.

b) It may also be obtained by cyclodehydrogenation of 2, 2’ - dimethyl diphenyl.

c) Haworth synthesis:
Phenanthrene is prepared from naphthalene by Haworth synthesis as follows.

2. 2 | P a g e


Reactions of phenanthrene
i) Pheanthrene can be reduced by sodium and isoamyl alcohol or copperoxide and
chromic acid to 9, 10-dihydrophenanthrene. Its oxidation with sodium dichromate and
sulphuric acid yield 9,10-phenanthraquinone.

ii) It adds on bromine to give 9, 10-dibromophenanthrene and its ozonolysis yields
diphenic acid.

3. 3 | P a g e

iii) It also undergoes electrophilic substitution. It undergoes Friedel-Crafts reaction
mainly at position 3. On nitration it gives 3-nitrophenanthrene and on bromination it
gives 9-bromophenanthrene.

Structure of phenanthrene
1) The molecular formula of phenanthrene is C
14
H
10
.
2) It is isomeric with anthracene.
3) On oxidation it gives phenanthraquinone which on further oxidation gives diphenic
acid.

4) Diphenic acid on decarboxylation gives diphenyl.

5) Hence phenanthrene must contain the following skeleton.

4. 4 | P a g e


6) On closing the middle ring and filling the hydrogen to satisfy the valencies of
various carbon atoms phenanthrene gets the following structure.

7) Evidence in support of the above structure of phenanthrene:
It gives five monosubstitution and twenty five disubstitution derivatives.
8) Resonance concept:
Phenanthrene is a resonance hybride of the following five contributing structures.

The existence of resonance in phenanthrene is proved by the following facts.
a) Resonance energy of phenanthrene is 387 kJ/mole.
b) Phenanthrene is a flat molecule.

5. 5 | P a g e

c) X-ray diffraction studies show that all the (C-C) bonds in phenanthrene are not
equivalent. C
1
-C
2
has 3/5 double bond character. C
2
- C
3
has 2/5 double bond
character and C
9
- C
10
has 4/5 double bond character.
9) Finally the sturcutre of phenanthrene is proved by Haworth synthesis:



Prepared by
Dr. A. Syed Mohamed
HoD and Senior Assistant Professor, Research Dept. of Chemistry
Sadakathullah Appa College (Autonomous)
Tirunelveli, Tamilnadu, India. [email protected]
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.