This document discusses different types of polynuclear hydrocarbons including isolated and condensed systems. It provides details on the preparation, properties, and reactions of several isolated polynuclear hydrocarbons including diphenyl, diphenylmethane, triphenylmethane, and stilbene. The key reactions used to synthesize these compounds involve Friedel-Crafts alkylation, Grignard reactions, and reductions. Atropisomerism is also discussed for substituted diphenyls.

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Polynuclear Hydrocarbons
Compounds containing more than one aromatic ring are known as polynuclear
hydrocarbons. They may be divided into two groups as isolated system and
condensed system.
Isolated system
Compounds in which the aromatic rings are either linked directly or through one
or more carbon atoms are known as isolated systems.
Example: Diphenyl, Diphenyl methane, Dibenzyl etc.
1. Diphenyl or Biphenyl
Preparation of Diphenyl or Biphenyl (Phenyl benzene)
i) Fittig’s reaction : Bromobenzene reacts with sodium in ether solution to give
diphenyl.
ii) It is also prepared by refluxing bromobenzene with hydrazine in alcoholic KOH in
the presence of palladium catalyst.
2C6H5 Br
Pd
2NH2NH
  
C6H5 - C6H5 + 2HBr
iii) Ullmann’s biaryl synthesis: When iodobenzene is heated with copper powder in
sealed tube diphenyl is obtained.
iv) Industrial preparation : Industrially it is prepared by passing benzene vapours
through heated iron tubes.

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2C6H6
 
Fe
C6H5 - C6H5 + H2
Atrop isomerism:
When 2, 2`, 4, 4` positions of diphenyl are occupied by bulky groups (NH2, CH3
group) it will exhibit optical activity.
The two benzene rings of the substituted diphenyl occupy different planes. Thus
the whole molecule is assymmetric and it is optically active.
The optical activity of substituted diphenyl is due to restricted rotation developed
by steric effects of the groups in the ortho, ortho` positions. This type of stereo
isomerism arising from restricted rotation along a single bond is called atrop
isomerism. The isomers are known as atrop isomers.
2. Diphenyl methane
Preparation of diphenyl methane
i) Friedel-Craft’s reaction:
Diphenyl methane is prepared by the Friedel-Craft’s condensation between benzyl
chloride and benzene.
C6H5CH2Cl + C6H6   3AlCl
C6H5-CH2-C6H5+ HCl
Benzyl chloride Diphenyl methane
ii) The condensation between one molecule of formaldehyde and two molecules of
benzene in the presence of conc.H2SO4 gives diphenyl methane.
2C6H6 + CH2O   42SOH
C6H5-CH2-C6H5+H2O

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iii) Heating benzophenone with HI and red phosphorus at 160oC under pressure, by
Wolff-Kishner reduction or by lithium-Aluminium hydride and AlCl3 gives diphenyl
methane.
C6H5COC6H5
3AlCl/LAH)or(
P/HI
 
C6H5CH2C6H5
Benzophenone Diphenylmethane
iv) Diphenylmethane is also prepared by the following Grignard reaction.
C6H5MgBr + C6H5CH2Cl  C6H5CH2C6H5+MgBrCl
Grignard reagent
3. Triphenyl methane
Preparation of triphenyl methane (Tritane)
i) Friedel-Craft’s reaction:
Triphenyl methane is prepared by the Friedel - Craft’s condensation between
benzal chloride and benzene.
C6H5 CH Cl2 + 2C6H6
  3AlCl
(C6H5)3 CH + 2HCl
Benzal chloride Triphenyl methane
ii) The condensation between benzene and chloroform gives triphenyl methane.
3C6H6 + CHCl3
  3AlCl
(C6H5)3 CH + 3HCl
iii) The condensation between benzaldehyde and benzene also gives triphenyl
methane.
C6H5CHO + 2 C6H6   2ZnCl
(C6H5)3CH+H2O
4. Stilbene
Preparation of stilbene
i) Benzyl magnesium bromide reacts with benzaldehyde followed by dehydration
gives stilbene.

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ii) Stilbene is prepared by reducing benzoin with Zn/Hg and ethanolic solution of HCl.
C6H5CHOHCOC6H5
Hg/Zn
HCl/OH5H2C
 
C6H5 - CH = CH - C6H5
iii) Stilbene is obtained when -phenyl cinnamic acid is heated in quinoline in the
presence of copper chromite.
C6H5CH=C-(C6H5)COOH

  chromiteCopper
C6H5 CH = CH - C6H5
-Phenyl cinnamic acid Stilbene
iv) Meerwein reaction:
Benzene diazonium chloride reacts with cinnamic acid to give stilbene.
C6H5N2Cl + C6H5CH=CH-COOH  C6H5CH=CH-C6H5+ N2 + CO2 + HCl
Benzene Cinnamic acid
diazonium chloride
Stilbene exhibits geometrical isomerism as follows.
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.