DAMIETTA
UNIVERSITY
CHEM-323:
Polynuclear Aromatic
Hydrocarbon
LECTURE 1
Dr Ali El-Agamey
1
LEARNING OUTCOMES
LECTURES 1-2
 Understand the principles of biphenyl nomenclature and the different
methods used in its synthesis.
 Understand the various methods involved in the synthesis of the
different isolated polynuclear aromatic compounds.
 Appreciate the effect of phenyl group on the stability of various
intermediates.
 Appreciate the steric effect of phenyl and benzyl groups.
 Understand the role of phenyl and benzyl groups in electrophilic
substitution reactions.
2
 Lecture 1-2: -Nomeclature, synthesis and reactions of biphenyl.
-Synthesis and reactions of diphenylmethane,
triphenylmethane, triphenylmethyl chloride, triphenylmethanol, transstilbene and benzoin.
-Nomenclature of fused systems.
 Lecture 3-6: -Aromatic character of fused systems.
-Structure elucidation of naphthalene
-Reactions of naphthalene.
 Lecture 7-11: - Synthesis of naphthalene.
- Synthesis and reactions of anthracene and
phenanthrene.
 Lecture 12: -General revision for the whole course.
Reading
RT Morrison and RN Boyd, Organic Chemistry, Allyn and Bacon, Inc., 1987.
Mundy, BP et al, Name reactions and reagents in organic synthesis, Wiley, 2005
H Meislich et al, Organic Chemistry, McGraw-Hill Book Company, 1980.
IL Finar, Organic Chemistry, Volume 1 (1985) and Volume 2 (1977).
3
Polynuclear Aromatic Hydrocarbons
Isolated rings
(1) Biphenyl
Nomenclature
4
Homework: Give name for the
following compounds
Cl
Br
Br
(b)
(a)
CO2H
(H3C)2N
Cl
N(CH3)2
(c)
5
(1) Biphenyl
Synthesis
(a) Fittig’s reaction
(b) Gattermann reaction
(c) Gomberg reaction
More examples
6
(d) Ullmann coupling reaction
More examples
(e) Benzidine rearrangement
7
(1) Biphenyl
Reactions
Although –NO2 is m-directing group, p-O2NC6H4 is p-directing!!
According to the charge distribution approach, a p-nitro group
would have been expected to direct to the m-position in the other ring.
O
O
N
N
O
O
O
N
O
8
(1) Biphenyl
Reactions
However, the stabilities of the carbocation intermediates generated
following o- and p-substitutions are greater than that generated from
m-substitution due to the greater dispersal of charge in the former
substitutions. Therefore, o-and p-substitutions are preferred.
O2N
NO2
O2N
NO2
p–substitution
NO2
NO2
O2N
O2N
m–substitution
9
(2) Diphenylmethane
Synthesis and reactions
(3) Triphenylmethane (Tritane)
Synthesis and reactions
10
Homework: write the product of the
following reaction
11
DAMIETTA
UNIVERSITY
CHEM-323:
Polynuclear Aromatic
Hydrocarbon
LECTURE 2
Dr Ali El-Agamey
12
(3) Triphenylmethyl chloride
(Trityl chloride)
2 Ph3CCl
Ag
C6H6, no air
Dimer
(White solid)
2 Ph3C
Yellow colour
was believed to be
Actually
Ph3C
Ph3C–CPh3
H
Hexaphenylethane
Real dimer product
13
(4) Triphenylmethanol
(Triphenylcarbinol)
Synthesis and reactions
(5) trans-Stilbene
(trans-1,2-Diphenylethene)
Synthesis and reactions
(6) Benzoin
Synthesis and reactions
14
Homework: Complete the following equation
Homework: Write the mechanism of the following reaction
15
Fused rings
Naphthalene
 Naphthalene, anthracene and phenanthrene are obtained
from coal tar, naphthalene being the most abundant
(5%) of all constituents of coal tar.
 Uses: moth balls, which contain naphthalene, are used as
insect repellants (naphthalene is very volatile).
 Many synthetic dyes contain naphthalene moiety.
16
Fused rings
Nomenclature of naphthalene, anthracene and
phenanthrene
 How many isomers for monosubstitution of naphthalene
(C8H7X)?
 How many isomers for monosubstitution of anthracene
(C14H9X)?
 How many isomers for monosubstitution of phenanthrene
(C14H9X)?
17
Homework
 How many isomers for disubstitution of naphthalene
(C8H6X2)?
 How many isomers for disubstitution of naphthalene
(C8H6XY)?
 How many isomers for disubstitution of anthracene
(C14H8X2)?
 How many isomers for disubstitution of phenanthrene
(C14H8X2)?
 In each case write the name of each isomer?
18
Nomenclature of naphthalene, anthracene and
phenanthrene
19
Homework: Give names for the following
compounds
20
DAMIETTA
UNIVERSITY
CHEM-323:
Polynuclear Aromatic
Hydrocarbon
LECTURE 3
Dr Ali El-Agamey
21
Aromatic character of naphthalene, anthracene
and phenanthrene
Naphthalene, anthracene and phenanthrene are classified as
aromatic because of their properties, which resemble those
of benzene.
(i) Experimental point of view
 Undergo electrophilic substitution reactions.
 Like benzene, they are unusually stable as indicated from
their low heats of hydrogenation and combustion.
22
Resonance Energy
23
Aromatic character of naphthalene, anthracene
and phenanthrene
 Resonance energy of A = 61 kcal mol-1
 Resonance energy of B = 84 kcal mol-1
 Resonance energy of C = 92 kcal mol-1
 Resonance energy of benzene = 36 kcal mol-1
24
Aromatic character of naphthalene, anthracene
and phenanthrene
(ii) Theoretical point of view: they have the structure required for
aromatic compounds
 Flat (sp2-hybidized); cyclic
 p-cloud above and below the plane due to ???
 HÜckel`s rule ???
25
Resonance structures and bond lengths of
naphthalene, anthracene and phenanthrene
There are n+1 principal resonance structures for polynuclear
aromatic hydrocarbon containing n benzene rings fused
together in a linear manner.
Double bond character
Bond length
The different carbon-carbon bond lengths reveal the
decreased aromaticity of fused polynuclear aromatic
hydrocarbons.
Phenanthrene is an angular polynuclear aromatic
hydrocarbon.
26
Fused rings
(1) Naphthalene
(C10H8)
Naphthalene, anthracene and phenanthrene are obtained from
coal tar.
Naphthalene being the most abundent (5%) of all constituents
of coal tar.
(a) Uses
 In moth balls as insect repellants.
 Many synthetic dyes contain naphthalene moiety.
(b) Structure Elucidation
27
Reactivity of naphthalene as compared to benzene
In naphthalene, two fused rings share a pair of p–electrons,
therefore the aromaticity and the resonance energy per ring
is less than that of benzene itself.
 Resonance energy of naphthalene = 61 kcal mol-1
 Resonance energy per ring = 30.5 kcal mol-1
 Resonance energy of benzene = 36 kcal mol-1
Reactivity
decreases
As R.E. increases
Resistance to destroy
aromatic character
increases
Stability of the
compound increases
28
Reactivity of naphthalene as compared to benzene
What happened when aromatic character of one ring of
naphthalene is destroyed?
Therefore, naphthalene undergoes oxidation or reduction more
readily than benzene but only to the stage where a substituted
benzene is formed; further oxidation or reduction requires
more vigorous conditions.
29
Reactions of naphthalene
(1) Oxidation
30
Reactions of naphthalene
(1) Oxidation
 Oxidation by V2O5 is important industrial process (naphthalene is
available from coal tar).
 Vitamins K1 and K2 are derivatives of 1,4-naphthoquinone.
 Because of this tendency to form quinones, it is not always feasible
to prepare naphthalenecarboxylic acids by oxidation of methyl side
chains.
31
Reactions of naphthalene
(2) Reduction
32
Reactions of naphthalene
(3) Electrophilic susbstitution reactions
Fries rule: The most stable arrangement of a polynuclear
compound is that form which has the maximum number
of rings in the benzenoid condition.
1-Substitution versus 2-substitution
Number of resonance structures
Aromatic sextet is retained
(benzene has large R. E.)
Carbocation and T.S. resulting from attack at the 1-position
are much more stable than those resulting from attack
at the 2-position . Therefore, 1-susbstitution will be
33
preferred.
Reactions of naphthalene
(i) Nitration
34
DAMIETTA
UNIVERSITY
CHEM-323:
Polynuclear Aromatic
Hydrocarbon
LECTURE 4
Dr Ali El-Agamey
35
LEARNING OUTCOMES
LECTURE 4
 Reactions of naphthalene: Nitration; Halogenation;
Chloromethylation; Friedel-Crafts reactions (alkylation and
acylation).
 Sulfonation.
 Naphthols and naphthylamines.
 Synthesis of 1-substituted naphthalenes.
 Synthesis of 2-substituted naphthalenes.
 Summary of naphthalene reactions.
36
Reactions of naphthalene
(i) Nitration
37
Reactions of naphthalene
(ii) Halogenation
Introduction of NO2 and X groups opens the way to the
preparation of a series of 1-substituted naphthalenes.
38
Reactions of naphthalene
(iii) Chloromethylation
(iv) Friedel-Crafts reactions
Friedel-Crafts reactions is carried out under mild conditions.
Friedel-Crafts alkylation is of little use because (1) polyalkylation
and (2) side reactions.
Alkylnaphthalenes can be prepared via acylation or ring closure.
39
Reactions of naphthalene
(iv) Friedel-Crafts reactions
Acetylation affords access to the preparation of a series of
2-substituted naphthalenes.
40
Reactions of naphthalene
(v) Sulfonation
41
Reactions of naphthalene
(v) Sulfonation
42
E2 > E1
Δ H2 > Δ H1
k1 > k2
E-2 (E2 + Δ H2) >> E-1 (E1 + Δ H1)
k-1 >> k-2
At low T, 1-S is formed faster than 2-S
Therefore, 1-S is called kinetically controlled product
However, at high T, which speeds up a reaction, or if the reaction is left for
long time, 2-S becomes the main product.
1-S reforms N much faster than does 2-S and hence 2-S accumulates at the
expense of 1-S i.e. the equilibrium is established in favor of the formation 43
of 2-S,
which is called the thermodynamically controlled product.
(v) Sulfonation
The lower stability of 1-S is attributed to the steric interaction between the
sulfonic group and the hydrogen atom in the 8-position.
44
Reactions of naphthalene
45
Reactions of naphthalene
Naphthols and naphthylamines
Bucherer reaction
46
Reactions of naphthalene
Naphthols and naphthylamines
47
Synthesis of 1-substituted naphthalenes
48
Synthesis of 2-substituted naphthalenes
Acetylation and sulfonation affords access to the
preparation of a series of 2-substituted naphthalenes.
Summary of naphthalene reactions
50