Chapter 30
Polynuclear Aromatic Compounds
Fused-ring aromatic compounds
Two aromatic rings that share a pair of carbon atoms are said to be fused.
In this chapter we shall study the chemistry of the simplest and most important of
the fused-ring hydrocarbons, naphthalene, C10H8 ,
and look briefly at two others of
formula C14H10 ,
anthracene and phenanthrene.
All three of these hydrocarbons are obtained from coal tar, naphthalene being
the most abundant (5%) of all constituents of coal tar.
If diamond (p. 285) is the ultimate polycyclic aliphatic system, then the other allow
tropic form of elemental carbon, graphite, might be considered the ultimate in fused-ring
aromatic systems. X-ray analysis shows that the carbon atoms are arranged in layers.
Each layer is a continuous network of planar, hexagonal rings; the carbon atoms.
Nomenclature of naphthalene derivatives
Positions in the naphthalene ring system are designated as in I. Two monosubstituted naphthalene's are differentiated by the prefixes 1- and 2-, or and /?-. The arrangement of groups in more highly substituted naphthalene's is
indicated by numbers. For example:
Structure of naphthalene
Naphthalene is classified as aromatic because its properties resemble those of
benzene (see Sec. 10.10). Its molecular formula, C| H8 , might lead one to
expect a high degree of unsaturation; yet naphthalene is resistant (although less* so
than benzene) to the addition reactions characteristic of unsaturated compounds. Instead, the typical reactions of naphthalene are electrophilic substitution reactions, in which hydrogen is displaced as hydrogen ion and the naphthalene
ring system is preserved. Like benzene, naphthalene is unusually stable: its heat of
combustion is 61 kcal lower than that calculated on the assumption that it is
aliphatic (see Problem 10.2, p.).
From the experimental standpoint, then, naphthalene is classified as aromatic
an the basis of its properties. From a theoretical standpoint, naphthalene has the
structure required of an aromatic compound: it contains flat six-membered rings,
and consideration of atomic orbitals shows that the structure can provide rr clouds
containing six electrons the aromatic sextet (Fig. 30.1). Ten carbons lie at the
:orners of two fused hexagons. Each carbon is attached to three other atoms by <r
:orners of two fused hexagons. Each carbon is attached to three other atoms by <r
bonds; since these a bonds result from the overlap of trigonal sp
2 orbitals, all
:arbon and hydrogen atoms lie in a single plane. Above and below this plane there
s a cloud of * electrons formed by the overlap of p orbitals and shaped like a
igure 8. We can consider this cloud as two partially overlapping sextets that have
i pair of TT electrons in common.
Reactions of naphthalene
Like benzene, naphthalene typically undergoes electrophilic substitution;
this is one of the properties that entitle it to the designation of "aromatic." An
electrophilic reagent finds the TT cloud a source of available electrons and attaches
itself to the ring to form an intermediate carbonium ion; to restore the stable
aromatic system, the carbonium ion then gives up a proton.
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. Naphthalene is stabilized by
resonance to the extent of 61 kcal/mole; benzene is stabilized to the extent of 36
kcal/mole. When the aromatic character of one ring of naphthalene is destroyed,
only 25 kcal of resonance energy is sacrificed; in the next stage, 36 kcal has to be
sacrificed.
Oxidation of naphthalene
Oxidation of naphthalene by oxygen in the presence of vanadium pentoxide
destroys one ring and yields phthalic anhydride. Because of the availability of
naphthalene from coal tar, and the large demand for phthalic anhydride (for
example, see Sees. 30.18 and 32.7), this is an important industrial process.
Oxidation of certain naphthalene derivatives destroys the aromatic character of one ring in a somewhat different way, and yields Diket compounds known as
quinones (Sec. 27.9). For example:
Reduction of naphthalene
In contrast to benzene, naphthalene can be reduced by chemical reducing
agents. It is converted by sodium and ethanol into 1 ,4-dihydronaphthalene, and
by sodium and isopentyl alcohol into 1,2,3,4-tetrahydronaphthalene (teralitre).
The temperature at which each of these sodium reductions is carried out is the
boiling point of the alcohol used; at the higher temperature permitted by isopentyl
alcohol (b.p. 132), reduction proceeds further than with the lower boiling ethyl
alcohol (b.p. 78).
(a) Build models of these compounds and see that they differ from one another.
Locate in the models the pair of hydrogen atoms, attached to the fused carbons, that
are cis or trans to each other.
(b) In /rcwj-decalin is one ring attached to the other by two equatorial bonds, by
two axial bonds, or by one axial bond and one equatorial bond? In m-decalin?
Remembering (Sec. 9.12) that an equatorial position gives more room than an axial
position for a bulky group, predict which should be the more stable isomer, cis- or
/ra/w-decalin.
(c) Account for the following facts: rapid hydrogenation of tetralin over a
platinum black catalyst at low temperatures yields cu-decalin, while slow hydrogenation of tetralin over nickel at high temperatures yields /ra/w-decalin. Compare this
with 1,2- and 1,4-addition to conjugated dienes (Sec. 8.22), Friedel-Crafts alkylation
of toluene (Sec. 12.1 1), sulfonation of phenol (Problem 24.13, p. 803), and sulfonation
of naphthalene (Sec. 30.11).
Dehydrogenation of hydroaromatic compounds. Aromatization
Compounds like 1 ,4-dihydronaphthalene, tetralin, and decalin, which contain
the carbon skeleton of an aromatic system but too many hydrogen atoms for
aromaticity, are called hydroaromatic compounds. They are sometimes prepared,
as we have seen, by partial or complete hydrogenation of an aromatic system.
More commonly, however, the process is reversed, and hydroaromatic compounds are converted into aromatic compounds. Such a process is called aromatization.
One of the best methods of aromatization is catalytic dehydrogenation,
accomplished by heating the hydroaromatic compound with a catalyst like
platinum, palladium, or nickel. We recognize these as the catalysts used for
hydrogenation; since they lower the energy barrier between hydrogenated and
dehydrogenated compounds, they speed up reaction in both directions (see Sec. 6.3).
The position of the equilibrium is determined by other factors: hydrogenation is favored by an excess of hydrogen under pressure; dehydrogenation is favored by
sweeping away the hydrogen in a stream of inert gas. For
In an elegant modification of dehydrogenation, hydrogen is transferred from
the hydroaromatic compound to a compound that readily accepts hydrogen. For
example:
Nitration and halogenation of naphthalene
Nitration and halogenation of naphthalene occur almost exclusively in the
1 -position. Chlorination or bromination takes place so readily that a Lewis acid
is not required for catalysis.
As we would expect, introduction of these groups opens the way to the
preparation of a series of <z//?/KZ-substituted naphthalenes: from 1-nitronaphthalene
via the amine and diazonium Salts, and from 1-bromonaphthalene via the Grignard
reagent.
Orientation of electrophilic substitution in naphthalene
Nitration and halogenation of naphthalene take place almost exclusively in
the a-position. Is this orientation of substitution what we might have expected?
In our study of electrophilic substitution in the benzene ring (Chap. 11), we
found that we could account for the observed orientation on the following basis:
(a) the controlling step is the attachment of an electrophilic reagent to the aromatic
ring to form an intermediate carbonium ion; and
(b) this attachment takes plaque in
such a way as to yield the most stable intermediate carbonium ion. Let us see if
this approach can be applied to the nitration of naphthalene.
Attack by nitreniums ion at the a-position of naphthalene yields an intermediate carbonium ion that is a hybrid of structures I and II in which the positive
charge is accommodated by the ring under attack, and several structures like III
in which the charge is accommodated by the other ring.
But there are two of these stable contributing structures (I and II) for attack
at the a-position and only one (IV) for attack at the ^-position. On this basis we
would expect the carbonium ion resulting from attack at the a-position (and also
the transition state leading to that ion) to be much more stable than the carbonium
ion (and the corresponding transition state) resulting from attack at the /J-position,
and that nitration would therefore occur much more rapidly at the a-position.
Throughout our study of polynuclear hydrocarbons, we shall find that the
matter of orientation is generally understandable on the basis of this principle: of
the large number of structures contributing to the intermediate carbonium ion,
the important ones are those that require the smallest sacrifice of resonance
stabilization. Indeed, we shall find that this principal accounts for orientation not
only in electrophilic substitution but also in oxidation, reduction, and addition.
Friedel-Ctafts acylation of naphthalene
Naphthalene can be acetylated by acetyl chloride in the presence of aluminum
chloride. The orientation of substitution is determined by the particular solvent
used: predominantly alpha in carbon disulfide or solvents like tetrachloroethane,
predominantly beta in nitrobenzene. (The effect of nitrobenzene has been attributed to its forming a complex with the acid chloride and aluminum chloride
which, because of its bulkiness, attacks the roomier beta position.)
Sulfonation of naphthalene
Sulfonation of naphthalene at 80 yields chiefly 1-naphthalenesulfonic acid;
sulfonation at 160 or higher yields chiefly 2-naphthalenesulfonic acid. When
1-naphthalenesulfonic
acid is heated in sulfuric acid at 160, it is largely converted
into the 2-isomer. These facts become understandable when we recall that sulfonation is readily reversible (Sec. 11.12).
Sulfonation, like nitration and halogenation, occurs more rapidly at the
a-position, since this involves the more stable intermediate carbonium ion. But,
for the same reason, attack by hydrogen ion, with subsequent dislocations,
also occurs more readily at the a-position. Sulfonation at the ^-position occurs more
slowly but, once formed, the )3-sulfonic acid tends to resist sulfonating. At low
temperatures dislocations is slow and we isolate the product that is formed faster,
the alpha naphthalene sulfonic acid. At higher temperatures, dislocations become important, equilibrium is more readily established, and we isolate the
product that is more stable, the beta naphthalene sulfonic.
Naphthol's
Like the phenols we have already studied, naphthol's can be prepared from the
corresponding sulfonic acids by fusion with alkali. Naphthol's can also be
Orientation of electrophilic substitution in naphthalene derivatives
We have seen that naphthalene undergoes 'nitration and halogenation chiefly
at the a-position, and sulfonation and Friedel-Crafts acylation
at either the a- or
0-position, depending upon conditions. Now, to what position
will a second
substituent attach itself, and how is the orientation influenced by the group already
present?
Orientation of substitution in the naphthalene series is more complicated than
in the benzene series. An entering group may attach itself either to the ring that
already carries the first substituent, or to the other ring; there are seven different positions open to attack, in contrast to only three positions in a monosubstituted
benzene.
The major products of further substitution in a monosubstituted naphthalene
can usually be predicted by the following rules. As we shall see, these rules are
reasonable ones in light of structural theory and our understanding of electrophilic
aromatic substitution.
(a) An activating group (electron-releasing group) tends to direct further
substitution into the same ring. An activating group in position 1 directs further
substitution to position 4 (and, to a lesser extent, to position 2). An activating
group in position 2 directs further substitution to position 1.
(b) A deactivating group (electron-withdrawing group) tends to direct further
substitution into the other ring: at an a-position in nitration or halogenation, or at
an a- or /9-position (depending upon temperature) in sulfonation.
For example:
However, we can see that only the structures like III preserve an aromatic sextet; these are much more stable than the structures like IV and are the important ones. It is not surprising, therefore, that substitution occurs almost entirely at position 1.
Synthesis of naphthalene derivatives by ring closure. The Haworth synthesis
Derivatives of benzene, we have seen, are almost always prepared from a
compound that already contains the benzene ring: benzene itself or some simple substituted benzene. One seldom generates the benzene ring in the course of a
synthesis*.
While compounds containing other aromatic ring systems, too, are often
prepared from the parent hydrocarbon, there are important exceptions: syntheses
in which the ring system, or part of it, is actually generated. Such syntheses usually
involve two stages: ring closure (or cyclization) and aromatization.
As an example, let us look at just one method used to make certain naphthalene
derivatives: the Haworth synthesis (developed by R. D. Haworth at the University
of Durham, England). Figure 30.2 (p. 987) shows the basic scheme, which would
yield naphthalene itself (not, of course, actually prepared in this way).
All the steps are familiar ones. The reaction in which the second ring is
formed is simply Friedel-Crafts acylation that happens to involve two parts of the
same molecule. Like many methods of ring closure, this one does not involve a
new reaction, but merely an adaptation of an old one.
The success of this reaction depends upon the fact that a ketone reacts much faster
than an ester with a Bringard.
Nomenclature of anthracene and phenanthrene derivatives
The positions in anthracene and phenanthrene are designated by numbers as
shown
Structure of anthracene and phenanthrene
Like naphthalene, anthracene and phenanthrene are classified as aromatic on
the basis of their properties. Consideration of atomic orbitals follows the same
pattern as for naphthalene and leads to the same kind of picture: a flat structure
with partially overlapping
IT clouds lying above and below the plane of the molecule.
In terms of valence bonds, anthracene is considered to be a hybrid of structures
I-IV,
Reactions of anthracene and phenanthrene
Anthracene and phenanthrene are even less resistant toward oxidation or
reduction than naphthalene. Both hydrocarbons are oxidized to the 9,10-quinones
and reduced to the 9,10-dihydro compounds. Both the orientation of these reactions and the comparative ease with which they take place are understandable on
the basis of the structures involved. Attack at the 9- and 10-positions leaves two
benzene rings intact; thus there is a sacrifice of only 12 kcal of resonance energy
(84
-
2 x 36) for anthracene, and 20 kcal (92
-
2 x 36) for phenanthrene.
Both anthracene and phenanthrene undergo electrophilic substitution, with
a few exceptions, however, these reactions are of little value in synthesis because
of the formation of mixtures and Poly substitution products.
Derivatives of these
two hydrocarbons are usually obtained in other ways: by electrophilic substitution
in 9.10-anthraquinone or 9,10-dihydrophenanthrene, for example, or by ring
closure methods (Sees. 30.18 and.
Preparation of anthracene derivatives by ring closure. Anthraquinones
.18 Preparation of anthracene derivatives by ring closure. Anthraquinones
Derivatives of anthracene are seldom prepared from anthracene itself, but
rather by ring-closure methods. As in the case of naphthalene, the most important
method of ring closure involves adaptation of Friedel-Crafts acylation. The
products initially obtained are anthraquinones, which can be converted into corresponding anthracenes by reduction with zinc and alkali. This last step is seldom
carried out, since the quinones are by far the more important class of compounds.
The following reaction sequence shows the basic scheme. (Large amounts of
anthraquinones are manufactured for the dye industry in this .
Preparation of phenanthrene derivatives by ring closure
Starting from naphthalene instead of benzene, the Haworth succinic anhydride
synthesis (Sec. 30.14) provides an excellent route to substituted phanaeine's.
The basic scheme is outlined in Fig. 30.3. Naphthalene is acylated by succinic
anhydride at both the 1- and 2-positions; the two products are separable, and
either can be converted into phenanthrene. We notice that y-(2^naphthyl) butyric
acid undergoes ring closure at the l-position to yield phenanthrene rather than
at the 3-position to yield anthracene; the electron-releasing side chain at the
2-position directs further substitution to the 1-position (Sec. 30.13).
Substituted phenanthrenes are obtained by modifying the basic scheme in the
ways already described for the Haworth method (Sec. 30.14).
Carcinogenic hydrocarbons
Much of the interest in complex polynuclear hydrocarbons has arisen because
a considerable number of them have cancer-producing properties. Some of the
most powerful carcinogens are derivatives of 1,2-benzanthracene:
The relationship between carcinogenic activity and chemical properties is far
from clear, but the possibility of uncovering this relationship has inspired a
tremendous amount of research in the fields of synthesis and of structure and
reactivity.
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