Part 7.
The chemistry of
AROMATIC COMPOUNDS
Doc
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
K12 grade 11 grade 12 organic chemistry
Part 7.5 Electrophilic substitution -
nitration of benzene & methylbenzene, properties and uses of
nitro-aromatics
Sub-index for this page
7.5.1
The reagents, reaction conditions,
products and
equations for the nitration of benzene and methylbenzene
7.5.2
The electrophilic
substitution mechanism of
nitration of arenes like benzene and methylbenzene
7.5.3
The physical properties of some
nitro-aromatic compounds obtained from arenes
7.5.4
Selected chemical reactions of some
nitro-aromatic compounds obtained from arenes
7.5.5
The uses of some nitro-aromatic
compounds obtained from nitrating arenes
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7.5.1 The reagents, reaction conditions and equations for
the nitration of aromatic compounds
particularly benzene and
methylbenzene
Methylbenzene is more reactive towards
electrophiles than benzene itself.
The methyl group has a small, but not
insignificant effect of raising the electron density around the ring,
particularly at the ring carbons 2 (= 6) and 4 (the 'fuzzy' sketch above
tries to illustrate the idea).
Nitration of an arene involves heating
the compound with a mixture of a concentrated nitric acid and sulfuric
acid mixture.
However, the temperature and heating time
are important to control whether you want a mono-nitro-substituted compound
or an aromatic molecule containing two or more nitro groups in the benzene
ring.
Examples of aromatic
nitration substitution reactions
(a)
+ HNO3 ===>
+ H2O
benzene + nitric acid
===> nitrobenzene + water
The conc. nitric acid, conc.
sulfuric acid and benzene are heated together in a flask fitted with
a reflux condenser, at ~60oC, taking less than an hour.
If the temperature rises above
65oC, a 2nd nitro group is substituted in the ring yielding the main
product to be 1,3-dinitrobenzene - see equation (e).
You need a temperature of 90oC
to complete the reaction to mainly 1,3-dinitrobenzene see (b).
(b)
+ HNO3
===>
+ H2O
nitrobenzene + nitric
acid ===> 1,3-dinitrobenzene + water
1,3-dinitrobenzene is
the majority product, BUT, you will still get some
small quantities 1,2-dinitrobenzene and 1,4-dinitrobenzene.
(c)
+ HNO3 ===>


+ H2O
methylbenzene + nitric acid ===> methyl-2/3/4-nitrobenzene + water
Three
structural-positional isomers C7H7NO2 formed in different
proportions.
Methylbenzene reacts faster than
benzene, because the methyl group activates the ring by increasing
its electron density.
The nitration reactivity order
is: methylbenzene > benzene > nitrobenzene
see section
7.14 for reactivity explanations and
orientation of products.
(d)
+ 3HNO3 ===>
+ 3H2O
The explosive TNT has the structure , the
acronym comes from its historic-trivial name of 2,4,6-trinitotolune, toluene was
the old name for methylbenzene.
A more systematic name for TNT is
2,4,6-trinitromethylbenzene.
I've now included the nitration of other
types of aromatic compound for completeness and variety, but I'll only cover
the electrophilic substitution mechanisms for the nitration of benzene and
methylbenzene.
(e)
+
HNO3 ===>
+ H2O
benzoic acid + nitric
acid ===> 3-nitrobenzoic acid + water
the 3-nitrobenzoic acid
is the majority product, BUT, you will also get some 2-nitrobenzoic
acid and 4-nitrobenzoic acid.
(f)
+ HNO3 ===>


+ H2O
chlorobenzene +
nitric acid ===> chloronitrobenzenes + water
3 structural-positional
isomers of C6H4NO2Cl,
1-chloro-2-nitrobenzene (chloro-2-nitrobenzen), 1-chloro-3-nitrobenzene
(chloro-3-nitrobenzene),
1-chloro-4-nitrobenzene (chloro-4-nitrobenzene), formed in different proportions.
TOP OF PAGE and sub-index
7.5.2 The electrophilic mechanism of
nitration of arenes like benzene and methylbenzene
The nitrating mixture
consists of concentrated nitric acid (source of the nitro group -NO2)
and concentrated sulphuric acid which acts as a catalyst
and as a
strong acid.
The overall nitration
reaction is the substitution of -H in the benzene ring by the nitro
group -NO2
Mechanism diagram 19
(above) - illustrates the electrophilic substitution in the nitration of the benzene ring
-R can be H, alkyl or
other group including -COOH, -Cl, -Br and even -NO2
itself.
For mono-nitration of the benzene
ring in a C6H5-R compound, apart from when R =
H, there are three possible substitution products i.e. substitution
at the 2, 3 or 4 position in the benzene ring, R is considered
position 1 here.
Step
(1) The
sulphuric acid protonates the nitric acid (strong acid, but
weaker than H2SO4)
Step
(2) The protonated nitric acid loses a water molecule
via a sulphuric acid molecule, to generate the electrophile,
the nitronium ion,
NO2+.
The nitronium cation is
a much more powerful electrophile, i.e. its a positive ion and a
stronger electron pair acceptor,
more so
than the original nitric acid, and the NO2+ is needed to attack the very
stable aromatic ring of benzene.
Steps
(1) and (2) can be written as:
2H2SO4
+ HNO3 ==> NO2+ + H3O+
+ 2HSO4-
Step
(3) The positive
nitronium ion attacks the electron rich pi orbital benzene rings of
the aromatic compound.
An electron pair from the
delocalised
pi
electrons of the benzene ring forms a C-N bond with the electron
pair accepting nitronium ion forming a highly unstable carbocation.
It is very unstable because the stable electron arrangement of the
benzene ring is partially broken to give a 'saturated' C (top right
of ring).
Step
(4) The
hydrogensulfate ion (HSO4-, formed in step
(1), abstracts a proton from the highly unstable intermediate
carbocation and simultaneously the nitro-aromatic product
(and stable aromatic ring) is formed.
Thus, simultaneously,
deprotonation,
the -H proton is abstracted by a base (hydrogensulfate ion)
reforming the sulfuric acid catalyst and reforming the stable aromatic
ring of pi orbitals in the substituted product nitrobenzene.
The
hydrogensulfate ion, HSO4- has
been shown in the style of -:O-SO2-OH,
to emphasize the importance a lone pair on the oxygen
abstracting the proton from the aromatic carbocation.
Note:
Like alkenes, arenes are susceptible to electrophilic attack because of the
high electron density of the delocalised electrons of the pi
orbitals involved in the carbon-carbon bonding.
So both
show little reactivity towards nucleophilic reagents - electron pair
donors that would tend to be repelled.
However two points
should be considered because of the particular stability of the
aromatic (benzene) ring.
(i) This makes
aromatic compounds less reactive than alkenes, which readily
undergo addition rather than substitution.
(ii) Unlike alkenes, aromatic
compounds do
not usually undergo addition, because this will remove the
stability conferred on the molecule by the benzene ring.
By
under going substitution rather than addition, the stable
aromatic ring is preserved.
Having introduced the general
electrophilic substitution mechanism for introducing a nitro group
(NO2) into a benzene ring, I've now drawn diagrams to
illustrate four specific nitrations of aromatic compounds and kept
the notes to a minimum, since there is a detailed description above.
To add more notes to the
diagrams below
Mechanism diagram 79A shows the
electrophilic substitution mechanism for nitrating benzene to yield
nitrobenzene.
Mechanism diagram 79E shows the reaction
progress
profile for the final two stages of the nitration of benzene.
First the NO2+
electrophile attacks the pi electron cloud of benzene to give the
unstable carbocation in which the aromatic pi orbital rings are broken.
Then, simultaneously, the -H proton
is abstracted by a base (hydrogensulfate ion) and the stable aromatic
ring of pi orbitals is re-formed to yield the substituted product
nitrobenzene.
Ea1
= the higher activation energy for the initial electrophile attack on
the pi orbitals of the benzene ring - which is the rate determining step
of the mechanism - the change is from stable benzene ring to highly
unstable carbocation.
Ea2
= the much lower activation energy, for the unstable carbocation, as the
proton is expelled, re-forming the stable pi orbital rings of the
benzene product (or the benzene ring of any aromatic compound undergoing
electrophilic substitution).
This diagram applies to ALL
electrophilic substitution nitration reactions of aromatic compounds.
Mechanism diagram 79B shows the electrophilic
substitution mechanism for nitrating methylbenzene to yield
methyl-2-nitrobenzene.
Mechanism diagram 79C shows the electrophilic
substitution mechanism for nitrating benzoic acid to yield 3-nitrobenzoic acid.
Mechanism diagram 79D shows the electrophilic
substitution mechanism for nitrating nitrobenzene to yield 1,2-dinitrobenzene.
Mechanism diagram 79F shows the electrophilic
substitution mechanism for nitrating chlorobenzene to yield chloro-4-nitrobenzene.
TOP OF PAGE and sub-index
7.5.3
Physical properties of some
nitro-aromatic compounds obtained from arenes
Nitroarenes and other nitroaromatic compounds
Notes on physical properties
(a) They are all more dense than
water.
(b) They are generally insoluble in
water.
(c) The lower members are colourless
or pale yellow liquids at room temperature or low melting solids.
TOP OF PAGE and sub-index
7.5.4
Selected chemical reactions of some
nitro-aromatic compounds obtained from arenes
(a) Reduction to amines
(i) In industry nitro-compounds are
reduced by mixing with hydrogen and passing the mixture over heated
nickel catalyst.
e.g. methyl-2-nitrobenzene to
methyl-2-phenylamine
+ 6[H]
== Ni/H2 ==>
+ 2H2O
(ii) Nitro-compounds can be reduced in laboratory
using lithium tetrahydridoaluminate(III)
NaBH4, is not a powerful enough reducing agent to reduce
nitro–aromatic compounds.
LiAlH4 is a more
powerful reducing agent than NaBH4 and in ether solvent readily
reduces nitro–aromatics to primary aromatic amines, the simplified equation
for nitrobenzene to phenylamine is
...
C6H5NO2 + 6[H]
===> C6H5NH2 + 2H2O
and methylnitrobenzenes would
be reduced to methylphenylamine primary amines, i.e.
CH3C6H4NO2 + 6[H]
===> CH3C6H4NH2
+ 2H2O
as will any aromatic
compound with a nitro group (–NO2) attached directly to the benzene
ring.
(iii) Nitro-aromatic compounds
are reduced by refluxing with tin and hydrochloric acid.
C6H5NO2 + 6[H]
===> C6H5NH2
+ 2H2O
but the 'real' equations
are rather more complicated, the simplest redox equation I can come up with is
2C6H5NO2(aq)
+ 14H+(aq) + 3Sn(s) ===> 2C6H5NH3+(aq)
+ 3Sn4+(aq) + 4H2O(l)
The nitro group is reduced and the
tin oxidised.
The phenylamine can be separated from the
'messy' reaction mixture by steam distillation.
For full details see
preparation of phenylamine from
nitrobenzene
Amines are very important
compounds for the manufacture products as diverse as drugs and dyes.
(b) Electrophilic substitutions in the
benzene ring
(i)
Nitration
This involves further nitration
of an already nitrated aromatic compound using the concentrated
nitric acid and sulfuric acid mixture.
Previously nitrated aromatic
compounds can be further nitrated to introduce another nitro group
into the benzene ring e.g.
+ HNO3
===>
+ H2O
nitrobenzene + nitric
acid ===> 1,3-dinitrobenzene + water
1,3-dinitrobenzene is
the majority product, BUT, you will still get some
1,2-dinitrobenzene and 1,4-dinitrobenzene.
(ii)
Halogenation
Nitro-aromatic compounds will
undergo halogenation when refluxed with aluminium chloride and
chlorine passed into the mixture e.g. starting with nitrobenzene.
+ Cl2 ===>
+ HCl
The principal product is
chloro-3-nitrobenzene.
You can synthesise
bromo-3-nitrobenzene by refluxing nitrobenzene with bromine and
iron(III) bromide catalyst.
TOP OF PAGE and sub-index
7.5.5 The uses of some nitro-aromatic
compounds obtained from arenes
Nitroaromatics are one of the
most important groups of intermediate aromatic compounds used in
industrial organic synthesis.
Reduction to amines, see
7.5.4 reaction (a), which are used in many pharmaceutical products,
dyes and polyamide polymers..
Nitro-aromatics are used in
explosives, the best known being TNT.
Need
x-reference with phenols and aromatic amines.
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