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Part 7.5
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 the preparation of
1-methyl-2-nitrobenzene apparatus reagents separating funnel fractional
distillation
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Part 7.5 Electrophilic substitution -
nitration of benzene and methylbenzene, properties and uses of
nitro-aromatics
(brief mention of nitration of chlorobenzene, benzoic acid and naphthalene)
Sub-index for this page
7.5.1
Reagents, reaction conditions,
products and
equations for the nitration of benzene, methylbenzene and naphthalene
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
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 round-bottomed 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 (b).
Nitrobenzene is a pale yellow liquid
mpt. 6oC and bpt. 211 oC with a strong smell of
almonds.
It is a very important intermediate
in the synthesis of dyes and pharmaceutical products including
paracetamol.
You need a temperature of 90oC
to complete the reaction to mainly 1,3-dinitrobenzene see equation (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 ===>
1-methyl-2/3/4-nitrobenzene + water
Three
structural-positional isomers C7H7NO2
are formed in different
proportions.
The main products are
1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene
Methylbenzene reacts faster than
benzene, because the methyl group activates the benzene ring by increasing
its electron density (+I effect).
This preparation can be done
using the same apparatus as above for preparing nitrobenzene,
but you don't heat the water in the beaker water bath, in fact
you may need to employ ice as a cooling agent and you don't want
to make 2,4,6-trinitomethylbenzene, otherwise known as the
explosive TNT !!!
The nitration reactivity order
is: methylbenzene > benzene > nitrobenzene
Note that if the atom of the initial substituent group is directly bonded to
the benzene ring does not have any π
bonding the ring is usually activated compared to benzene
itself. The methyl group tends to increase the electron density of the ring
and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions.
Therefore in methylbenzene, the 2 and 4 positions become the preferred
substitution points for nitration in the benzene ring of methylbenzene.
see section
7.14 for reactivity theory explanations and
orientation of products.
More
details on preparing 1-methyl-2-nitrobenzene (2-methylnitrobenzene)
 1. Methylbenzene is nitrated with an
equal volume mixture of concentrated nitric and sulfuric acids.
Methylbenzene is more reactive
than benzene, so a lower reaction temperature is used.
2. The acid and methylbenzene mixture
is gently heated to around 30oC in the reflux water bath system
illustrated above - a high temperature will lead to the formation of
dinitromethylbenzenes and at a lower temperature the reaction is too
slow.
 3.
After completion, water is added to dilute and dissolve the acids. Then
the mixture is poured into a separating funnel and the bottom layer
containing the 1-methyl-2-nitrobenzene drained off via the tap into a
beaker.
4. Using the separating funnel the organic liquid is washed with sodium
carbonate solution to remove any remaining acid. The organic liquid is
further washed with water to remove traces of the sodium carbonate
solution.
5. The oily organic layer is dried
with lumps of anhydrous calcium chloride.
6. The organic liquid is decanted
into a round-bottom flask and fractionally distilled collecting the
fraction ~223-225oC the boiling point of
1-methyl-2-nitrobenzene is 225oC. 1-methyl-4-nitrobenzene boils at ~230oC
and should condense back into the flask (theoretically!).
The product of further nitration of
the benzene ring - beware!
(d)
+ 3HNO3 ===>
+ 3H2O
If the nitration of methylbenzene
is allowed to continue you will obtain a high yield of
2,4-dinitrobenzene and some 2,6-dinitrobenzene and eventually
2,4,6-trinitromethylbenzene.
This preparation should not be
attempted because the products are highly explosive!
The explosive TNT has the structure , the
acronym comes from its historic-trivial name of 2,4,6-trinitotolune,
(or just trinitrotoluene) and toluene was
the old name for methylbenzene.
A more systematic name for TNT is
2,4,6-trinitromethylbenzene.
It is very unstable substance and
readily explodes if not handled carefully.
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.
NOTE if the atom of the initial
substituent group is directly bonded to the benzene ring does have
any π
bonding the ring is usually deactivated compared to benzene
itself. In this case the -COOH group tends to decrease the
electron density of the ring and more so at the 2, 4 and 6 positions,
compared to the 3 and 5 positions. Therefore the 3- position become the
preferred 2nd substitution point in the benzene ring of benzoic acid.
(See
section 7.14 for more details on this electrophilic substitution
reaction).
(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.
Major products.
The more electronegative
chlorine atom deactivates the benzene ring, but less so at the
2, 4 and 6 positions, hence the 3- substitution yields the minor
product.
(For more details on the
theory of electrophilic substitution see
section 7.14).
The nitration of naphthalene
At room temperature, naphthalene reacts directly with
concentrated nitric acid to yield mainly 1-nitronaphthalene.
This reaction is in principle the same electrophilic substitution reaction
undergone by benzene and methylbenzene.
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 than nitric acid, 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 covalent
(sigma) 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
pi electron ring 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 electron orbitals in the substituted product nitrobenzene.
The
hydrogensulfate ion, HSO4- has
been shown in the style of -:O-SO2-OH,
to emphasize the importance of 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 of pi
electrons.
The greater
delocalisation in the benzene ring makes it more stable
with a lower electron density system for the
electrophile to attack.
By
under going substitution rather than addition, the stable
aromatic ring of pi electrons 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.
Mechanism diagram 79A shows the
electrophilic substitution mechanism for nitrating benzene to yield
nitrobenzene.
Initially the generation of the
nitronium ion, NO2+,
a powerful electrophile - electron pair acceptor, more so than the
original nitric acid.
The pi electron cloud donates a pair
of electrons to the nitronium ion to form a covalent C-N bond, so one of
the ring carbons is saturated with loss of the extra stability of the
original benzene ring of pi electrons.
A proton is expelled and abstracted
by the hydrogensulfate ion to give final product of nitrobenzene as the
stable aromatic benzene ring of pi electrons is reformed and the sulfuric acid catalyst
is also regenerated.
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.
Initially the generation of the
nitronium ion, NO2+,
a powerful electrophile, a more powerful electron pair acceptor than the
original nitric acid.
The pi electron cloud donates a pair
of electrons to the nitronium ion to form a covalent C-N bond, so one of
the ring carbons is saturated with loss of the extra stability of the
original complete benzene ring of pi electrons.
A proton is expelled and abstracted
by the hydrogensulfate ion to give final product of
methyl-2-nitrobenzene as the stable aromatic benzene ring of pi
electrons is reformed
and the sulfuric acid catalyst is also regenerated.
The final substitution product
has now regained the stable ring of pi electrons of the benzene
ring.
You can also get substitution in the
3 and 4 positions, but the mechanism details are the same.
The 2 and 4 positions are favoured in
the electrophilic substitution of methylbenzene.
Mechanism diagram 79C shows the electrophilic
substitution mechanism for nitrating benzoic acid to yield 3-nitrobenzoic acid.
The 3 position is favoured in the
electrophilic substitution of benzoic acid.
The mechanism description is identical to
those already described for the nitration of benzene and methylbenzene.
Mechanism diagram 79D shows the electrophilic
substitution mechanism for nitrating nitrobenzene to yield 1,2-dinitrobenzene.
The 3 position is favoured in the
electrophilic substitution of nitrobenzene.
The mechanism description is identical to
those already described for the nitration of benzene and methylbenzene.
Mechanism diagram 79F shows the electrophilic
substitution mechanism for nitrating chlorobenzene to yield chloro-4-nitrobenzene.
The 2 and 4 positions are favoured in the
electrophilic substitution of chlorobenzene.
The mechanism description is identical to
those already described for the nitration of benzene and methylbenzene.
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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