Doc Brown's GCE Chemistry - GCE A Level Revision Notes PART 10
Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
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Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions
Part 10.8 AROMATIC HYDROCARBONS (ARENES) -
introduction to arene electrophilic substitutions.
Nitration to give
nitro-aromatics like nitrobenzene.
The orientation of products in aromatic substitution (1,2-; 1,3-; and
positions for two substituents in the benzene ring, old names - ortho/meta/para substitution products).
include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions
and other con-current reaction pathways and products are also explained
for the reaction mechanisms of aromatic hydrocarbons like benzene and
Part 10.8 AROMATIC
10.8.1 Introduction to the reactivity of aromatic
e.g. the arenes benzene and methyl benzene
Why do aromatic hydrocarbon
molecules primarily react via electrophilic substitution reaction?
reactions described Part 10.8 are electrophilic substitution reactions involving the
generation of a powerful electrophile (electron pair acceptor) which
subsequently attacks the electron rich
π (pi) electron system of the
to undergo substitution, rather than addition, because substitutions allows
the very stable benzene ring to remain intact.
10.8.2 The electrophilic substitution of an arene - nitration
Organic synthesis of nitro aromatic compounds by reaction of conc.
sulfuric acid & nitric acid with benzene/methylbenzene
Examples of aromatic
nitration substitution reactions
+ HNO3 ==>
+ HNO3 ==>
nitrobenzene + nitric
acid ==> 1,3-dinitrobenzene + water
the majority product, BUT, you will still get some
1,2-dinitrobenzene and 1,4-dinitrobenzene.
+ HNO3 ==>
nitric acid ==> chloronitrobenzenes
isomers of C6H4NO2Cl,
1-chloro-4-nitrobenzene, formed in different proportions.
What is the mechanism
for nitrating benzene? or methyl benzene?
benzene : C6H6 + HNO3
==> C6H5NO2 + H2O
+ HNO3 ==> O2NC6H4CH3
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
- electrophilic substitution in the nitration of the benzene ring
19 above] Benzene is converted into nitrobenzene,
when R = H.
When R = CH3,
methylbenzene will form a mixture of the three possible substitution
sulphuric acid protonates the nitric acid (strong acid, but
weaker than H2SO4)
The protonated nitric acid loses a water molecule
via a sulphuric acid molecule, to generate the electrophile,
the nitronium ion, NO2+. This is
a much more powerful electrophile, i.e. electron pair acceptor,
than the original nitric acid, and is needed to attack the very
stable aromatic ring of benzene.
(3) An electron pair from the
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
The hydrogensulphate ion (HSO4-,
formed in step (1), abstracts a proton from the highly unstable
intermediate carbocation to give the nitro-aromatic product and
reform the sulphuric acid catalyst as well as the stable benzene
orientation of products in aromatic
electrophilic substitution reactions
groups, already present, can increase the electron density of
the benzene ring and make the aromatic compound more reactive
such as those described above. However
the effect seems to enhance the reactivity at the 2 and 4
substitution positions more than the 3 substitution position.
that increase reactivity are e.g. -CH3,
-Cl, -OH, -NH2, -NHCOCH3, and
favour substitution at the 2 and 4 positions (typically
by some means, have a small, but significant, electron
donating (+I inductive effect) on the ring of
example, methyl benzene is significantly more reactive
than benzene and when nitrated, over 90% of the products
are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.
groups, already present, can decrease the electron density of
the benzene ring and make the aromatic compound less reactive
such as described above.
However the effect seems to decrease the reactivity at the 2 and
4 substitution positions more than the 3 substitution position.
that decrease reactivity, by some means, are e.g.
-NO2, COOH, -CHO, -SO2OH, and favour
substitution at the 3 position (typically 70-90%) and their
effect does fit in with them all being strongly
electronegative groupings giving a
-I inductive effect.
example, nitrobenzene is much less reactive than benzene
and on nitration, 93% of the product is 1,3-dinitrobenzene.
ALL OF THESE ARE NOW ON SEPARATE PAGES
10.8.3 The electrophilic substitution of an arene -
(example of aromatic halogenation)
10.8.4 The electrophilic substitution of an arene -
10.8.5 The electrophilic substitution of an arene -
mechanism (Friedel-Crafts reaction)
10.8.6 The electrophilic substitution of an arene -
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms electrophilic substitution nitration
methylbenzene benzene C6H6 +
HNO3 ==> C6H5NO2 + H2O nitration of methylbenzene C7H8 C6H5CH3 + HNO3 ==> O2NC6H4CH3 + H2O 2H2SO4 + HNO3 ==>
NO2+ + H3O+ + 2HSO4-
and Organic Synthesis INDEX
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