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.
Sulfonation/sulfonation to give a
sulfonic/sulfonic acid like benzenesulfonic acid/benzenesulfonic acid.
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.
10.8.6 The electrophilic substitution of an arene -
The organic synthesis of sulfonic/sulfonic acids by reaction of
benzene/methylbenzene with conc. sulfuric/sulfuric acid
Examples of aromatic
sulfonation substitution reactions
+ H2SO4 ==>
+ H2SO4 ==>
What is the mechanism
for sulfonating benzene or methyl benzene?
for R =
H, benzene: C6H6
+ SO3 ==> C6H5SO2OH
[see mechanism 44
+ H2SO4 ==> C6H5SO2OH
Benzene is heated with
concentrated sulfuric acid or even better, 'fuming' sulfuric acid,
which has a higher sulfur trioxide content and more efficient at
introducing the sulfonic acid group into the benzene ring.
mechanism 25 - electrophilic
substitution by a sulfonate group in the benzene ring
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.
keywords phrases: reaction conditions formula intermediates
organic chemistry reaction mechanisms electrophilic substitution benzene
methylbenzene C6H6 + SO3 ==> C6H5SO2OH or C6H6 + H2SO4 ==> C6H5SO2OH + H2O R =
CH3, benzene: C6H5CH3 + H2SO4 ==> CH3C6H4SO2OH + H2O sulfonation reaction is the
substitution of -H by -SO2OH Sulfonation
and Organic Synthesis INDEX
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