Doc Brown's GCE Chemistry
Revision Notes PART 10
Summary of organic reaction mechanisms -
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions
Part 10.8 AROMATIC HYDROCARBONS (ARENES) -
introduction to arene electrophilic substitutions. Alkylation to give alkyl-aromatics like methylbenzene
[Friedel-Crafts reaction]. The orientation of products in aromatic substitution
(1,2-; 1,3-; and 1,4-
positions for two substituents in the benzene ring, old names -
ortho/meta/para substitution products). The revision
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 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 double bond. Arenes tend
to undergo substitution, rather than addition, because substitutions allows
the very stable benzene ring to remain intact.
10.8.4 The electrophilic substitution of an arene -
Organic synthesis of alkyl substituted aromatic compounds by reaction of
halogenoalkanes (haloalkanes) with benzene/methylbenzene
Examples of aromatic
Friedel Crafts alkylation substitution reactions
What is the mechanism
for alkylating benzene? or methyl benzene?
+ R3C-Cl ==> C6H5-CR3
+ HCl (R = H, alkyl, aryl)
The arene is refluxed
with a chloroalkane and anhydrous aluminium chloride catalyst.
mechanism 23 -
electrophilic substitution by an alkyl group in the benzene ring
23 above] If R' = H, benzene would form
methylbenzene if chloromethane was used.
(1) The weakly polar and uncharged
halogenoalkane molecule is not a strong enough an electrophile to
of the benzene ring. The aluminium chloride reacts with the
halogenoalkane molecule to form a carbocation which is a much
electron pair accepting electrophile than the original acid
chloride (either this or an AlCl3-R3Cl complex
- details not needed for A level).
(2) An electron pair from the
electrons of the
benzene ring forms a C-C bond with the electron pair accepting
carbocation forming a second 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).
(3) is a proton transfer, as the
tetrachloroaluminate(III) ion [formed in step (1)], abstracts a
proton from the highly unstable intermediate carbocation to give the
alkyl-aromatic product, hydrogen chloride gas and reform the
aluminium chloride catalyst.
If R' = CH3
+ R3C-Cl ==> R3C-C6H4CH3
A mixture of
polysubstituted alkyl aromatic compounds are formed.
chloromethane, 1,2- or 1,3- or 1,4-dimethylbenzene will be
reaction is the substitution of -H by -CR3
can also be used for alkylation, but more expensive. Similar catalysts
can be used e.g. anhydrous AlBr3 or FeBr3.
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
methylbenzene benzene C6H6 + R3C-Cl ==> C6H5-CR3 + HCl R' = CH3 electrophilic substitution
alkylation methylbenzene C7H8: C6H5CH3 + R3C-Cl ==> R3C-C6H4CH3 + HCl benzene
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