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
- CHLORINATION & BROMINATION
Part 10.8 AROMATIC HYDROCARBONS (ARENES) -
introduction to arene electrophilic substitutions. Chlorination/bromination to aromatics like
chlorobenzene, bromobenzene etc. 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.
10.8.3 The electrophilic substitution of an arene -
(example of aromatic halogenation)
Organic synthesis of chloro/bromo aromatic compounds by reaction of
chlorine/bromine with benzene/methylbenzene
Examples of aromatic
chlorination/bromination substitution reactions
What is the mechanism
for chlorinating/bromination benzene? or chlorinating/brominating methyl benzene?
via ring substitution reactions.
TO ADD BROMINATION
+ Cl2 ==> C6H5Cl + HCl
Chlorine is bubbled
into a mixture of the arene and anhydrous aluminium chloride catalyst.
Other catalysts like anhydrous iron(III) chloride can be used, and they
are collectively known as halogen carriers.
mechanism 21 -
electrophilic substitution by halogen in a benzene ring
21 above] When R = H, benzene forms
(1) The non-polar and uncharged
chlorine molecule is not a strong enough an electrophile to disrupt
of the benzene ring. The aluminium chloride reacts with a chlorine
molecule to form a positive chlorine ion Cl+
which is a much stronger electron pair accepting
electrophile and a tetrachloroaluminate(III) ion (either this or an
AlCl3-Cl2 complex - details not needed for A
(2) An electron pair from the
electrons of the
benzene ring forms a C-Cl bond with the electron pair accepting
positive chlorine 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
tetrachloroaluminate(III) ion, formed in step (1), abstracts a
proton from the highly unstable intermediate carbocation to give the
chloro-aromatic product, hydrogen chloride gas and reform the
aluminium chloride catalyst.
+ Cl2 ==> ClC6H4CH3
when R = CH3,
methylbenzene forms a mixture of chloro-2/3/4-methylbenzene.
reaction is the substitution of -H by -Cl
can be carried in the same way by mixing bromine, the aromatic
hydrocarbon (arene) with a halogen carrier catalyst such as
Why do aromatic
compounds tend to react by electrophilic substitution BUT
alkenes tend to react by electrophilic addition?
interact with electrophiles because they both have 'electron
rich' electron pair donating bonding systems i.e. the >C=C<
double bond in alkenes and the delocalised ∏
the benzene ring, but the benzene ring has a particularly high
stability which is preserved on substitution. For the same
reason alkenes are generally more reactive than arenes.
If methyl benzene
is reacted with chlorine in the presence of uv light, substitution
takes place in the alkyl side chain. In other words it behaves like
an alkane and undergoes a free radical substitution reaction.
The initial product is chloromethylbenzene, C6H5CH2Cl,
and further substitution products can be formed C6H5CHCl2
and C6H5CCl3. This illustrates the
significance of changing reaction conditions which function via a
different mechanism to give a different product.
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 + Cl2 ==> C6H5Cl + HCl initiation: free radical
substitution chlorination chain propagations: Cl. + C6H5CH3 ==> HCl + C6H5CH2
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