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Revising organic chemistry
Doc Brown's
Summary of organic reaction mechanisms
Part IV Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions
Revision
notes
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.
Revision notes for GCE Advanced
Subsidiary Level AS Advanced Level A2 IB
Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters
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(equal to US grade 11 and grade 12 and Honours/honors level courses)
Part IV AROMATIC
HYDROCARBONS (Arenes)
The five
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.
The electrophilic substitution of an arene - nitration

mechanism 19
- electrophilic substitution in the nitration of the benzene ring
-
[mechanism
19 above] Benzene is converted into nitrobenzene,
when R = H.
-
When R = CH3,
methylbenzene will form a mixture of the three possible substitution
products methyl-2/3/4-nitrobenzene,
-
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+. 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.
-
Step
(3) An electron pair from the
delocalised
pi
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
of ring).
-
Step
(4)
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
ring.
The electrophilic substitution of an arene -
chlorination
(example of aromatic halogenation)

mechanism 21 -
electrophilic substitution by halogen in a benzene ring
-
[mechanism
21 above] When R = H, benzene forms
chlorobenzene.
-
Step
(1) The non-polar and uncharged
chlorine molecule is not a strong enough an electrophile to disrupt
the
pi
electron system
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
level).
-
Step
(2) An electron pair from the
delocalised
pi
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
ring).
-
Step
(3) The
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.
-
Also consider
C6H5CH3
+ Cl2 ==> ClC6H4CH3
+ HCl
-
when R = CH3,
methylbenzene forms a mixture of chloro-2/3/4-methylbenzene.
-
FURTHER COMMENTS
-
The overall
halogenation
reaction is the substitution of -H by -Cl
-
Bromination
can be carried in the same way by mixing bromine, the aromatic
hydrocarbon (arene) with a halogen carrier catalyst such as
anhydrous AlBr3
or FeBr3.
-
Why do aromatic
compounds tend to react by electrophilic substitution BUT
alkenes tend to react by electrophilic addition?
-
They both
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 ∏
electrons of
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.
-
initiation:
-
chain
propagations:
-
chain
terminations:
The electrophilic substitution of an arene -
alkylation
(Friedel-Crafts reaction)

mechanism 23 -
electrophilic substitution by an alkyl group in the benzene ring
-
[mechanism
23 above] If R' = H, benzene would form
methylbenzene if chloromethane was used.
-
Step
(1) The weakly polar and uncharged
halogenoalkane molecule is not a strong enough an electrophile to
disrupt the
pi
electron system
of the benzene ring. The aluminium chloride reacts with the
halogenoalkane molecule to form a carbocation which is a much
stronger
electron pair accepting electrophile than the original acid
chloride (either this or an AlCl3-R3Cl complex
- details not needed for A level).
-
Step
(2) An electron pair from the
delocalised
∏
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).
-
Step
(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
methylbenzene: C6H5CH3
+ R3C-Cl ==> R3C-C6H4CH3
+ HCl
-
A mixture of
polysubstituted alkyl aromatic compounds are formed.
-
e.g. using
chloromethane, 1,2- or 1,3- or 1,4-dimethylbenzene will be
formed,
-
FURTHER COMMENTS
-
The overall
alkylation
reaction is the substitution of -H by -CR3
-
Bromoalkanes
can also be used for alkylation, but more expensive. Similar catalysts
can be used e.g. anhydrous AlBr3 or FeBr3.
The electrophilic substitution of an arene -
acylation
(Friedel-Crafts reaction)

mechanism 25 -
electrophilic substitution by an acyl group in the benzene ring
-
[mechanism
25 above] If ethanoyl chloride, CH3COCl, was
used (R=CH3-), benzene forms phenylethanone, C6H5-CO-CH3.
-
Step
(1)
Although the acid chloride molecule is polar, it is still not a strong
enough electrophile to disrupt the
pi
electron system of the benzene ring. The aluminium chloride reacts
with an acid chloride molecule to form a
carbocation (acylonium ion, RCO+) which is a
much stronger electron pair accepting electrophile
than the original acid chloride (either this or an AlCl3-RCOCl
complex - details not needed for A level).
-
Step
(2) An electron pair from the
delocalised
pi
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.
-
Step
(3) is a proton transfer, as the
tetrachloroaluminate(III) ion [formed in step (1)], abstracts a
proton from the second highly unstable intermediate carbocation to
give the ketone product, hydrogen chloride gas and reforming the
aluminium chloride catalyst.
-
for R =
CH3, benzene: C6H5CH3
+ R'COCl ==> R'COC6H4CH3 +
HCl
-
FURTHER
COMMENTS
The electrophilic substitution of an arene -
sulphonation

mechanism 25 -
electrophilic substitution by an acyl group in the benzene ring
The
orientation of products in aromatic
electrophilic substitution reactions
-
Certain
groups, already present, can increase the electron density of
the benzene ring and make the aromatic compound more reactive
towards electrophiles
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.
-
Groups
that increase reactivity are e.g. -CH3,
-Cl, -OH, -NH2, -NHCOCH3, and
favour substitution at the 2 and 4 positions (typically
90-100% combined).
-
They all,
by some means, have a small, but significant, electron
donating (+I inductive effect) on the ring of
pi
electrons.
-
For
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.
-
Certain
groups, already present, can decrease the electron density of
the benzene ring and make the aromatic compound less reactive
towards electrophiles
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.
-
Groups
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.
-
For
example, nitrobenzene is much less reactive than benzene
and on nitration, 93% of the product is 1,3-dinitrobenzene.
-
-
| (french) Réviser la
chimie organique Doc Brown Résumé des mécanismes des
réactions organiques Partie IV Hydrocarbures aromatiques -
Arènes - réactions de substitution électrophile. Notes de
révision complète, dont des diagrammes et l'explication des
mécanismes moléculaires et les «équation et des conditions
de réaction et d'autres en cours de réaction voies-con et
les produits sont également expliqués. Notes de révision
pour subsidiaire avancée Niveau Réviser Chimie révision des
cours pour les étudiants pré-universitaires (égal à US 11 e
et 12 e année et distinctions / cours de niveau spécialisé)
Partie IV HYDROCARBURES AROMATIQUES - Introduction à la
arène substitutions électrophiles Nitration de donner nitro-aromatiques
comme le nitrobenzène La chloration de chloro-aromatiques
comme le chlorobenzène Alkylation de donner alkyl-aromatiques
comme-[Artisanat] Réaction de Friedel méthylbenzène
Acylation de donner cétones aromatiques Friedel Crafts
réaction Sulfonation / sulfonation de donner une sulfonique
/ acide sulfonique comme l'acide benzène / acide
benzènesulfonique. L'orientation des produits de
substitution aromatique (1,5 / 3,5 / 4 positions, ortho /
meta / produits de substitution par) * |
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