Electrophilic Substitution: Benzene and its derivatives react with an electrophile which replaces
a hydrogen atom on benzene.
1) Nitration of Benzene: Benzene reacts with nitric acid to form nitrobenzene
in electrophilic substitution. Reaction catalysed by conc. sulfuric acid and heated to 50O
C.
Water bath used to maintain steady temperature. Important to control temperature. If
temperature above 50O
C, further substitution reactions may occur, e.g. dinitrobenzene, provided
enough nitric acid.
Nitrobenzene: Nitrobenzene used in dyes, pharmaceuticals and fibres.
Reaction Mechanism of Nitration of Benzene: Include three all steps in question. Arrows out
then in.
Step 1- The electrophile is nitronium ion, NO2
+
, which is produced in reaction with acid catalyst.
Step 2- NO2
+
accepts a pair of electrons from the benzene ring to form a dative covalent bond.
The intermediate is unstable and breaks down to form stable nitrobenzene and H+
ion.
Step 3- Catalyst sulfuric acid is regenerated, HSO4
–
is from step 1.
Electrophile: An electrophile accepts a pair of electrons.
2) Halogenation of Benzene: Halogens don’t react with benzene unless
catalyst called halogen carrier present e.g. AlCl3, FeCl3, AlBr3 and FeBr3, which are genenrated in
situ.
Bromination of Benzene: Benzene reacts with bromine in electrophilic substitution reaction. At
room temperature and pressure and in presence of halogen carrier. Bromine only
monosubstituted unless stated otherwise- unlike bromiation of phenol (tri-) and cyclohexene
(di-).
Reaction Mechanism of Bromination of Benzene:
Step 1- Benzene too stable to react with non- polar Br. The electrophile, bromonium ion Br +
produced from reaction of halogen carrier catalyst with bromine.
Chlorination of Benzene: Chlorine will react the same as bromine. Halogen carrier used is FeCl 3,
or AlCl3. An iron metal and chlorine may be reacted to make FeCl3.
Halogen Carrier Catalyst: Halogen carrier introduces a permanent dipole on Cl2 as forms Cl+
which allows reaction to take place.
3) Alkylation Reactions: AKA Friedel- Crafts alkylation. Alkylation of benzene is
when an alkyl group substitutes a H atom in benzene ring.
React benzene with haloalkane. In presence of AlCl3, a halogen carrier (Friedel- Crafts) catalyst,
to generate an electrophile. Example of electrophilic substitution = same mechanism.
Alkylation increases number of carbon atoms in compound.
Electrophile formed in any reaction always positive- CH3+ here.
If CH3Cl reacts, the electrophile CH3+ formed rather than Cl+. Since Cl more electronegative than
C so when bond breaks by heterolytic fission- both electrons go to Cl not CH3.
Make sure + charge is on the carbon that has lost a chlorine/ bromine. And this is the bit that is
attached. So if + charge in middle of chain, the carbon chain attached to benzene will have
branches.
4) Acylation Reactions: Acylation reaction is when acyl chloride reacts with
benzene in presence of an AlCl3 catalyst. Product is aromatic ketone.
Example of electrophilic substitution. Also increases number of C atoms.
Ethanoyl chloride CH3COCl is first member of acyl chloride homologous series.
Even if not benzene, carbon chain takes priority over Br or Cl when substituting in. If addition
would be both carbon chain and Br/ Cl.
+ charge must be on carbon of CH3CO and that carbon is what is attached.
An aromatic ketone.
Comparing Reactivity of Alkenes with Arenes…
Alkenes: The electrophilic addition learnt in AS. The alkene/ cycloalkane decolourises bromine.
In benzene pi electrons are delocalised, in alkenes electrons are localised between two C atoms.
Benzene has lower electron density than alkenes. Benzene polarises and attract Br 2 less (not able
to induce a dipole in bromine). More energy needed to break the π bond, more stable.
Localised electrons in pi bond induce dipole in non- polar bromine molecule, making one end
slightly positive and other slightly negative, which acts like an electrophile.
Benzene does not react with bromine unless halogen carrier present and it’s electrophilic
substitution.
Can use this reaction to test for alkene/ double bond.
Methyl groups attached also increase electron density of benzene so are more reactive than
benzene. If in position 1, then methyl group direct groups into position 2 and 4.