At first sight benzene seems surprisingly unreactive for an unsaturated hydrocarbon. Benzene does not decolourise bromine water when shaken with it unlike the far more reactive alkenes.
Benzene undergoes electrophilic substitution reactions which preserve the stable aromatic ring rather than the electrophilic addition reactions of alkenes. Electophiles are positively charges ions or molecules with a partial positive charge that are attracted to the electron density of the benzene ring, and they react by accepting a pair of electrons to form a new covalent bond.
In the case of benzene, we need a very strong electrophile to coax it to react. The reaction happens via 2 steps:
Step 1 has a higher activation enthalpy and is the slow step in the reaction mechanism. The aromatic ring is broken as a new C-electrophile bond forms using delocalised ∏ electrons from the ring. Note also that the intermediate carbocation has a higher energy than either the reactants or products as it is not stabilised by a complete aromatic ring.
1. halogenation
Benzene reacts with liquid bromine in the presence of an anhydrous iron(III) bromide catalyst, and with gaseous chlorine in the presence of an anhydrous iron(III) chloride or aluminium chloride catalyst (these catalysts are often referred to as ‘halogen carriers’). The catalyst is needed to react with the bromine or chlorine to form a more powerful electrophile in a preliminary step to the mechanism shown above.
The C-Br or C-Cl bond in a halogenoarene is stronger than in a halogenoalkane because a lone pair of electrons on the halogen atom overlaps slightly with the delocalised ring of electrons in the benzene.
2. nitration
Refluxing benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid at 55°C gives us nitrobenzene. The electrophile is the nitronium ion, NO2+, which is produced in situ.
HNO3 + 2H2SO4 ⇾ NO2+ + 2HSO4– + H3O+
3. sulfonation
Benzenesulfonic acid is formed as a result of the reaction between benzene and fuming sulfuric acid which is a mixture of concentrated sulfuric acid and sulfur trioxide. The electrophile is HSO3+.
Benzenesulfonic acid is a strong acid, forming ionic salts with alkalis such as sodium hydroxide.
Attaching a sulfonate group, SO3–, to a benzene ring allows us to form soluble salts of aromatic compounds such as large dye molecules which will bind strongly to positively charged groups in the fibres of fabrics.
Practice questions
- 2-chloromethylbenzene is synthesised from methylbenzene as shown below:
(a) Write an equation for the formation of the electrophile in this reaction.
(b) Explain the role of anhydrous aluminium chloride in the reaction.
(c) Complete the mechanism for the reaction, including all the relevant curly arrows and charges.
2. Benzoic acid reacts with concentrated nitric acid in the presence of a concentrated sulfuric acid catlayst at a temperature of 100°C.
(a) Write an equation for the generation of the electrophile.
(b) Outline a mechanism for this reaction, showing how the catalyst is regenerated.
(c) The same nitrating mixture can be used to nitrate benzene with the reaction being carried out at 55°C. Explain the difference in the reaction conditions for the formation of nitrobenzene and 3-nitrobenzoic acid with reference to the bonding in each arene.
Answers
- (a) Cl2 + AlCl3 ⇾ Cl+ + AlCl4–
(b) AlCl3 is a catalyst in the reaction. It polarises the Cl2 molecule creating a stronger Cl+ electrophile allowing the reaction between chlorine and benzene to happen at room temperature.
(c)
2. (a) HNO3 + 2H2SO4 ⇾ NO2+ + 2HSO4– + H3O+
(or HNO3 + H2SO4 ⇾ NO2+ + HSO4– + H2O)
(b)
(c) Benzene is more reactive than benzoic acid hence the lower temperature needed for nitration.
The carboxylic acid group in benzoic acid is electron withdrawing and this lower electron density in the ring means that it is less susceptible / less open to attack by an electrophile.