Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. You are also familiar with naphthalene balls which are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. They burn with sooty flame.
Arenes are characterised by electrophilic substitution reactions. However, under special conditions they can also undergo addition and oxidation reactions.
Electrophilic substitution reactions
The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E+)
(i) Nitration: A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture).
(ii) Halogenation: Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes.
(iii) Sulphonation: The replacement of a hydrogen atom by a sulphonic acid group in a ring is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum).
(iv) Friedel-Crafts alkylation reaction: When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.
Why do we get isopropyl benzene on treating benzene with 1-chloropropane instead of n-propyl benzene?
(v) Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (AlCl3) yields acyl benzene.
If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous AlCl3 can be chlorinated to hexachlorobenzene (C6Cl6)
Mechanism of electrophilic substitution reactions:
According to experimental evidences, SE (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps:
(a) Generation of the eletrophile
(b) Formation of carbocation intermediate
(c) Removal of proton from the carbocation intermediate
(a) Generation of electrophile E
During chlorination, alkylation and acylation of benzene, anhydrous AlCl3, being a Lewis acid helps in generation of the elctrophile Cl+ , R+ , RC+O (acylium ion) respectively by combining with the attacking reagent.
In the case of nitration, the electrophile, nitronium ion, NO2+ is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner:
It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base. Thus, it is a simple acid-base equilibrium.
(b) Formation of Carbocation (arenium ion): Attack of electrophile results in the formation of σ-complex or arenium ion in which one of the carbon is sp3 hybridised.
Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at sp3 hybridised carbon.
(c) Removal of proton: To restore the aromatic character, σ-complex releases proton from sp3 hybridised carbon on attack by [AlCl4] – (in case of halogenation, alkylation and acylation) and [HSO4] – (in case of nitration).
Under vigorous conditions, i.e., at high temperature and/ or pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane.
Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C6H6Cl6 which is also called gammaxane.
Combustion: When heated in air, benzene burns with sooty flame producing CO2 and H2O