Question

Halogenation of benzene in presence of anhy. $AlC{{l}_{3}}$is
A. nucleophilic substitution
B. nucleophilic addition
C. electrophilic substitution
D. free radical substitution

Answer 1

Hint: The concept of alkylation of benzene using anhydrous $AlC{{l}_{3}}$ is to be used in this question. $AlC{{l}_{3}}$ acts as a Lewis acid, that means, it is deficient in electrons and helps in generating species.

Complete step by step answer:
In order to answer our question, we need to study the reactions given by arenes. Arenes are expected to behave like unsaturated hydrocarbons and must be reactive chemically. However, they show characteristics similar to the saturated hydrocarbons (i.e., alkanes). They take part in the substitution reactions easily and resist addition and oxidation. The stability of benzene and arenes is because of the delocalisation of the $\pi $electrons due to resonance. Let us talk about the substitution reactions arenes show:
Halogenation: Chlorination or bromination of benzene is carried by reacting with the halogen in the absence of sunlight and in the presence of some ferric or aluminium salts (in anhydrous state) called halogen carriers. These are Lewis acids and they carry the halogen to the aromatic ring.
Nitration of benzene is done by heating with a nitrating mixture of conc. $HN{{O}_{3}}$ and conc. ${{H}_{2}}S{{O}_{4}}$ to about 330 K. As a result, a hydrogen atom in the ring gets substituted by the nitro ($-N{{O}_{2}}$) group. The nitration is carried by nitric acid while sulphuric acid is added to catalyse the reaction
Sulphonation: Sulphonation of benzene is carried by heating with fuming sulphuric acid or oleum (conc. ${{H}_{2}}S{{O}_{4}}$ containing dissolved $S{{O}_{3}}$) at nearly 330 K. As a result, a hydrogen atom in the ring gets substituted by a sulphonic acid ($S{{O}_{3}}H$) group.
Alkylation and acylation: Alkylation means the substitution of a hydrogen atom in the ring by an alkyl (R) group while acylation means the substitution by an acyl (RCO) group. These reactions are carried by reacting benzene with either alkyl halide or acyl chloride in the presence of anhydrous,$AlC{{l}_{3}}$. The reaction is called Friedel-Crafts reaction.

Benzene donates a pair of electrons to the electrophile, as it is electron rich, hence, the hydrogen atom is replaced by a halogen.

Hence, it is electrophilic aromatic substitution and we obtain our correct answer as option C.

Note: Direct iodination of benzene is not successful because the HI formed in the reaction is a powerful reducing agent and will combine with iodobenzene to form benzene again. Thus, the reaction will be reversible.