Aromatic hydrocarbons are highly unsaturated molecules but behave like saturated hydrocarbons. Those which contain a benzene ring are called benzenoids but those which contain some highly unsaturated ring other than benzene are called non-benzenoids. Their aromatic character can be ascertained through Hückel rule. Aromatic hydrocarbons are, however, more reactive than alkanes but are less reactive than alkenes and alkynes. The lower reactivity of arenes is because of the extra stability associated with these molecules due to delocalization of -electrons. Their stability is measured in terms of resonance energy which can be estimated from either heat of combustion or heat of hydrogenation data. They normally undergo electrophilic substitution reactions. In presence of a Lewis acid catalyst, nuclear halogenation occurs but in absence of Lewis acid catalyst and in presence of light, halogens add to the benzene ring. If an aromatic hydrocarbon contains an alkyl side chain, then in presence of heat/light side chain halogenation occurs in preference to addition of halogens to the benzene ring. The reactivity of aromatic hydrocarbons towards electrophilic substitution reactions depends upon the electron density in the benzene ring. Electron-donating groups favour while electron-withdrawing groups retard these reactions. Orientation of electrophilic substitution reactions is governed by the nature of the substituent already present in the ring. Although aromatic hydrocarbons are resistant to oxidising agents `(KMnO_4, K_2Cr_2O_7`, etc.) they do undergo ozonolysis.
Reaction of benzene with excess of `Cl_2` in presence of light and in presence of anhydrous `AlCl_3` and in dark give respectively

Options

A. hexachlorobenzene and benzene hexachloride
B. benzene hexachloride and hexachlorobenzene
C. both give benzene hexachloride
D. none of the above

Answer

To solve the question regarding the reaction of benzene with excess of Cl₂ in the presence of light and anhydrous AlCl₃, we need to analyze the two scenarios separately:

### Step-by-Step Solution:

1. **Reaction of Benzene with Cl₂ in Presence of Light:**
- When benzene reacts with chlorine (Cl₂) in the presence of light, a free radical mechanism occurs. This leads to the substitution of hydrogen atoms in the benzene ring with chlorine atoms.
- The product formed in this reaction is **Benzene Hexachloride (C₆H₆Cl₆)**, also known as **Lindane**. This is because the benzene ring can undergo multiple substitutions, resulting in the formation of a fully chlorinated compound.

2. **Reaction of Benzene with Cl₂ in Presence of Anhydrous AlCl₃:**
- In the presence of anhydrous aluminum chloride (AlCl₃), which acts as a Lewis acid, the reaction proceeds via an electrophilic aromatic substitution mechanism.
- The chlorine molecule is polarized by AlCl₃, forming a chloronium ion (Cl⁺), which then attacks the benzene ring.
- The product formed in this case is **Chlorobenzene (C₆H₅Cl)**. This is because the reaction typically leads to the substitution of one hydrogen atom on the benzene ring with a chlorine atom.

### Final Products:
- **In presence of light:** Benzene Hexachloride (C₆H₆Cl₆)
- **In presence of anhydrous AlCl₃:** Chlorobenzene (C₆H₅Cl)