Questions & explanations
1. Which of the following polyhalogen compounds was widely used as an insecticide but is now banned in many countries due to its persistence in the environment and harmful effects on wildlife?
- Freon-12
- BHC (benzene hexachloride)
- DDT
- Chloroform
Answer: DDT
DDT (dichlorodiphenyltrichloroethane) is a synthetic insecticide that was extensively used but is now banned or restricted because of its environmental persistence, bioaccumulation, and toxicity to non-target organisms. Freon-12 is a refrigerant that depletes the ozone layer; BHC is also an insecticide but its ban is more region-specific; chloroform is an anaesthetic and solvent.
2. Which of the following reactions is used to prepare biphenyl from bromobenzene?
- Wurtz reaction
- Fittig reaction
- Wurtz-Fittig reaction
- Sandmeyer reaction
Answer: Fittig reaction
The Fittig reaction involves the coupling of two aryl halides with sodium metal in dry ether to form a biaryl compound like biphenyl. The Wurtz reaction couples alkyl halides, the Wurtz-Fittig reaction couples an alkyl and an aryl halide to give an alkylbenzene, and the Sandmeyer reaction converts diazonium salts to aryl halides.
3. Why are haloarenes much less reactive than haloalkanes towards nucleophilic substitution reactions?
- Due to the presence of a strong C–X bond
- Due to the resonance stabilization of the aryl halide which gives partial double bond character to the C–X bond
- Due to the sp³ hybridized carbon in the C–X bond
- Due to the inductive effect of the benzene ring
Answer: Due to the resonance stabilization of the aryl halide which gives partial double bond character to the C–X bond
In haloarenes, the lone pair on the halogen participates in resonance with the benzene ring, imparting partial double bond character to the C–X bond. This makes the bond stronger and difficult to break in nucleophilic substitution. The other options do not address the key resonance effect.
4. Which of the following products is predominantly formed when chlorobenzene is nitrated?
- o-Nitrochlorobenzene
- m-Nitrochlorobenzene
- p-Nitrochlorobenzene
- A mixture of o- and p-nitrochlorobenzene
Answer: A mixture of o- and p-nitrochlorobenzene
Chlorine is ortho-para directing (though deactivating) in electrophilic aromatic substitution. Nitration of chlorobenzene therefore gives mainly o- and p-nitrochlorobenzene, with the ortho product being slightly less favoured due to steric hindrance. The meta isomer is a minor product.
5. The conversion of chlorobenzene to phenol is carried out by treating chlorobenzene with:
- Aqueous NaOH at room temperature
- Aqueous NaOH at 300 °C and high pressure
- Alcoholic NaOH at room temperature
- Aqueous KOH at 100 °C
Answer: Aqueous NaOH at 300 °C and high pressure
Haloarenes are extremely unreactive towards nucleophilic substitution; hence drastic conditions (high temperature and high pressure) are required. The Dow process uses aqueous NaOH at about 300 °C to convert chlorobenzene to phenol. Mild conditions do not effect the reaction.
6. Which factor is primarily responsible for the partial double bond character of the C–X bond in haloarenes?
- Inductive effect of the halogen
- Resonance effect of the halogen with the benzene ring
- Hyperconjugation
- Electromeric effect
Answer: Resonance effect of the halogen with the benzene ring
The lone pair on the halogen atom delocalises into the π-electron system of the benzene ring, creating resonance structures that give the C–X bond partial double bond character. Inductive, hyperconjugation, and electromeric effects are not the primary cause here.
7. Which of the following is the most suitable method for the preparation of chlorobenzene from benzene?
- Chlorination with Cl₂ in presence of UV light
- Chlorination with Cl₂ in presence of FeCl₃
- Chlorination with Cl₂ in presence of sunlight
- Chlorination with Cl₂ in presence of peroxide
Answer: Chlorination with Cl₂ in presence of FeCl₃
Benzene undergoes electrophilic substitution with Cl₂ in the presence of a Lewis acid catalyst like FeCl₃ to give chlorobenzene. UV light, sunlight, or peroxide conditions lead to addition or radical reactions, not substitution.