i. Which catalyst is commonly used in the halogenation of benzene with Cl₂ or Br₂?
Answer: b) FeCl₃
Explanation: FeCl₃ (or FeBr₃ for bromination) is a Lewis acid catalyst used in the electrophilic substitution halogenation of benzene. It generates the electrophile (Cl⁺ or Br⁺) by polarizing the halogen molecule.
ii. In the reaction of benzene with Cl₂ in the presence of a catalyst, the product is:
Answer: b) Chlorobenzene
Explanation: The electrophilic substitution of benzene with chlorine in the presence of a Lewis acid catalyst (like AlCl₃ or FeCl₃) replaces one hydrogen atom with a chlorine atom, forming chlorobenzene (C₆H₅Cl).
iii. What is the major product of the reaction between benzene and Br₂ with FeBr₃ as a catalyst?
Answer: b) Bromobenzene
Explanation: The reaction of benzene with Br₂ in the presence of FeBr₃ is an electrophilic substitution that yields bromobenzene (C₆H₅Br) and HBr as the major product.
iv. Which mechanism do primary haloalkanes typically follow in nucleophilic substitution reactions?
Answer: b) SN₂
Explanation: Primary haloalkanes have minimal steric hindrance around the carbon bearing the halogen, making them ideal for the bimolecular SN₂ mechanism, where the nucleophile attacks from the backside.
v. What type of bond is broken in the nucleophilic substitution reaction of a haloalkane?
Answer: c) C-X (where X is a halogen)
Explanation: In a nucleophilic substitution reaction, the nucleophile attacks the electrophilic carbon and replaces the leaving group, which is the halide ion (X⁻). Therefore, the C-X bond is broken.
vi. Which factor does NOT significantly affect the rate of SN₁ reactions?
Answer: c) Strength of the nucleophile
Explanation: SN₁ reactions are unimolecular, and the rate-determining step is the formation of the carbocation intermediate. The rate depends on the substrate concentration, solvent polarity, and leaving group ability, but not on the concentration or strength of the nucleophile.
vii. Which haloalkane is more reactive in an SN₂ reaction?
Answer: a) Chloromethane
Explanation: In SN₂ reactions, steric hindrance is a major factor. Chloromethane (CH₃Cl) has the least steric hindrance (only three hydrogen atoms around the carbon), making it the most reactive. Chloroethane (CH₃CH₂Cl) has slightly more hindrance. Chlorobenzene is unreactive due to the C-Cl bond's partial double bond character and the stability of the aromatic ring.
viii. The C-Cl bond in chlorobenzene is less reactive towards nucleophilic substitution because:
Answer: c) Resonance effect makes the bond partially double
Explanation: In chlorobenzene, the lone pair of electrons on the chlorine atom can delocalize into the benzene ring through resonance. This gives the C-Cl bond partial double bond character, making it stronger and shorter, and thus less reactive towards nucleophilic substitution.
