Question

Which of the following is an example of electrophilic substitution reaction?

• A. \(\text{CH}_3\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH(OH)CN}\)
• B. \((\text{CH}_3)_3\text{CX} + \text{H}_2\text{O} \rightarrow (\text{CH}_3)_3\text{C-OH} + \text{HX}\)
• C. \(\text{C}_6\text{H}_6 + \text{CH}_3\text{COCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5(\text{COCH}_3) + \text{HCl}\)
• D. \(\text{Br CH}_2\text{CH}_2\text{Br} + \text{Zn} \xrightarrow{\text{alcohol}, \Delta} \text{CH}_2=\text{CH}_2 + \text{ZnBr}_2\)

Hint:

Reactions involving Benzene rings where a group replaces a Hydrogen atom (Nitration, Sulphonation, Halogenation, Friedel-Crafts) are almost always Electrophilic Aromatic Substitutions.

Answer 1

The Correct Option is C

Step 1: Analyze Mechanism of Each Option:

• (A) Nucleophilic Addition:
  Reaction of Acetaldehyde with HCN. The nucleophile (\(\text{CN}^-\)) attacks the electrophilic carbonyl carbon. \[ \text{CH}_3\text{-C(=O)H} \xrightarrow{\text{CN}^-} \text{CH}_3\text{-CH(O}^-\text{)-CN} \xrightarrow{\text{H}^+} \text{CH}_3\text{-CH(OH)-CN} \]
• (B) Nucleophilic Substitution (\(\text{S}_{\text{N}}1\)):
  Hydrolysis of tertiary butyl halide. The nucleophile (\(\text{H}_2\text{O}\)) substitutes the leaving group (Halide \(\text{X}^-\)).
• (C) Electrophilic Aromatic Substitution:
  This is Friedel-Crafts Acylation. The Lewis acid catalyst (\(\text{AlCl}_3\)) reacts with acetyl chloride to generate an electrophile (Acylium ion, \(\text{CH}_3\text{-C}^+\text{=O}\)). \[ \text{CH}_3\text{COCl} + \text{AlCl}_3 \rightarrow \text{CH}_3\text{C}^+\text{O} + \text{AlCl}_4^- \] This electrophile attacks the electron-rich benzene ring, substituting a hydrogen atom. \[ \text{C}_6\text{H}_6 + \text{E}^+ \rightarrow \text{C}_6\text{H}_5\text{E} + \text{H}^+ \] This matches the definition of electrophilic substitution.
• (D) Elimination:
  Reaction of vicinal dihalide with Zinc dust. It removes two bromine atoms to form a double bond (Debromination). This is an elimination reaction.

Final Answer:
The reaction in option (C) is an electrophilic substitution.