NEET 2019 · ChemistryPrevious Year Question

Among the following, the reaction that proceeds through an electrophilic substitution is:

Answer: (B) $C X_{6} H X_{6} + Cl X_{2} AlCl X_{3} C X_{6} H X_{5} - Cl + HCl$ . \textbf{Answer:} (b) $C X_{6} H X_{6} + Cl X_{2} AlCl X_{3} C X_{6} H X_{5} - Cl + HCl$ \textbf{Solution:} Option (a) is a Sandmeyer-type reaction (decomposition of the diazonium salt, Cu-mediated), not electrophilic substitution.

A. $C X_{6} H X_{5} - N X_{2} X^{+} Cl X^{-} + Cu X_{2} Cl X_{2} C X_{6} H X_{5} - Cl + N X_{2}$
B. $C X_{6} H X_{6} + Cl X_{2} AlCl X_{3} C X_{6} H X_{5} - Cl + HCl$ ✓
C. $C X_{6} H X_{6} + Cl X_{2} UV light C X_{6} H X_{6} Cl X_{6}$
D. $C X_{6} H X_{5} - CH X_{2} - OH + HCl heat C X_{6} H X_{5} - CH X_{2} - Cl + H X_{2} O$

Correct Answer

(B) $C X_{6} H X_{6} + Cl X_{2} AlCl X_{3} C X_{6} H X_{5} - Cl + HCl$

Solution & Explanation

\textbf{Answer:} (b) $C X_{6} H X_{6} + Cl X_{2} AlCl X_{3} C X_{6} H X_{5} - Cl + HCl$ \textbf{Solution:} Option (a) is a Sandmeyer-type reaction (decomposition of the diazonium salt, Cu-mediated), not electrophilic substitution. Option (b): $AlCl X_{3}$ generates the electrophile $Cl X^{+}$ , which attacks the benzene ring. This is electrophilic aromatic substitution. Option (c): addition of $Cl X_{2}$ to benzene under UV light is a free-radical addition giving hexachlorocyclohexane (BHC), not substitution. Option (d): conversion of benzyl alcohol to benzyl chloride is a nucleophilic substitution at the $s p^{3}$ carbon. Hence only (b) is an electrophilic substitution.

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