Q10 / 150 · MHT CET 2025 (21 Apr Shift 2) · Chemistry

What type of arenes are obtained when arene diazonium chloride is treated with fluoroboric acid and then heated further?

  • A
    $\mathrm{Ar}-\mathrm{NO}_2$
  • Correct answer
    B
    $\mathrm{Ar}-\mathrm{F}$
  • C
    $\mathrm{Ar}-\mathrm{Cl}$
  • D
    Ar-H

Solution

The reaction is the Balz-Schiemann reaction, which synthesizes fluoroarenes from arene diazonium salts.

Arenediazonium fluoroborate forms when arenediazonium chloride $\mathrm{ArN_2^+Cl^-}$ reacts with fluoroboric acid $\mathrm{HBF_4}$:

$\mathrm{ArN_2^+Cl^- + HBF_4 \to ArN_2^+BF_4^- + HCl}$

Thermal decomposition of arenediazonium fluoroborate produces fluoroarene $\mathrm{ArF}$, nitrogen gas, and boron trifluoride:

$\mathrm{ArN_2^+BF_4^- \xrightarrow{heat} ArF + N_2 \uparrow + BF_3}$

The product is a fluoroarene $\mathrm{ArF}$, corresponding to option B.

Hence, the correct answer is option: 'B'.

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