Question

Identify the set of reagents (X) in the given reaction sequence

• A. (i) $\text{HBF}_4$ ; (ii) $\text{Conc. }\text{HNO}_3 + \text{H}_2\text{SO}_4$
• B. (i) $\text{HBF}_4$ ; (ii) $\text{NaNO}_2$, $\text{Cu}, \Delta$
• C. (i) $\text{BF}_3$ ; (ii) $\text{NaNO}_2$, $\text{Cu}, \Delta$
• D. (i) $\text{H}_2\text{O}, 283\text{ K}$ ; (ii) $\text{Conc. }\text{HNO}_3$

Hint:

Fluoroboric acid ($\text{HBF}_4$) is widely used to prepare stable, isolable diazonium fluoroborates.
This salt can either be heated alone to form fluorobenzene (Schiemann reaction) or heated with $\text{NaNO}_2/\text{Cu}$ to form nitrobenzene.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We are given a reaction map starting with aniline.
Aniline is converted to benzene diazonium chloride (A), then to benzene (B), and then to nitrobenzene (C).
We need to identify the single set of reagents "X" that can convert benzene diazonium chloride (A) directly to nitrobenzene (C).

Step 2: Key Formula or Approach:
The direct conversion of a diazonium salt to a nitro compound is achieved by first precipitating the diazonium fluoroborate salt using fluoroboric acid ($\text{HBF}_4$).
This stable intermediate is then heated in the presence of sodium nitrite ($\text{NaNO}_2$) and copper powder (Cu) as a catalyst.

Step 3: Detailed Explanation:

• Let us analyze the reaction sequence shown:
Aniline ($\text{C}_6\text{H}_5\text{NH}_2$) reacts with $\text{NaNO}_2 + \text{HCl}$ at $273-278\text{ K}$ to form benzene diazonium chloride (A):
\[ \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2 + \text{HCl}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \quad (\text{A}) \]

• Benzene diazonium chloride (A) can be reduced to benzene (B) using hypophosphorous acid ($\text{H}_3\text{PO}_2$):
\[ \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{H}_3\text{PO}_2/\text{H}_2\text{O}} \text{C}_6\text{H}_6 \quad (\text{B}) \]

• Benzene (B) undergoes nitration with concentrated nitric and sulfuric acids to yield nitrobenzene (C):
\[ \text{C}_6\text{H}_6 \xrightarrow{\text{Conc. }\text{HNO}_3/\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2 \quad (\text{C}) \]

• Now, for the direct path A $\rightarrow$ C:
Benzene diazonium chloride is treated with fluoroboric acid ($\text{HBF}_4$) to form benzene diazonium fluoroborate:
\[ \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{HBF}_4 \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{BF}_4^- \downarrow + \text{HCl} \] This fluoroborate precipitate is then heated with aqueous sodium nitrite ($\text{NaNO}_2$) solution in the presence of copper powder (Cu):
\[ \text{C}_6\text{H}_5\text{N}_2^+\text{BF}_4^- + \text{NaNO}_2 \xrightarrow{\text{Cu}, \Delta} \text{C}_6\text{H}_5\text{NO}_2 + \text{N}_2 \uparrow + \text{NaBF}_4 \] This synthetic route is the standard preparation method for nitrobenzene from diazonium salts.


Step 4: Final Answer:
The correct set of reagents X is (i) $\text{HBF}_4$; (ii) $\text{NaNO}_2, \text{Cu}, \Delta$.