Which of the following reagents will bring out the following transformation ? (A) \(\text{NaNO}_2\)/HCl and Cu/HCl (B) \(\text{NaNO}_2\)/NaOH and Cu/HCl (C) \(\text{NaNO}_2\)/HCl and \(\text{Cu}_2\text{Cl}_2\)/HCl (D) \(\text{NaNO}_2\)/NaOH and \(\text{Cu}_2\text{Cl}_2\)/HCl Choose the correct answer from the options given below:
Show Hint
Diazotisation needs acid, not NaOH. Then Sandmeyer (Cu2Cl2/HCl) or Gattermann (Cu/HCl) gives chlorobenzene.
Step 1: Understanding the Question:
Aniline is converted to chlorobenzene. The -NH2 group must be replaced by -Cl. This is done in two stages: diazotisation, then replacement of the diazonium group by chlorine.
Step 2: Stage one, diazotisation.
Aniline reacts with sodium nitrite and hydrochloric acid at 273-278 K to give benzenediazonium chloride. The acid is needed because it makes nitrous acid from NaNO2. \[ \text{C}_6\text{H}_5\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{273-278\ \text{K}} \text{C}_6\text{H}_5\text{N}_2^{+}\text{Cl}^{-} + \text{NaCl} + 2\text{H}_2\text{O} \]
Step 3: Stage two, replacement by chlorine.
The diazonium salt gives chlorobenzene in two named reactions. In the Sandmeyer reaction we use cuprous chloride, Cu2Cl2 in HCl. In the Gattermann reaction we use copper powder in HCl. Both give C6H5Cl and nitrogen gas.
Step 4: Check (A).
NaNO2/HCl then Cu/HCl is the Gattermann reaction. It works, so (A) is correct.
Step 5: Check (B).
NaNO2 with NaOH does not make nitrous acid, because the medium is basic. A diazonium salt is not formed from aniline under these conditions. So (B) is wrong.
Step 6: Check (C).
NaNO2/HCl then Cu2Cl2/HCl is the Sandmeyer reaction. So (C) is correct.
Step 7: Check (D).
Same problem as (B): NaOH prevents diazotisation. So (D) is wrong.
Final Answer:
Only (A) and (C) give chlorobenzene from aniline. This is option 1.
\[ \boxed{\text{(A) and (C) only (option 1)}} \]
