Question

Consider the given sequence of reactions

The compound \(D\) cannot be obtained by

• A. Etard reaction
• B. Friedel--Crafts reaction
• C. Rosenmund reaction
• D. Gattermann--Koch reaction

Hint:

Important methods for preparing benzaldehyde: \[ \boxed{ \begin{aligned} &\text{Etard reaction} &\text{Rosenmund reduction} &\text{Gattermann--Koch formylation} \end{aligned} } \] Friedel--Crafts reaction does not directly give benzaldehyde.

Answer 1

The Correct Option is B

Step 1: Identify compounds \(A,\;B,\;C,\;D\). \[ 3\mathrm{C_2H_2} \rightarrow \boxed{\mathrm{C_6H_6}} \] Hence, \[ A=\text{Benzene} \] \[ \mathrm{C_6H_6} \xrightarrow[\mathrm{AlCl_3}]{\mathrm{CH_3Cl}} \boxed{\mathrm{C_6H_5CH_3}} \] Thus, \[ B=\text{Toluene} \] Side-chain chlorination with two moles of chlorine gives \[ \boxed{\mathrm{C_6H_5CHCl_2}} \] Hydrolysis of benzal chloride gives \[ \boxed{\mathrm{C_6H_5CHO}} \] Therefore, \[ D=\text{Benzaldehyde} \]

Step 2: Identify the method which cannot prepare benzaldehyde. Benzaldehyde can be prepared by \[ \boxed{\text{Etard reaction}} \] \[ \boxed{\text{Rosenmund reaction}} \] \[ \boxed{\text{Gattermann--Koch reaction}} \] Friedel--Crafts reaction introduces alkyl or acyl groups into benzene but does not directly prepare benzaldehyde. Hence, \[ \boxed{(B)} \] is the correct answer.