Question

Which of the following reaction represents Rosenmund reduction ?

• A. \(\text{R}-\text{CO}-\text{Cl}\rightarrow _{\text{Pd}-\text{BaSO}_4\,}^{\text{H}_2\,}\text{R}-\text{CHO}+\text{HCl}\)
• B. \(\text{R}-\text{CN}\rightarrow _{\text{H}_3\text{O}^+\,}^{\text{SnCl}_2,\text{HCl}\,}\text{R}-\text{CHO}+\text{NH}_4\text{Cl}\)
• C. \(\text{R}-\text{CHO}\rightarrow _{\Delta \,}^{\text{Zn}-\text{Hg conc.HCl}\,}\text{R}-\text{CH}_3+\text{H}_2\text{O}\)
• D. \(\text{R}-\text{CO}-\text{R}\rightarrow _{\text{ii) KOH, HO}-\text{CH}_2-\text{CH}_2-\text{OH}, \Delta \,}^{\text{i) H}_2\text{N}-\text{NH}_2\,}\text{R}-\text{CH}_2-\text{R}\)

Hint:

Rosenmund reduction converts an acyl chloride to an aldehyde using H2 over poisoned Pd-BaSO4.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Rosenmund reduction is the partial hydrogenation of an acyl chloride (acid chloride) to an aldehyde.

Step 2: Key Details:
The catalyst is palladium on barium sulphate, and it is partially poisoned (with quinoline or sulphur) so the aldehyde is not reduced further to an alcohol.

Step 3: Detailed Explanation:
\[ \text{R-CO-Cl} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4} \text{R-CHO} + \text{HCl} \]
This is option (A).

Step 4: Why the other options are wrong.
Option (B) is the Stephen reduction (nitrile to aldehyde with \(\text{SnCl}_2/\text{HCl}\)). Option (C) is Clemmensen reduction (Zn-Hg with conc. HCl). Option (D) is Wolff-Kishner reduction (hydrazine then KOH in ethylene glycol).

Final Answer:
Option (A) represents the Rosenmund reduction. \[ \boxed{\text{(A)}} \]