Identifying Compounds A and B in the Reaction

The given reaction involves the conversion of compound A to compound B through a series of steps. The reaction scheme is:

Reaction scheme

This is a classic example of the Rosenmund reduction followed by a nucleophilic addition reaction.

Step-by-Step Explanation:

Step 1: Identify the reaction conditions.

The first step uses $H_{2}$ gas with a poisoned catalyst (Pd/BaSO₄). This is characteristic of the Rosenmund reduction, where an acyl chloride (acid chloride) is selectively reduced to an aldehyde. The reaction is:

$R - C (= O) - Cl \overset{H_{2}, Pd/BaSO 4_{}}{\to} R - C (= O) - H$

Therefore, compound A must be an acyl chloride.

Step 2: Analyze the second step.

The second step involves the compound formed from the first step (the aldehyde) reacting with $H C N$ in the presence of a base (Na₂CO₃). This is a standard nucleophilic addition reaction to form a cyanohydrin. The reaction is:

$R - C (= O) - H + H C N \overset{{Na}^{+} {CO}_{3}^{2-}}{\to} R - C H (O H) C N$

Therefore, compound B is a cyanohydrin.

Step 3: Match the structures to the options.

Looking at the options, we need to find the pair where A is an acyl chloride and B is the corresponding cyanohydrin.

Option 1:
A is an acyl chloride, B is a cyanohydrin. This is correct.
Option 2:
A is an acid, not an acyl chloride.
Option 3:
A is an ester, not an acyl chloride.
Option 4:
A is an amide, not an acyl chloride.

Final Answer: The correct option is the first one. Compound A is $C H_{3} C H C H C O C l$ and compound B is $C H_{3} C H C H C H (O H) C N$ .

Related Topics:

Rosenmund Reduction
Nucleophilic Addition Reactions
Preparation of Aldehydes
Cyanohydrin Formation
Carbonyl Chemistry

Important Formulae and Theory:

1. Rosenmund Reduction:
$R - C O C l + H_{2} \overset{Pd/BaSO 4_{}, S}{\to} R - C H O + H C l$
This reaction selectively reduces acid chlorides to aldehydes without further reduction to alcohols. The catalyst is "poisoned" with sulfur (S) to moderate its activity.

2. Cyanohydrin Formation:
$R - C H O + H C N \overset{{CN}^{-}}{\to} R - C H (O H) C N$
This is a nucleophilic addition reaction where the cyanide ion ( ${CN}^{-}$ ) attacks the electrophilic carbonyl carbon. A base is often used to generate the nucleophilic cyanide ion from HCN.