`CH_(3)COCl+H_(2)overset(Pd//BaSO_(4))underset("quinoline")rarrCH_(3)CHO.`
Here quinoline acts as

Options

A. Catalyst
B. Catalyst poison
C. Promoter
D. Medium

Answer

To solve the question regarding the role of quinoline in the reaction of acetyl chloride with hydrogen in the presence of a Lindlar catalyst, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Identify the Reaction**: The given reaction is:
\[ CH_3COCl + H_2 \overset{Pd/BaSO_4}{\underset{\text{quinoline}}{\rightarrow}} CH_3CHO \]
Here, acetyl chloride (CH₃COCl) is being reduced to acetaldehyde (CH₃CHO) using hydrogen gas (H₂) in the presence of a catalyst.

2. **Understand the Role of the Catalyst**: The catalyst used here is a Lindlar catalyst, which is a palladium catalyst supported on barium sulfate. It is used for the partial hydrogenation of alkynes and can also facilitate the reduction of acyl chlorides to aldehydes.

3. **Analyze the Function of Quinoline**: Quinoline is added to the reaction mixture. Its role is crucial because it affects the activity of the catalyst. Specifically, quinoline acts to inhibit or suppress the further reduction of acetaldehyde (the product) to ethanol.

4. **Define the Term 'Catalyst Poison'**: A catalyst poison is a substance that decreases the activity of a catalyst. In this case, quinoline prevents the Lindlar catalyst from fully reducing acetaldehyde to ethanol, allowing the reaction to stop at the aldehyde stage.

5. **Conclude the Role of Quinoline**: Since quinoline suppresses the activity of the Lindlar catalyst, we conclude that quinoline acts as a catalyst poison in this reaction.

### Final Answer:
Quinoline acts as a catalyst poison in the given reaction.

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