In Rosenmund reduction, which of the following does not poison the catalyst `Pd`?

Options

A. a.`BaSO_(4)`
B. b.S
C. c.Quinoline
D. d.Xylene

Explanation

To solve the question regarding which substance does not poison the catalyst palladium in the Rosenmund reduction, we can follow these steps:

### Step 1: Understand the Rosenmund Reduction
The Rosenmund reduction is a specific reaction where acyl chlorides are reduced to aldehydes using hydrogen gas in the presence of a palladium catalyst. The reaction is typically carried out on palladium supported on barium sulfate (Pd/BaSO4), which acts as a catalyst.

### Step 2: Identify the Role of the Poison
In this reaction, a "poison" is a substance that reduces the activity of the catalyst, preventing it from over-reducing the acyl chloride to an alcohol. Barium sulfate (BaSO4) is commonly used as a poison because it provides a low surface area, which limits the catalytic activity of palladium and stops the reaction at the aldehyde stage.

### Step 3: List Possible Options
The question provides several options to choose from regarding which substance does not poison the palladium catalyst. Common poisons include:
- Barium sulfate (BaSO4)
- Calcium carbonate (CaCO3)
- Sulfur
- Quinoline

### Step 4: Analyze Each Option
- **Barium sulfate (BaSO4)**: Acts as a poison in the Rosenmund reduction.
- **Calcium carbonate (CaCO3)**: Generally does not act as a poison in this context.
- **Sulfur**: Can act as a poison by adsorbing onto the catalyst surface.
- **Quinoline**: Also acts as a poison.

### Step 5: Conclusion
From the analysis, it is clear that **Calcium carbonate (CaCO3)** does not poison the palladium catalyst in the Rosenmund reduction. Therefore, the correct answer is:

**Answer: Calcium carbonate (CaCO3)**

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