Which of the following reagents is used to step up the C-chain in alkyl halides?

Options

A. HCN
B. KCN
C. `NH_(4)`CN
D. `Ag_(2)CO_(3)`

Answer

To determine which reagent is used to step up the carbon chain in alkyl halides, we need to analyze the options provided in the question. The goal is to find a reagent that can effectively add one carbon atom to the carbon chain of the alkyl halide.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a reagent that can increase the carbon chain length by one carbon atom in alkyl halides. This means we are looking for a reagent that can facilitate the addition of a carbon atom to the existing carbon chain.

2. **Analyzing Option A (HCN)**:
- When an alkyl halide (RX) is treated with hydrogen cyanide (HCN), the reaction typically leads to the formation of a nitrile (RCN).
- However, this does not effectively add a carbon atom to the main chain; instead, it replaces the halogen with a cyano group. Therefore, this option is **incorrect**.

3. **Analyzing Option B (KCN)**:
- When an alkyl halide (RX) reacts with potassium cyanide (KCN), the product formed is a nitrile (RCN) where the carbon chain is effectively increased by one carbon atom.
- This reaction can be summarized as:
\[ RX + KCN \rightarrow RCN + KX \]
- Here, the carbon from KCN is added to the carbon chain, making this option **correct**.

4. **Analyzing Option C (Ag2CO3)**:
- Silver carbonate (Ag2CO3) does not add any carbon to the carbon chain. Instead, it can participate in other types of reactions but does not contribute an additional carbon atom to the alkyl halide.
- Thus, this option is also **incorrect**.

5. **Conclusion**: Based on the analysis, the only reagent that successfully steps up the carbon chain in alkyl halides by adding one carbon atom is **KCN**.

### Final Answer:
The correct reagent to step up the carbon chain in alkyl halides is **KCN**.

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