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. The term "step up the carbon chain" means to increase the number of carbon atoms in the alkyl chain by one.

### Step-by-Step Solution:

1. **Understanding the Question**:
- The question asks which reagent can increase the carbon chain length in alkyl halides (R-X) by one carbon atom.

2. **Analyzing the Options**:
- The options given are:
- A) HCN
- B) KCN
- C) NH4CN
- D) H2CO3

3. **Evaluating HCN**:
- HCN (hydrogen cyanide) is a gaseous compound.
- It is weakly acidic and does not react effectively with alkyl halides because it evaporates quickly when heated, preventing the reaction from occurring.
- **Conclusion**: HCN is not suitable for stepping up the carbon chain.

4. **Evaluating KCN**:
- KCN (potassium cyanide) can react with alkyl halides through an SN2 mechanism.
- The reaction can be represented as:
\[ R-X + KCN \rightarrow R-CN + KX \]
- In this reaction, the carbon chain (R) increases by one carbon atom (from R to R-CN).
- **Conclusion**: KCN is the correct reagent that steps up the carbon chain.

5. **Evaluating NH4CN**:
- NH4CN (ammonium cyanide) is an ionic compound and is unstable.
- It decomposes into NH3 and HCN, both of which are gases and do not facilitate the reaction with alkyl halides.
- **Conclusion**: NH4CN is not suitable for stepping up the carbon chain.

6. **Evaluating H2CO3**:
- H2CO3 (carbonic acid) does not react with alkyl halides to increase the carbon chain.
- It is primarily involved in acid-base reactions and does not provide a nucleophile that can attack the alkyl halide.
- **Conclusion**: H2CO3 is not suitable for stepping up the carbon chain.

7. **Final Conclusion**:
- After evaluating all options, the only reagent that effectively increases the carbon chain in alkyl halides is **KCN**.

### Answer:
The correct answer is **B) KCN**.