An alkyl halide, RX reacts with KCN to give propane nitrile. RX is

Options

A. `C_3H_7Br`
B. `C_4H_9Br`
C. `C_2H_5Br`
D. `C_5H_(11)Br`

Explanation

To determine the alkyl halide (RX) that reacts with KCN to produce propane nitrile (C3H7CN), we can follow these steps:

### Step 1: Understand the Product
The product given is propane nitrile, which has the molecular formula C3H7CN. This indicates that the compound has three carbon atoms (C3) and a nitrile functional group (CN).

### Step 2: Analyze the Reaction
When an alkyl halide (RX) reacts with KCN, the KCN acts as a nucleophile. The nucleophile attacks the carbon atom that is bonded to the halogen (X) in the alkyl halide, leading to the substitution of the halogen with the cyanide group (CN).

### Step 3: Determine the Structure of the Alkyl Halide
Since the product is propane nitrile (C3H7CN), we need to find an alkyl halide that, when reacted with KCN, will yield this product. The alkyl halide must have two carbon atoms (C2) because the reaction will add one carbon from KCN to the two carbons in the alkyl halide to form the three-carbon product.

### Step 4: Evaluate the Options
Let's evaluate the given options:
1. **C3H7Br**: This would yield a four-carbon product (C4H9CN) when reacted with KCN.
2. **C4H9Br**: This would yield a five-carbon product (C5H11CN) when reacted with KCN.
3. **C2H5Br**: This would yield propane nitrile (C3H7CN) when reacted with KCN.
4. **C5H11Br**: This would yield a six-carbon product (C6H13CN) when reacted with KCN.

### Step 5: Identify the Correct Alkyl Halide
The only option that, when reacted with KCN, produces propane nitrile (C3H7CN) is **C2H5Br**. Therefore, the correct answer is:

**RX = C2H5Br**

### Final Answer
The alkyl halide RX that reacts with KCN to give propane nitrile is **C2H5Br**.

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