Which of the following is not an allylic halide?

Options

A. 4-Bromopent-2-ene
B. 3-Bromo-2-methylbut-1-ene
C. 1-Bromobut-2-ene
D. 4-Bromobut-1-ene

Answer

To determine which of the given compounds is not an allylic halide, we first need to understand what an allylic halide is. An allylic halide is a compound where a halogen atom is attached to a carbon atom that is adjacent to a carbon-carbon double bond (the allylic position).

Let's analyze each of the given options step by step:

### Step 1: Identify the Structures of Each Compound

1. **4-bromopent-2-ene**
- Structure:
- C1: CH3
- C2: CH
- C3: CH
- C4: CH2Br
- C5: CH3
- The Br is on C4, which is adjacent to the double bond between C2 and C3. Thus, this is an allylic halide.

2. **3-bromo-2-methyl-but-1-ene**
- Structure:
- C1: CH2
- C2: CH(CH3)
- C3: CHBr
- C4: CH
- The Br is on C3, which is adjacent to the double bond between C1 and C2. Thus, this is an allylic halide.

3. **1-bromo-but-2-ene**
- Structure:
- C1: CHBr
- C2: CH
- C3: CH2
- C4: CH3
- The Br is on C1, which is not adjacent to the double bond between C2 and C3. Thus, this is not an allylic halide.

4. **4-bromo-but-1-ene**
- Structure:
- C1: CH2
- C2: CH
- C3: CH2
- C4: CHBr
- The Br is on C4, which is not adjacent to the double bond between C1 and C2. Thus, this is not an allylic halide.

### Step 2: Conclusion
From the analysis above, we find that:
- **4-bromopent-2-ene** is an allylic halide.
- **3-bromo-2-methyl-but-1-ene** is an allylic halide.
- **1-bromo-but-2-ene** is **not** an allylic halide.
- **4-bromo-but-1-ene** is **not** an allylic halide.

Thus, the answer to the question "Which of the following is not an allylic halide?" is **1-bromo-but-2-ene**.