When but `-3-en-2-ol` reacts with `aq HBr` , we get

Options

A. 3 - bromobut -1- ene
B. 1 - bromobut -2- ene
C. a mixture of both A and B
D. 2 - bromobut -2- ene

Answer

To solve the problem of what happens when but-3-en-2-ol reacts with aqueous HBr, let's break it down step by step.

### Step 1: Understand the Structure of But-3-en-2-ol
But-3-en-2-ol has four carbon atoms, with a double bond between the 3rd and 4th carbon and a hydroxyl (OH) group on the 2nd carbon. The structure can be represented as follows:

```
CH3-CH=CH-CH2OH
```

### Step 2: Reaction with Aqueous HBr
When but-3-en-2-ol reacts with aqueous HBr, the HBr dissociates into H⁺ and Br⁻ ions. The H⁺ ion will attack the hydroxyl (OH) group, leading to the formation of a carbocation.

### Step 3: Formation of Carbocation
The reaction will lead to the protonation of the alcohol group, resulting in the following structure:

```
CH3-CH=CH-CH2OH → CH3-CH=CH-CH2^+ (carbocation)
```

### Step 4: Carbocation Stability and Rearrangement
The carbocation formed can undergo rearrangement due to its stability. The positive charge can shift to the adjacent carbon, allowing for resonance stabilization:

```
CH3-CH^+-CH=CH2 → CH3-CH=CH-CH2^+
```

### Step 5: Nucleophilic Attack by Br⁻
The Br⁻ ion will attack the carbocation, leading to the formation of two possible products due to the two different carbocations formed during the reaction:

1. **3-Bromobut-1-ene**:
- Formed when Br⁻ attacks the carbocation at the 3rd carbon.
- Structure:
```
CH3-CH=CH-Br-CH2
```

2. **1-Bromobut-2-ene**:
- Formed when Br⁻ attacks the carbocation at the 1st carbon.
- Structure:
```
CH3-CH2-CH=CH-Br
```

### Step 6: Naming the Products
The products can be named as follows:
- The first product is **3-bromobut-1-ene**.
- The second product is **1-bromobut-2-ene**.

### Conclusion
Thus, the reaction of but-3-en-2-ol with aqueous HBr yields two products: **3-bromobut-1-ene** and **1-bromobut-2-ene**.