`CH_(3)-underset(O)underset(||)C-CH=CH-CH_(3)overset(HBr)rarr`
find the major product

Options

A. `CH_(3)-underset(O)underset(||)C-CH_(2)-underset(Br)underset(|)CH-CH_(3)`
B. `CH_(3)-underset(O)underset(||)C-underset(Br)underset(|)CH-CH_(2)-CH_(3)`
C. `CH_(3)-overset(Br)overset(|)underset(OH)underset(|)C-CH=CH-CH_(3)`
D. none of these

Answer

To solve the problem of finding the major product when 3-penten-2-one (CH₃-CO-CH=CH-CH₃) reacts with HBr, we will follow these steps:

### Step 1: Identify the Reactant
The reactant is 3-penten-2-one, which has a carbon-carbon double bond (C=C) and a carbonyl group (C=O). The structure can be represented as follows:
```
CH₃
|
CO
|
CH=CH-CH₃
```

### Step 2: Understand the Reaction Mechanism
When an alkene reacts with HBr, the hydrogen (H) will add to one of the carbons in the double bond, while the bromine (Br) will attach to the other carbon. The addition follows Markovnikov's rule, which states that the more substituted carbon will preferentially bond with the halogen (Br) because it leads to a more stable carbocation.

### Step 3: Forming the Carbocation
In our case, the double bond between the two carbons can lead to the formation of two possible carbocations:
1. If H adds to the carbon adjacent to the carbonyl (C=O), we form a secondary carbocation.
2. If H adds to the terminal carbon, we form a primary carbocation.

The secondary carbocation is more stable due to the inductive effect of the adjacent carbonyl group and the neighboring carbon atoms.

### Step 4: Determine the Major Product
The more stable carbocation will lead to the major product. The secondary carbocation formed will be:
```
CH₃
|
CO
|
CH-CH₃
|
Br
```
Here, the Br will attach to the carbon that was part of the double bond, leading to the product:
```
CH₃
|
CO
|
CHBr-CH₃
```

### Final Product
The major product of the reaction is 3-bromo-3-pentan-2-one.