Which of the following will be the major product when 3 - phenylpropene reacts with HBr ?

Options

A. `C_6H_5CH_2CHBrCH_3`
B. `C_6H_5CHBrCHCH_2`
C. `C_6H_5CH_2CH_2CH_2Br`
D. `C_6H_5CHBrCH_2CH_3`

Answer

To solve the problem of determining the major product when 3-phenylpropene reacts with HBr, we can follow these steps:

### Step 1: Identify the structure of 3-phenylpropene
3-phenylpropene is an alkene with a phenyl group attached to the third carbon of a three-carbon chain. The structure can be represented as follows:

```
Ph
|
H2C=C-CH2-CH3
```
Where "Ph" represents the phenyl group (C6H5).

### Step 2: Understand the reaction with HBr
When 3-phenylpropene reacts with HBr, the double bond will react with HBr, leading to the formation of a carbocation. The H+ from HBr will add to one of the carbons involved in the double bond.

### Step 3: Determine the carbocation formation
The double bond between the first and second carbon atoms can form a carbocation. The addition of H+ can occur at either carbon, but we need to consider the stability of the resulting carbocation.

If H+ adds to the first carbon (C1), we will get a secondary carbocation at C2:
```
Ph
|
CH2
|
C+ - CH3
```
If H+ adds to the second carbon (C2), we will get a primary carbocation at C1, which is less stable.

### Step 4: Stability of the carbocation
The secondary carbocation formed at C2 is more stable due to hyperconjugation and resonance stabilization from the phenyl group. Therefore, the H+ will preferentially add to C1, leading to the more stable secondary carbocation at C2.

### Step 5: Rearrangement of the carbocation
In this case, the secondary carbocation can undergo a hydride shift (1,2-hydride shift) to form a more stable tertiary carbocation:
```
Ph
|
CH - CH2 - CH3
|
C+
```
This rearrangement increases stability due to the resonance with the phenyl group.

### Step 6: Nucleophilic attack by Br-
The Br- ion will then attack the carbocation, leading to the formation of the major product. The Br- will attach to the carbon that carries the positive charge (C2).

### Step 7: Write the final product
The final product will be:
```
Ph
|
CH - CH2 - CH3
|
Br
```
This compound is 1-bromo-1-phenylpropane.

### Conclusion
The major product when 3-phenylpropene reacts with HBr is 1-bromo-1-phenylpropane.

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