Which of the following alkenes is most reactive towards addition of `HBr`:

Options

A. `C_(6)H_(5)-CH=CH_(2)`
B.
C.
D.

Answer

To determine which alkene is most reactive towards the addition of HBr, we need to analyze the factors affecting the reactivity of alkenes, particularly the electron density around the double bond. The reactivity of alkenes in electrophilic addition reactions, such as with HBr, is influenced by the stability of the carbocation that forms during the reaction. More electron-dense alkenes can stabilize the positive charge better, leading to a faster reaction.

### Step-by-Step Solution:

1. **Identify the Alkenes**:
We need to examine the structures of the alkenes provided in options A, B, C, and D.

2. **Understand the Role of Electrophiles**:
In the case of HBr, the hydrogen (H+) acts as an electrophile. The double bond (π bond) in the alkene acts as a nucleophile and will attack the H+, leading to the formation of a carbocation.

3. **Analyze the Electron Density**:
The more electron density present in the alkene, the more reactive it will be towards HBr. Electron-donating groups increase electron density, while electron-withdrawing groups decrease it.

4. **Evaluate Each Option**:
- **Option A**: Contains a phenyl group (C6H5) attached to the alkene. The phenyl group can donate some electron density through resonance, but it is not as effective as alkyl groups.
- **Option B**: Contains a methyl group (CH3) attached to the alkene and a phenyl ring. The methyl group is an electron-donating group through hyperconjugation, which increases the electron density at the double bond.
- **Option C**: Contains a nitro group (NO2), which is a strong electron-withdrawing group. This will decrease the electron density at the double bond, making it less reactive.
- **Option D**: Contains a phenyl ring and an alkene. Similar to option A, the phenyl group can donate some electron density, but not as effectively as alkyl groups.

5. **Determine the Most Reactive Alkene**:
Comparing the options, option B has the highest electron density due to the presence of the methyl group, which stabilizes the carbocation formed during the reaction with HBr. Therefore, option B is the most reactive alkene towards the addition of HBr.

### Conclusion:
The alkene that is most reactive towards the addition of HBr is **Option B**.