Propene undergo electrophilic addition with HB r, but not with HCN. The reason is

Options

A. `Br^-` is better nucleophile than `CN^-`
B. HB r being better source of proton as it is stronger acid than HCN
C. HCN attacks preferentially via lone pair of nitrogen
D. the C – Br bond belong stronger is formed easily as compared to C – CN bond

Answer

To understand why propene undergoes electrophilic addition with HBr but not with HCN, we need to analyze the nature of the reactants and the mechanism of the reaction.

### Step-by-Step Solution:

1. **Understanding Propene**:
- Propene (C3H6) is an alkene with a double bond between two carbon atoms. This double bond is a site of high electron density, making it susceptible to electrophilic attack.

2. **Electrophilic Addition Reaction**:
- In electrophilic addition, an electrophile (a species that seeks electrons) reacts with a nucleophile (a species that donates electrons). The double bond in propene acts as a nucleophile.

3. **Reaction with HBr**:
- HBr is a strong acid and dissociates into H⁺ and Br⁻ ions in solution. The H⁺ ion acts as the electrophile.
- When propene reacts with HBr, the H⁺ ion attacks the double bond, leading to the formation of a carbocation (positively charged carbon atom).
- The Br⁻ ion then attacks the carbocation, resulting in the formation of bromo-propane.

4. **Reaction with HCN**:
- HCN is a weaker acid compared to HBr. It dissociates into H⁺ and CN⁻ ions, but the H⁺ ion is less electrophilic than that from HBr.
- The weaker acidity of HCN means that the H⁺ produced is not as effective at attacking the double bond in propene. Therefore, the electrophilic addition does not occur.

5. **Comparison of Acidity**:
- HBr is a stronger acid than HCN. The strength of an acid is determined by its ability to donate protons (H⁺ ions). Since HBr can donate protons more readily, it is more effective in facilitating the electrophilic addition reaction with propene.

6. **Conclusion**:
- The primary reason propene undergoes electrophilic addition with HBr but not with HCN is that HBr is a stronger acid, providing a more effective electrophile (H⁺) for the reaction.

### Final Answer:
Propene undergoes electrophilic addition with HBr but not with HCN because HBr is a stronger acid than HCN, making H⁺ from HBr a more effective electrophile for the reaction.