Propene is more reactive than ethene towards HBr because:

Options

A. propene can more readily undergo a free radical chain reaction.
B. propene gives rise to more stable carbonium ion
C. the double bond in case of propene is unstable
D. the methyl group attached to double bond withdraws electrons and facilitates the attack.

Answer

To understand why propene is more reactive than ethene towards HBr, we can break down the explanation into several steps:

### Step 1: Structure of Propene and Ethene
- **Propene (C3H6)** has the structure CH3-CH=CH2, which contains a double bond between the second and third carbon atoms.
- **Ethene (C2H4)** has the structure CH2=CH2, which contains a double bond between the two carbon atoms.

**Hint:** Identify the structures of the compounds involved to understand their reactivity.

### Step 2: Reaction with HBr
- When propene reacts with HBr, the double bond acts as a nucleophile and attacks the electrophilic hydrogen (H+) from HBr.
- The reaction can be represented as:
\[ CH3-CH=CH2 + HBr \rightarrow CH3-CH^+-CH3 \]
- This results in the formation of a secondary carbocation (CH3-CH^+-CH3).

**Hint:** Recognize that the double bond in alkenes acts as a nucleophile, attacking electrophiles.

### Step 3: Carbocation Stability
- The carbocation formed from propene is a **secondary carbocation**, which is more stable due to the presence of two alkyl groups (methyl groups) that can donate electron density.
- In contrast, when ethene reacts with HBr, it forms a primary carbocation:
\[ CH2=CH2 + HBr \rightarrow CH3-CH^+-H \]
- This results in a primary carbocation, which is less stable than a secondary carbocation.

**Hint:** Stability of carbocations is crucial; secondary carbocations are more stable than primary ones.

### Step 4: Markovnikov's Rule
- According to Markovnikov's rule, in the addition of HX to an alkene, the hydrogen atom will attach to the carbon with more hydrogen atoms already attached (the less substituted carbon), while the halide (Br) will attach to the carbon with fewer hydrogen atoms.
- In propene, the hydrogen adds to the terminal carbon (CH2), leading to the more stable secondary carbocation.

**Hint:** Remember Markovnikov's rule when predicting the products of alkene reactions.

### Step 5: Conclusion
- The increased reactivity of propene compared to ethene towards HBr is primarily due to the formation of a more stable secondary carbocation in propene, whereas ethene forms a less stable primary carbocation.
- Therefore, propene is more reactive than ethene towards HBr because it gives rise to a more stable carbocation.

**Final Answer:** Propene is more reactive than ethene towards HBr because propene gives rise to a more stable carbocation.