Which of the following is most stable ?

Options

A. 1-butene
B. 1-pentene
C. 2-butene
D. 2-pentene

Explanation

To determine which of the given alkenes is the most stable, we will analyze the structures of 1-butene, 1-pentene, 2-butene, and 2-pentene based on the degree of substitution of the double bond and the concept of hyperconjugation.

### Step 1: Identify the structures of the alkenes
1. **1-butene**: CH2=CH-CH2-CH3
2. **1-pentene**: CH2=CH-CH2-CH2-CH3
3. **2-butene**: CH3-CH=CH-CH3
4. **2-pentene**: CH3-CH=CH-CH2-CH3

### Step 2: Determine the degree of substitution
- **1-butene**: The double bond is between the first and second carbon, making it **monosubstituted** (only one carbon attached to the double bond).
- **1-pentene**: Similar to 1-butene, it is also **monosubstituted**.
- **2-butene**: The double bond is between the second and third carbon, making it **disubstituted** (two carbons attached to the double bond).
- **2-pentene**: Similar to 2-butene, it is also **disubstituted**.

### Step 3: Apply the stability rule
According to the stability rule for alkenes, more substituted alkenes are more stable. Therefore, both 2-butene and 2-pentene (disubstituted) are more stable than 1-butene and 1-pentene (monosubstituted).

### Step 4: Compare the stability of 2-butene and 2-pentene
To determine which is more stable between 2-butene and 2-pentene, we consider the hyperconjugation effect:
- **2-butene** has 6 alpha hydrogens (3 from each of the two CH3 groups).
- **2-pentene** has 5 alpha hydrogens (3 from one CH3 and 2 from the CH2 group).

### Step 5: Conclusion
Since 2-butene has more alpha hydrogens contributing to hyperconjugation compared to 2-pentene, it is more stable.

**Final Answer**: **2-butene** is the most stable among the given options.

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