Which alkene in the following is most stable?

Options

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

Answer

To determine which alkene is the most stable among the given options, we will analyze the stability based on the degree of substitution and the presence of alpha-hydrogens which contribute to hyperconjugation.

### Step-by-Step Solution:

1. **Identify the Alkenes**:
- The alkenes provided are 1-butene, 2-butene, 1-pentene, and 2-pentene.

2. **Determine the Structure of Each Alkene**:
- **1-butene**: Structure is CH2=CH-CH2-CH3 (monosubstituted).
- **2-butene**: Structure is CH3-CH=CH-CH3 (disubstituted).
- **1-pentene**: Structure is CH2=CH-CH2-CH2-CH3 (monosubstituted).
- **2-pentene**: Structure is CH3-CH=CH-CH2-CH3 (disubstituted).

3. **Classify Each Alkene by Substitution**:
- 1-butene: Monosubstituted (1 alkyl group).
- 2-butene: Disubstituted (2 alkyl groups).
- 1-pentene: Monosubstituted (1 alkyl group).
- 2-pentene: Disubstituted (2 alkyl groups).

4. **Stability Based on Substitution**:
- More substituted alkenes are generally more stable due to the electron-donating effect of alkyl groups. Therefore, disubstituted alkenes (2-butene and 2-pentene) are more stable than monosubstituted alkenes (1-butene and 1-pentene).

5. **Count Alpha-Hydrogens**:
- Alpha-hydrogens are the hydrogens on the carbon adjacent to the double bond. They contribute to hyperconjugation.
- **1-butene**: 2 alpha-hydrogens.
- **2-butene**: 6 alpha-hydrogens.
- **1-pentene**: 2 alpha-hydrogens.
- **2-pentene**: 5 alpha-hydrogens.

6. **Evaluate Hyperconjugation**:
- The more alpha-hydrogens present, the more hyperconjugating structures can be formed, leading to increased stability.
- 2-butene has the highest number of alpha-hydrogens (6), making it the most stable alkene.

7. **Conclusion**:
- Based on the analysis, the most stable alkene among the options provided is **2-butene**.

### Final Answer:
**The most stable alkene is 2-butene.**