Which of the following is most stable?

Options

A. 2,3-Dimethyl-2-butene
B. 2-Butene
C. 2-Methyl-1,2-butene
D. 1-Butene

Answer

To determine which of the given alkenes is the most stable, we need to analyze the structures based on the number of alpha hydrogens present. The stability of alkenes generally increases with the number of alpha hydrogens attached to the carbons adjacent to the double bond.

### Step-by-Step Solution:

1. **Identify the Structures**:
We have four alkenes to analyze:
- A: 2,3-Dimethyl-2-butene
- B: 2-Butene
- C: 1,2-Dimethyl-1,2-butene
- D: 1-Butene

2. **Draw the Structures**:
- **A: 2,3-Dimethyl-2-butene**:
- Structure:
```
CH3 CH3
| |
CH3-C=C-CH2-CH3
```
- This structure has two methyl groups attached to the second carbon.

- **B: 2-Butene**:
- Structure:
```
CH3-CH=CH-CH3
```
- This structure has no substituents on the double bond.

- **C: 1,2-Dimethyl-1,2-butene**:
- Structure:
```
CH2=C(CH3)-C(CH3)=CH2
```
- This structure has two double bonds and two methyl groups on the first and second carbons.

- **D: 1-Butene**:
- Structure:
```
CH2=CH-CH2-CH3
```

3. **Count the Alpha Hydrogens**:
- **A (2,3-Dimethyl-2-butene)**:
- The carbons adjacent to the double bond (alpha carbons) are:
- Carbon 2: 3 alpha hydrogens (attached to CH3)
- Carbon 3: 3 alpha hydrogens (attached to CH3)
- Total: 12 alpha hydrogens.

- **B (2-Butene)**:
- The alpha carbons are:
- Carbon 1: 3 alpha hydrogens (attached to CH3)
- Carbon 3: 3 alpha hydrogens (attached to CH3)
- Total: 6 alpha hydrogens.

- **C (1,2-Dimethyl-1,2-butene)**:
- This structure is unstable due to exceeding the tetravalency of carbon. Hence, we disregard this option.

- **D (1-Butene)**:
- The alpha carbon is:
- Carbon 2: 2 alpha hydrogens (attached to CH2)
- Total: 2 alpha hydrogens.

4. **Determine Stability**:
- The stability of alkenes is directly proportional to the number of alpha hydrogens.
- A has 12 alpha hydrogens, B has 6, D has 2, and C is disregarded due to instability.

5. **Conclusion**:
- The most stable alkene among the options is **A: 2,3-Dimethyl-2-butene** due to having the highest number of alpha hydrogens (12).

### Final Answer:
**A: 2,3-Dimethyl-2-butene is the most stable alkene.**