Which among the given alkenes is most stable ?

Options

A. 1 - butene
B. 2,3 - dimethyl -2- butene
C. cis -2- butene
D. trans - 2- butene

Answer

To determine which among the given alkenes is the most stable, we will analyze the stability based on the number of alpha hydrogens and the steric factors involved. Here’s a step-by-step solution:

### Step 1: Identify the Alkenes
We have four alkenes to analyze:
- Option A: But-1-ene
- Option B: 2,3-dimethylbutene
- Option C: Cis-but-2-ene
- Option D: Trans-but-2-ene

### Step 2: Draw the Structures
1. **But-1-ene (Option A)**:
- Structure: CH2=CH-CH2-CH3
- Alpha Hydrogens: 6 (3 from CH2 and 3 from CH3)

2. **2,3-Dimethylbutene (Option B)**:
- Structure: CH3-CH=CH-CH3 with two methyl groups on the second and third carbons.
- Alpha Hydrogens: 12 (6 from each CH3 group adjacent to the double bond)

3. **Cis-but-2-ene (Option C)**:
- Structure: CH3-CH=CH-CH3 with both CH3 groups on the same side of the double bond.
- Alpha Hydrogens: 6 (3 from each CH3 group adjacent to the double bond)

4. **Trans-but-2-ene (Option D)**:
- Structure: CH3-CH=CH-CH3 with CH3 groups on opposite sides of the double bond.
- Alpha Hydrogens: 6 (3 from each CH3 group adjacent to the double bond)

### Step 3: Count the Alpha Hydrogens
- **But-1-ene**: 6 alpha hydrogens
- **2,3-Dimethylbutene**: 12 alpha hydrogens
- **Cis-but-2-ene**: 6 alpha hydrogens
- **Trans-but-2-ene**: 6 alpha hydrogens

### Step 4: Analyze Stability
The stability of alkenes is generally determined by the number of alpha hydrogens. More alpha hydrogens typically lead to greater stability due to hyperconjugation and the ability to stabilize the double bond.

- **2,3-Dimethylbutene (Option B)** has the highest number of alpha hydrogens (12), making it the most stable alkene among the options.
- For the other alkenes (Cis and Trans but-2-ene), they have the same number of alpha hydrogens (6), but trans is generally more stable than cis due to less steric hindrance.

### Conclusion
The most stable alkene among the given options is **Option B: 2,3-Dimethylbutene**.

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