Which of the following is acidic :
But-2-ene, But-2-yne, But-1-yne, But-1-ene

Answer

To determine which of the given compounds is acidic, we need to analyze the structures of each compound and their ability to donate a proton (H⁺). The compounds given are:

1. But-2-ene
2. But-2-yne
3. But-1-yne
4. But-1-ene

### Step 1: Draw the Structures
- **But-2-ene**: CH₃-CH=CH-CH₃ (double bond between the second and third carbon)
- **But-2-yne**: CH₃-C≡C-CH₃ (triple bond between the second and third carbon)
- **But-1-yne**: HC≡C-CH₂-CH₃ (triple bond between the first and second carbon)
- **But-1-ene**: HC=CH-CH₂-CH₃ (double bond between the first and second carbon)

### Step 2: Identify the Type of Hybridization
- **But-2-ene**: The double bond carbons are sp² hybridized.
- **But-2-yne**: The triple bond carbons are sp hybridized.
- **But-1-yne**: The terminal carbon is sp hybridized.
- **But-1-ene**: The double bond carbons are sp² hybridized.

### Step 3: Determine Acidity Based on Hybridization
The acidity of hydrocarbons generally increases with the s-character of the hybridized orbitals:
- **sp hybridization** (50% s-character) → most acidic
- **sp² hybridization** (33.3% s-character) → less acidic
- **sp³ hybridization** (25% s-character) → least acidic

### Step 4: Analyze Each Compound
- **But-2-ene**: No acidic protons available (sp²).
- **But-2-yne**: No acidic protons available (sp).
- **But-1-yne**: Has an acidic proton (terminal alkyne, sp hybridized).
- **But-1-ene**: No acidic protons available (sp²).

### Step 5: Conclusion
Among the compounds, **But-1-yne** is the only compound that has an acidic proton due to its terminal position and sp hybridization, which allows for the formation of a stable anion upon deprotonation.

**Final Answer**: But-1-yne is the most acidic compound among the given options.

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