Why is `CH_(2)=CH-COOH` a stronger acid than `CH_(3)CH_(2)COOH`?

Explanation

To understand why prop-2-enoic acid (CH₂=CH-COOH) is a stronger acid than propanoic acid (CH₃CH₂COOH), we can analyze the structural and electronic factors that influence acidity.

### Step-by-Step Solution:

1. **Identify the Structures**:
- Prop-2-enoic acid has the structure CH₂=CH-COOH, which contains a double bond between the first two carbon atoms.
- Propanoic acid has the structure CH₃CH₂COOH, which contains only single bonds.

2. **Hybridization of Carbon Atoms**:
- In prop-2-enoic acid, the carbon atom adjacent to the carboxylic group (the α-carbon) is sp² hybridized due to the presence of the double bond.
- In propanoic acid, the α-carbon is sp³ hybridized.

3. **Electronegativity Consideration**:
- sp² hybridized carbon atoms are more electronegative than sp³ hybridized carbon atoms. This is because sp² hybridized carbon has a higher s-character (33%) compared to sp³ hybridized carbon (25%).
- The increased electronegativity of the sp² carbon in prop-2-enoic acid allows it to better stabilize the negative charge that develops when the acid donates a proton (H⁺).

4. **Proton Donation**:
- The ability of an acid to donate a proton is a key factor in its strength. In prop-2-enoic acid, the greater electronegativity of the α-carbon facilitates the release of the proton from the carboxylic group.
- In contrast, in propanoic acid, the sp³ hybridized α-carbon does not stabilize the negative charge as effectively, making it less favorable for the acid to donate a proton.

5. **Conclusion**:
- Therefore, due to the higher electronegativity of the α-carbon in prop-2-enoic acid, the release of the proton is easier compared to propanoic acid. This makes prop-2-enoic acid a stronger acid than propanoic acid.

### Final Answer:
Prop-2-enoic acid (CH₂=CH-COOH) is a stronger acid than propanoic acid (CH₃CH₂COOH) because the α-carbon in prop-2-enoic acid is sp² hybridized, making it more electronegative and better able to stabilize the negative charge after proton donation, thus facilitating the release of the proton.