Which of the following is most acidic ?

Options

A. `H_(2)O`
B. `CH_(3)OH`
C. `C_(2)H_(5)OH`
D. `CH_(3)CH_(2)CH_(2)OH`

Explanation

To determine which of the given substances (water, methanol, ethanol, and propanol) is the most acidic, we need to analyze the acidity of each compound based on their ability to donate protons (H⁺ ions).

### Step-by-Step Solution:

1. **Understanding Acidity**: Acidity is defined by the ability of a substance to donate H⁺ ions. The more readily a substance can donate H⁺ ions, the more acidic it is.

2. **Analyzing Water (H₂O)**: Water can dissociate into H⁺ and OH⁻ ions:
\[ H_2O \rightleftharpoons H^+ + OH^- \]
Water is neutral but can act as an acid by donating H⁺ ions.

3. **Analyzing Alcohols (ROH)**: Alcohols like methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH) also can dissociate to form H⁺ ions:
\[ ROH \rightleftharpoons H^+ + RO^- \]
Here, RO⁻ is the alkoxide ion.

4. **Stability of Conjugate Bases**: The acidity of a compound is inversely related to the stability of its conjugate base. The more stable the conjugate base (RO⁻), the stronger the acid.

5. **Effect of Alkyl Groups**: The alkyl groups (R) in alcohols (like CH₃, C₂H₅, and C₃H₇) exert a negative inductive effect (−I effect). This effect increases the electron density on the oxygen atom of the alkoxide ion (RO⁻), making it less stable. Therefore, the presence of these alkyl groups decreases the acidity of the alcohols compared to water.

6. **Comparing Acidity**:
- Water has a stable conjugate base (OH⁻).
- The conjugate bases of alcohols (alkoxides) are destabilized by the electron-donating effects of the alkyl groups, making them less acidic.

7. **Conclusion**: Since water has a stable conjugate base and can donate H⁺ ions more readily than the alcohols, it is concluded that:
\[ \text{Water (H₂O) is the most acidic among the given options.} \]

### Final Answer:
**Water (H₂O) is the most acidic.**

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