Question

Among the following, the compound having the lowest value of \(pK_a\) is:

• A. Ethanol
• B. Water
• C. Phenol
• D. Propan-2-ol

Hint:

Greater the acidic strength, smaller is the value of \(pK_a\).

Answer 1

The Correct Option is C

Concept: Lower value of \(pK_a\) indicates stronger acidity. \[ pK_a = -\log K_a \] Hence: \[ \text{Smaller } pK_a \Rightarrow \text{Greater acidity} \] Acidic strength depends upon the stability of conjugate base formed after removal of proton.

Step 1: Compare alcohols and water. Alcohols contain alkyl groups which show \(+I\) effect and destabilize alkoxide ions. Therefore alcohols are less acidic than water. \[ \text{Propan-2-ol} < \text{Ethanol} < \text{Water} \]

Step 2: Analyze phenol. Phenol forms phenoxide ion: \[ \text{C}_6\text{H}_5\text{OH} \rightarrow \text{C}_6\text{H}_5\text{O}^- + H^+ \] The negative charge on oxygen gets delocalized over the benzene ring through resonance. Hence phenoxide ion becomes highly stable.

Step 3: Determine lowest \(pK_a\). Since phenol is most acidic among the given compounds, it has the lowest \(pK_a\). \[ \boxed{\text{Phenol}} \]