Question

Which of the following compounds will show maximum acidic strength?

• A. $\text{CH}_3\text{OH}$
• B. $\text{C}_2\text{H}_5\text{OH}$
• C. Phenol
• D. Ethane

Hint:

An aromatic hydroxyl group (like in phenol) is always more acidic than aliphatic hydroxyl groups (like in methanol or ethanol).
This is due to the resonance delocalization of the negative charge on the phenoxide oxygen into the benzene ring.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question requires us to determine which of the listed substances displays the highest acidic strength.

Step 2: Key Formula or Approach:
The acidic strength of a molecule depends on the ease of releasing a proton and the stability of its conjugate base.
\[ \text{Acidic Strength} \propto \text{Stability of Conjugate Base} \]

Step 3: Detailed Explanation:

• Acidic Dissociation of Phenol: When phenol loses a proton, it forms the phenoxide ion (\(C_6H_5O^-\)).
  
• Resonance Stabilization: The negative charge on the oxygen atom of the phenoxide ion is delocalized over the aromatic benzene ring, making the conjugate base highly stable.
  
• Acidic Dissociation of Methanol and Ethanol: Methanol (\(\text{CH}_3\text{OH}\)) and ethanol (\(\text{C}_2\text{H}_5\text{OH}\)) form methoxide and ethoxide ions, respectively.
  
• Inductive Destabilization: The alkyl groups (\(\text{–CH}_3\) and \(\text{–C}_2\text{H}_5\)) exert a \(+I\) inductive effect, which pushes electron density toward the oxygen atom, destabilizing the conjugate base.
  
• Comparison with Ethane: Ethane (\(\text{C}_2\text{H}_6\)) is an alkane with non-polar \(C\text{–}H\) bonds and has practically no acidic behavior (\(pK_a \approx 50\)).
  
• Comparison of pKa Values: Phenol has a \(pK_a\) value of approximately 10, whereas methanol and ethanol have \(pK_a\) values of around 15.5 and 15.9, making phenol the strongest acid.
  

Step 4: Final Answer:
Therefore, Phenol has the maximum acidic strength, which matches option (C).