Question

Which is the correct order of acid strength from the following?

• A. C$_6$H$_5$OH > H$_2$O > ROH
• B. C$_6$H$_5$OH > ROH > H$_2$O
• C. ROH > C$_6$H$_5$OH > H$_2$O
• D. H$_2$O > C$_6$H$_5$OH > ROH

Hint:

The acidity of alcohols, phenols, and water is governed by their ability to stabilize the negative charge after deprotonation.

Answer 1

The Correct Option is D

To address this, we will evaluate the acidity of phenol (C₆H₅OH), alcohol (ROH), and water (H₂O).

1. Understanding Acid Strength:
Acid strength is defined by a molecule's capacity to donate a proton (H⁺). This is contingent upon the stability of the conjugate base formed after proton dissociation.

2. Phenol (C₆H₅OH) vs Alcohol (ROH):
Phenol exhibits greater acidity than alcohol. This is due to the resonance stabilization of the negative charge on the oxygen within the phenoxide ion (C₆H₅O^-) through the aromatic ring. Conversely, the alkoxide ion (RO^-) lacks this stabilization, rendering alcohol less acidic than phenol.

3. Phenol (C₆H₅OH) vs Water (H₂O):
Water (H₂O) is a weaker acid than phenol. While water can relinquish a proton to form the hydroxide ion (OH^-), phenol's conjugate base is more stable due to resonance.

4. Conclusion:
The hierarchy of acid strength is as follows: Water > Phenol > Alcohol.

Final Answer:
The definitive option is (D) H₂O > C₆H₅OH > ROH.