Question

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

• A. C_6H_5OH > H_2O > ROH
• B. C_6H_5OH > ROH > H_2O
• C. ROH > C_6H_5OH > H_2O
• D. H_2O > C_6H_5OH > ROH
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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 A

To determine the correct order of acid strength among phenol (C_6H_5OH), water (H_2O), and alcohols (ROH), we must consider their ability to donate a proton (H+), which is indicative of acid strength. 

Phenol (C_6H_5OH): Phenol is a weak acid but stronger than both water and alcohols. The phenoxide ion, formed after donating a proton, is stabilized by resonance with the benzene ring.

Water (H_2O): Water is less acidic than phenol but more acidic than alcohols. The hydroxide ion formed is stable but lacks resonance stabilization found in phenoxide.

Alcohols (ROH): Alcohols are the weakest acids because the alkoxide ion formed after losing a proton is not stabilized by resonance or significant electronegativity effects.

Order of Acid Strength: The acid strength order based on their ability to donate protons and stabilization of resulting ions is:
C_6H_5OH > H_2O > ROH

This order is due to the resonance stabilization of the phenoxide ion, moderate stability of hydroxide ion, and the lack of stabilization in the alkoxide ion.