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 establish the relative acid strengths of phenol (C_6H_5OH), water (H_2O), and alcohols (ROH), we examine their proton (H+) donation capability.

Phenol (C_6H_5OH): Phenol exhibits weaker acidity than some compounds but is stronger than both water and alcohols. This is attributed to the resonance stabilization of the resulting phenoxide ion within the benzene ring after proton donation.

Water (H_2O): Water is less acidic than phenol but more acidic than alcohols. The formed hydroxide ion possesses moderate stability, lacking the resonance stabilization seen in phenoxide.

Alcohols (ROH): Alcohols are the least acidic due to the alkoxide ion, formed after proton loss, lacking resonance stabilization or significant electronegativity effects.

Order of Acid Strength: Based on proton donation and resulting ion stability, the order of acid strength is:
C_6H_5OH > H_2O > ROH

This sequence is a consequence of the resonance stabilization of the phenoxide ion, the moderate stability of the hydroxide ion, and the absence of significant stabilization in the alkoxide ion.