Question

Which of the following is the weakest acid?

• A. Phenol
• B. p-Nitrophenol
• C. p-Cresol
• D. Ethanol
• E. m-Cresol

Hint:

Acidity order: p-Nitrophenol $>$ Phenol $>$ p-Cresol $>$ m-Cresol $>$ Ethanol.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The acidity of a compound is determined by its ability to donate a proton (H\(^+\)). The strength of an acid is related to the stability of its conjugate base formed after donating the proton. A more stable conjugate base corresponds to a stronger acid.
Step 2: Detailed Explanation:
Let's compare the acidity of the given compounds. Options (A), (B), (C), and (E) are phenols or substituted phenols. Option (D) is an aliphatic alcohol.

Phenols vs. Alcohols: Phenols are generally more acidic than aliphatic alcohols. When a phenol loses a proton, it forms a phenoxide ion. This phenoxide ion is stabilized by resonance, as the negative charge is delocalized over the benzene ring. When an alcohol like ethanol loses a proton, it forms an ethoxide ion (CH\(_3\)CH\(_2\)O\(^-\)). The negative charge is localized on the oxygen atom. Furthermore, the ethyl group (-C\(_2\)H\(_5\)) is an electron-donating group (+I effect), which destabilizes the ethoxide ion by intensifying the negative charge. Because the phenoxide ion is much more stable than the ethoxide ion, phenol is a much stronger acid than ethanol. Therefore, ethanol is the weakest acid among all choices.

Comparing the Phenols:
Phenol (C\(_6\)H\(_5\)OH): This is our reference.
p-Nitrophenol: The nitro group (-NO\(_2\)) is a strong electron-withdrawing group (-R and -I effects). It delocalizes the negative charge of the phenoxide ion very effectively, greatly stabilizing it. Thus, p-nitrophenol is a much stronger acid than phenol.
p-Cresol and m-Cresol: The methyl group (-CH\(_3\)) is an electron-donating group (+I and hyperconjugation effects). It destabilizes the phenoxide ion by pushing electron density onto the ring, making it less stable than the simple phenoxide ion. Therefore, cresols are weaker acids than phenol.

The order of acidity is: p-Nitrophenol> Phenol> Cresols> Ethanol.
Step 3: Final Answer:
Ethanol is an aliphatic alcohol whose conjugate base is not resonance-stabilized, making it significantly less acidic than phenols. It is the weakest acid in the list. This corresponds to option (D).