Question

The strongest acid from the following is

• A.
• B.
• C. <
• D.

Answer 1

The Correct Option is A

Acidity of Substituted Phenols Analysis 

The strength of an acid is determined by the stability of its conjugate base.

Electron-withdrawing groups (-I and -M effects) stabilize the conjugate base by delocalizing the negative charge after the loss of \( \text{H}^+ \). Electron-donating groups (+I and +M effects) destabilize the conjugate base.

In the given options, all are substituted phenols. The substituents are:

• (1) No substituent (phenol)
• (2) \( \text{-Cl} \) (chloro) - electron-withdrawing through -I effect
• (3) \( \text{-CH}_3 \) (methyl) - electron-donating through +I effect
• (4) \( \text{-NO}_2 \) (nitro) - strong electron-withdrawing through -I and -M effects

Since the nitro group (\( \text{-NO}_2 \)) has the strongest electron-withdrawing effect (-I and -M), it stabilizes the conjugate base of the phenol most effectively. Therefore, the compound with the \( \text{-NO}_2 \) substituent (option 4) is the strongest acid among the given options.

Answer 2

The acid strength of a phenolic compound depends on the electron-withdrawing or electron-donating groups attached to the benzene ring. The electron-withdrawing group increases the acidity of the compound. Among the options, the nitro group (-NO\(_2\)) is a strong electron-withdrawing group through the inductive effect (-I), making the compound with -NO\(_2\) attached to the ring the most acidic.