Question

Decreasing order of acidic strength for the following is:

• A. (i) \(>\) (iii) \(>\) (ii) \(>\) (iv)
• B. (ii) \(>\) (iv) \(>\) (i) \(>\) (iii)
• C. (iii) \(>\) (i) \(>\) (iv) \(>\) (ii)
• D. (iii) \(>\) (i) \(>\) (ii) \(>\) (iv)

Hint:

Electron-withdrawing groups like \(-NO_2\) increase the acidity of phenols by stabilizing the phenoxide ion, while electron-donating groups like \(-CH_3\) decrease acidity by destabilizing the phenoxide ion.

Answer 1

The Correct Option is C

Step 1: Basis of phenolic acidity.
Acidity hinges on phenoxide ion stability; electron-withdrawing groups enhance acidity, while electron-donating groups diminish it.

Step 2: Impact of the nitro group.
–NO₂ exerts strong –I and –R effects, withdrawing electron density and stabilizing the phenoxide, thereby increasing acidity.

Step 3: Ortho vs meta nitrophenol.
o-nitrophenol benefits from both –I and –R effects for superior stabilization. m-nitrophenol mainly relies on –I effect, so o-nitrophenol is more acidic.

Step 4: Impact of the methyl group.
–CH₃ donates electrons via +I and hyperconjugation, destabilizing the phenoxide and making p-cresol less acidic than phenol.

Step 5: Overall acidity ranking.
Decreasing acidity: o-nitrophenol>m-nitrophenol>phenol>p-cresol, i.e., (iii)>(i)>(iv)>(ii).

Step 6: Conclusion.
Option (3) matches this order.