Question

For the given compounds, the correct order of increasing \(pK_a\) value :

• A. \((E)<(D)<(C)<(B)<(A)\)
• B. \((D)<(E)<(C)<(B)<(A)\)
• C. \((E)<(D)<(B)<(A)<(C)\)
• D. \((B)<(D)<(A)<(C)<(E)\)
• E. None of these

Answer 1

Answer: (E)

Explanation:

1. Definition of \( pK_a \): Lower \( pK_a \) corresponds to a stronger acid.
2. Factors affecting \( pK_a \):
   • Electron-withdrawing groups (EWG) increase acidity by stabilizing the conjugate base.
   • Electron-donating groups (EDG) decrease acidity by destabilizing the conjugate base.
3. Order Analysis:
   • (D) Dinitrophenol: Strongest acid due to two nitro (EWG) groups stabilizing the phenoxide ion.
   • (B) Nitrophenol: Less acidic than (D), but still acidic due to one nitro group.
   • (A) Phenol: Less acidic than (B) due to the absence of strong EWGs.
   • (C) Methoxyphenol: The methoxy group (EDG) destabilizes the phenoxide ion, making it less acidic than phenol.
   • (E) Methoxy-dinitrophenol: The methoxy group reduces the effect of the nitro groups, making it less acidic than dinitrophenol but more acidic than phenol.
4. Final Order: \[ (D) < (B) < (A) < (C) < (E). \]

Final Answer: None of these.