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)

None of these

Answer 2

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.