Question

The increasing order of pKa for the following phenols is 
(A) 2,4-Dinitrophenol 
(B) 4-Nitrophenol 
(C) 2,4,5-Trimethylphenol 
(D) Phenol 
(E) 3-Chlorophenol 
Choose the correct answer from the option given below :

• A. (C), (E), (D), (B), (A)
• B. (C), (D), (E), (B), (A)
• C. (A), (E), (B), (D), (C)
• D. (A), (B), (E), (D), (C)

Hint:

For analyzing pKa trends:
• Electron-withdrawing groups increase acidity (lower pKa).
• Electron-donating groups decrease acidity (higher pKa).
• Consider the position and number of substituents for their effect on the phenoxide ion.

Answer 1

The Correct Option is D

To determine the increasing order of pKa values for the given phenols, we need to understand how substituents on the benzene ring affect the acidity of phenols. The pKa value is a measure of the acidity of a compound: the lower the pKa, the stronger the acid.

1. 2,4-Dinitrophenol (A): Nitro groups are electron-withdrawing groups. They stabilize the negative charge on the phenoxide ion formed after losing a proton, hence increase the acidity. With two nitro groups, 2,4-dinitrophenol is a very strong acid with a low pKa.
2. 4-Nitrophenol (B): Similarly, the nitro group at the para position enhances acidity by stabilizing the phenoxide ion. However, having only one nitro group makes it less acidic than 2,4-dinitrophenol.
3. 3-Chlorophenol (E): Chlorine is also an electron-withdrawing group, but its effect is weaker than that of a nitro group. Hence, 3-chlorophenol is less acidic compared to 4-nitrophenol.
4. Phenol (D): Without any electron-withdrawing group, phenol is less acidic than its nitro- or chlorine-substituted counterparts.
5. 2,4,5-Trimethylphenol (C): Methyl groups are electron-donating, which decrease acidity. Therefore, 2,4,5-trimethylphenol has the highest pKa among the given options, indicating it is the least acidic.

Considering these effects, the increasing order of pKa values for the given phenols is as follows:

• 2,4-Dinitrophenol (A)
• 4-Nitrophenol (B)
• 3-Chlorophenol (E)
• Phenol (D)
• 2,4,5-Trimethylphenol (C)

Therefore, the correct increasing order of pKa values is: (A), (B), (E), (D), (C).