Question

Select the correct decreasing order of acid strength for the following compounds.
a) Ethanol b) 2-Methyl propan-2-ol   c) Phenol   d) p - Nitrophenol

• A. a > b > c > d
• B. c > d > b > a
• C. d > c > a > b
• D. d > b > c > a

Hint:

Phenols are more acidic than alcohols. A nitro group makes phenol more acidic. Tertiary alcohols are the weakest.

Answer 1

The Correct Option is C

Step 1: Rank the conjugate bases:
A more stable anion means a stronger parent acid.
p-Nitrophenoxide: charge delocalised into the ring and onto the nitro group, the most stable.
Phenoxide: charge delocalised into the ring.
Ethoxide: charge localised on oxygen, with one electron-releasing group.
tert-Butoxide: charge localised on oxygen, with three electron-releasing methyl groups, the least stable.

Step 2: Order:
d, then c, then a, then b. The approximate pKa values are 7.1, 10, 16 and 19, respectively.

Step 3: Match:
This is option (C). Options (A) and (B) put alcohols above phenols, and option (D) puts the tertiary alcohol above phenol.

Final Answer:
Option (C). \[ \boxed{d>c>a>b \text{ (C)}} \]