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: Understanding the Concept:
Acid strength depends on how stable the conjugate base is. Phenoxide ions are stabilised by resonance, so phenols are stronger acids than alcohols. Electron-donating alkyl groups destabilise the alkoxide ion and weaken acidity.

Step 2: Key Formula or Approach:
Compare in three groups: nitrophenol vs phenol, phenol vs alcohols, and primary vs tertiary alcohol.

Step 3: Detailed Explanation:
p-Nitrophenol: the \(-\text{NO}_2\) group withdraws electrons by resonance and induction and spreads the negative charge of the phenoxide further, so it is more acidic than phenol.
Phenol: the phenoxide ion is stabilised by resonance with the ring, so it is more acidic than any simple alcohol.
Ethanol: one alkyl group, so the alkoxide is only slightly destabilised.
2-Methylpropan-2-ol: three alkyl groups push electrons towards the oxygen of the alkoxide ion, so it is the weakest acid.
So the order is p-nitrophenol > phenol > ethanol > 2-methylpropan-2-ol, that is d > c > a > b.

Final Answer:
The decreasing order of acidity is d > c > a > b, option (C). \[ \boxed{d>c>a>b \text{ (C)}} \]