Question

Rank in the order of increasing acidity :

• A. $III<II<I$
• B. $III<I<II$
• C. $I<II<III$
• D. $II<I<III$

Answer 1

The correct option is D $II<I<III$

More the stability of conjugate base formed, more is the acidity.
The conjugate base so formed after removal of $H^{+}$ for each of the following compounds are :

III (phenoxide ion) is resonance stabilized, whereas there is no resonance in I and II.
So III is the most stable conjugate base and most acidic.
Comparing I and II
In I the negative charge is on more electronegative atom (oxygen), as compared to II where the negative charge is on less electronegative atom (nitrogen).
So, I is more stable than II, and consequently a stronger acid .
The correct order of increasing acidity is :
$II<I<III$