Question

Rank the following compounds in order of decreasing acidic strength :

• A. $1>2>3>4$
• B. $3>2>4>1$
• C. $4>1>3>2$
• D. $3>1>4>2$

Answer 1

The correct option is C $4>1>3>2$

More the stability of conjugate base, more is the acidic strength.

The conjugate base so formed after removal of $H^{+}$ is :




Between 1 and 4 :
In 1, the negative charge is delocalized and stabilized by only $-I$ effect of $-N{O}_2$. This is because $-M$ effect does not operate at meta-position.
In 4, $-M$ effect of $-N{O}_2$ increases the stability of conjugate base.
Since $-M$ effect is more dominating than $-I$ effect , 4 is more stable than 1
In 3, negative charge is delocalized with non-equivalent resonating structures, so it is less stable than 1 and 4.
In 2 , there is no delocalisation of negative charge, so it is the least stable, and least acidic.
So, the correct order is $4>1>3>2$