Question

Rank the following compounds in order of decreasing acidity (strongest acid first) :

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

Answer 1

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

Conjugate base formed after removal of $H^{+}$ are :


In 2, $-C{H}_3$ group attached at ortho and para positions, show $+I$ and $+H$ effect, which increase the electron density and thus making the conjugate base more unstable.
In 3. $-N{O}_2$ group shows both $-R$ and $-I$ effect which will decrease the electron density and make the conjugate base more stable. Since , $-N{O}_2$ has more electron withdrawing effect, it will stabilize the negative charge more, and hence make compound 3 the most acidic.
In 1, $-Cl$ group shows both $+R$ and $-I$ effect where $-I$ effect will decrease the electron density but +R effect will increase the electron density. For Cl, -I effect is dominating over +R effect hence make the conjugate base more stable than 2 but less stable than 3.
Stability of conjugate base $\propto$ Strength of acid
Hence, the correct decreasing order of acidity is :
$3>1>2$