Question

Arrange the following in increasing order of their $p{K}_a$ value :

• A. $\left(a\right)<\left(b\right)<\left(c\right)$
• B. $\left(a\right)<\left(c\right)<\left(b\right)$
• C. $\left(b\right)<\left(a\right)<\left(c\right)$
• D. $\left(c\right)<\left(b\right)<\left(a\right)$

Answer 1

The correct option is A $\left(a\right)<\left(b\right)<\left(c\right)$

Conjugate base of compound (a) has 3 equivalent resonating structures as shown :



Conjugate base of compound (b) has 2 equivalent resonating structures as shown :


More the number of equivalent resonating structures, more is the stability of conjugate base, and more is the acidic character.
So, (a) has more acidic character than (b)
For conjugate base of compound (c), there is $+I$ effect of $C{H}_3$ group, so it is the least stable, and thus least acidic.
Correct order of acidic character is :
$\left(c\right)<\left(b\right)<\left(a\right)$
$\text{Acidic strength}\propto \frac{1}{p{K}_a}$
So, the order of $p{K}_a$ value is :
$\left(a\right)<\left(b\right)<\left(c\right)$