Question

What is the correct order of decreasing acidic strengths?

• A. $H_b>H_c>H_a$
• B. $H_a>H_c>H_b$
• C. $H_c>H_b>H_a$
• D. $H_b>H_a>H_c$

Answer 1

The correct option is A $H_b>H_c>H_a$

To check the acidic strength, remove the $H^{+}$ ion and check the statbility of the conjugate base that will be formed.
In the case of $H_b$, the negative charge is delocalised in the vacant d-orbitals of $S$, which makes it stable.

$H_a$ and $H_c$ can be compared by the anion formed on $N$ and $O$ respectively. As $O$ is more electronegative than $N$, the negative charge will be more stable on $O$ than on $N$. So, the order of acidic strength is:
$H_b>H_c>H_a$