Question

Which of the following is the strongest Bronsted acid?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Bronsted acids are acids which donate $H^{+}$ to the solution.
Strongest Bronsted acids will have stable conjugate base.

In conjugate base of option (a) and (b) negative charge is on the nitrogen atom and in option (c) and (d) negative charge is on the oxygen atom.
Negative charge on more electronegative atom will stabilise the conjugate base. Since oxygen is more electronegative than nitrogen, option (c) and (d) are more stable or acidic.

Delocalisation of electron will increase the stability of conjugate base.
Comparing (c) and (d), phenoxide ion (c) is more stable because the negative charge is delocalised in the benzene ring. Hence conjugate base of (c) is more stable than (d).

$\text{Stability of conjugate base}\propto \text{strength of acid}$
Hence, (c) phenol is the strongest bronsted acid.