Question

Which of the following is correct regarding degrees of dissociation of benzoic acid, 2-hydroxybenzoic acid and 2,6-dihydroxy benzoic acid ?

• A. Degree of dissociation of benzoic acid is the minimum
• B. Degree of dissociation of 2-hydroxybenzoic acid is the highest
• C. Degree of dissociation of 2,6-dihydroxy benzoic acid is the highest
• D. All of them equal degree of dissociation

Answer 1

Answer: (A), (C)

Degree of dissociation of 2,6-dihydroxy benzoic acid is the highest

Degree of dissociation, $K_a$ $\propto$ Strength of acid

Stability of conjugate base $\propto$ Strength of acid

2,4-dihydroxy benzoic acid is the most acidic acid because it has more hydrogen bonding compare to other. The two OH group in ortho position make hydrogen bonding with carboxylate ion. This hydrogen bonding stabilise the conjugate base of this compoud. Hence it is more acid and will have a high $K_a$ value compare to the others.
In 2-hydroxybenzoic acid, one OH is present at one ortho position and it will have one hydrogen bonding with carboxylate ion.
Benzoic acid has no OH group in ortho position, so there is not any intramolecular hydrogen bonding hence its conjugate base is least stabilised and hence less $K_a$ value So, degree of dissociation is the least.

Order of stability of conjugate base :
A > B > C
Order of acidic strength and degree of dissociation :
A > B > C