Question

Which of the following has lowest $p{K}_a$ value?

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

Due to Ortho effect any group placed at an ortho place, irrespective of whether it is electron withdrawing or donating will be the most acidic in the case of aromatic carboxylic acids.

Ortho hydroxy benzoic acid is more acidic because in the ortho isomer, strong hydrogen bridge will be formed between the phenolic -OH and the carboxylate ion and this will tend to stabilise the conjugate carboxylate ion hence increasing the acidity.
For the para isomer, the mesomeric effect makes the compound less acidic in comparison with benzoic acid.
For the meta isomer a minor inductive effect operates, but no resonance effect, which makes it slightly more
acidic than benzoic acid.
Thus the correct increasing order of acidic strength is given by a>b>d>c.
Now $\text{Acidic strength}\propto \frac{1}{p{K}_b}$
Thus, option a has lowest $p{K}_a$ value.