The correct order of acidity for the following compounds is

Options

A. `I gt II gt III gt IV`
B. `III gt I gt II gt IV`
C. `III gt IV gt II gt I`
D. `I gt III gt IV gt II`

Answer

A

Explanation

We know that the strength of an acid depends upon the stability of its carboxylate anion. Greater the stability of the carboxylate anion more acidic is the carboxylic acid. Now the carboxylate ion from 2,6-dihydroxybenzoic acid (I) is mroe stable due to chelation with two OH groups than anion of salicylic acid which is stabilized by chelation with only one OH group.

Further, OH group has a strong +R-effect and a weak-I-effect. Since at m-position OH group cannot exert its +R-effect but can only exert its -I-effect, therefore, m-hydroxybenzoic acid (III) is a stronger acid than p-hydroxybenzoic acid (IV) where OH group exerts its strong +R-effect as shown.

Thus, the overall acid strength decreases in the order: 2,6-`underset((pK_(a)=2.30))("Dihydroxybenzoic acid (I)") gt underset((pK_(a)=2.98))("Salicylic acid (II)") gt underset((pK_(a)=4.08))("m-Hydroxybenzoic acid (III)") gt underset((pK_(a)=4.58))("p-Hydroxybenzoic acid (IV)")`