Benzoic acid-weak acid-stronger than acetic acid weaker than formic acid.

Why is benzoic acid a weak acid ?

In case of benzoic acid,the carboxylic acid group ( –COOH ) is directly attached to the sp2 hybridized carbon atom of aromatic benzene ring that is , attached with phenyl group.

Now, +R  effect of phenyl group is greater than –I effect ( +R > –I ). As a result of this , the O–H bond of benzoic acid becomes less polar than its expected value.

Consequently, the tendency of benzoic acid to donate proton in aqueous medium decreases.

For the above reason benzoic acid behave as a weak acid and this matter also proved by the measurement of pKa ( pKa = 4.19 ) value of benzoic acid.

Why is benzoic acid stronger than acetic acid but weaker than formic acid ?

In case of benzoic acid,the carboxylic acid group ( –COOH ) is directly attached to the sp2 hybridized carbon atom of aromatic benzene ring ,that is , attached with phenyl group.

Now, +R effect of phenyl group is greater than –I effect ( +R > –I ). As a result of this, the O–H bond of benzoic acid becomes less polar than its expected value.

Consequently, the tendency of benzoic acid to donate protonin aqueous medium decreases.For the above reason benzoic acid behave as a weak acid.

But , in case of formic acid, there are no such type of group that exhibit inductive effect or resonance effect.

So, benzoic acid behave weaker acid than formic acid, which also proved by their pKa value . The pKa value of benzoic acid is 4.19 ,while the pKa value of formic acid is 3.75.

Although, benzoic acid is a weaker acid than formic acid, but it is more acidic than acetic acid .

Because, in case of acetic acid , the carboxylic acid group (–COOH ) is directly attached to the methyl group ( –CH₃ ) which have +I effect.

Now, the +I effect of methyl group is more effective than the combined effect  due to +R  and –I effect of phenyl group.

Hence the order of acidic strength among benzoic acid, acetic acid and formic acid are , formic acid > benzoic acid > acetic acid.

Why is  chloro acetic acid is stronger than acetic acid ?

The strength of any acid depends on the stability of conjugate base of that acid.

Now, on ionization of both acetic acid and chloro acetic acid in aqueous solution, gives  the conjugate base , acetate and chloro acetate anion respectively.

Both the anion have two equivalent resonating structure. But, chloro acetate anion  becomes more stable than acetate anion.

 Because, in chloro acetate anion, due to – I effect of Cl-atom, the charge density on oxygen atom spread out upto chlorine atom.  

On the other hand , in case of acetate anion , due to + I effect of methyl group , the  electron density on oxygen atom increases.

As a result , the stability of acetate anion decreases . That is why chloro acetate anion is more stable than acetate anion. Consequently, chloro acetic acid is stronger than acetic acid.

Summary

• Why is benzoic acid a weak acid ?
• Why is benzoic acid stronger than acetic acid but weaker than formic acid ?
• Why is  chloro acetic acid is stronger than acetic acid ?

Benzoic acid definition in chemistry