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.

For the above reason benzoic acid behave as a weak acid and this matter also proved by the measurement of pKa 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,As a result of this , the +R effect of phenyl group is greater than -I effect ( +R > -I ) . O-H bond of benzoic acid becomes less polar than its expected value.

For the above reason benzoic acid behave as a weak acid.

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 .

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, givesthe conjugate base , acetate and chloro acetate anion respectively .

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

Because, in chloro acetate anion, due to- I effectof ‘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 .

Why is chloro acetic acid is stronger than acetic acid ?

Originally published at https://chemisfast.blogspot.com.