Why phenol or benzenol is acidic and Why carboxylic acid is more acidic than phenol ?

Why phenol or benzenol is acidic ?

Oxygen is more electronegative than hydrogen element . So, O – H bond of phenol is polar . Besides this, the lone pair on oxygen atom participate in resonance with the pi electron of benzene ring .

As a result, the oxygen atom of phenol gets partially positive charge . Hence it attracts the O – H bonded electron towards itself .

Therefore , O – H  bond becomes more weak and easily break to release H⁺ ion .

Consequently , phenol or benzenol is acidic in nature .

How would you prove that phenol is acidic ?

There are few chemical reaction which can prove the acidic nature of phenol .  

For example , reaction of phenol  with metallic sodium produced  hydrogen .

 This reaction indicate that phenol is acidic .  In addition to , reaction of  phenol with strong base sodium hydroxide  gives phenoxide salt and water which prove the acidic nature of phenol.

Why carboxylic acid is more acidic than phenol ?

Both carboxylic acid and phenol are ionized in aqueous solution and give carboxylate  and phenoxide anion respectively . Now , the strength of acid depends on relative stability of their concerned anion .

Carboxylate anion have two equivalent resonating structure . The negative charge on oxygen atom equally spread out between two oxygen atom.

On the other hand,in case of phenoxide anion , there are five resonating structure. But the negative charge carry the less electronegative carbon atom .

Consequently, the relative stability of carboxylate anion is higher than phenoxide anion. Therefore , carboxylic acid is more acidic than phenol .

Why is carboxylic acid more acidic than alcohol ?

If carboxylic acid is ionized in aqueous solution ,it gives H⁺ cation and carboxylate anion . On the other hand , on ionization , alcohol gives H⁺ cation and alkoxide anion .

Now, the strength of acid depends on the relative stability of their concerned anion ( that is conjugate base ) . 

Carboxylate anion have two equivalent resonating  structure through which it is stabilized . 

But alkoxide anion have no such type of resonating structure . In addition to, +I effect of alkyl group destabilized the alkoxide anion .

Consequently, carboxylic acid more acidic than alcohol .

Summary:

Why phenol or benzenol is acidic ?

Why carboxylic acid is more acidic than phenol ?

Why is carboxylic acid more acidic than alcohol ?

Why boiling point of alcohol is higher than ether ? Click here.