picric acid or 2,4,6-tri-nitro phenol more acidic than phenol .

Why is picric acid or 2,4,6-tri-nitro phenol more acidic than phenol?

2,4,6-tri nitro phenol is also known as picric acid .Picric acid is more stronger acid than phenol.

Because, picric acid or 2,4,6-tri nitro phenol contains three strong electron withdrawing – NO₂ group.

 Due to – I effect and – R effect of – NO₂ group, the density of electron on oxygen atom of O – H group very much decreases.

As a result, oxygen atom attract O – H bonded electron towards itself . Hence O – H  bond becomes more polar and more weak.

So, O – H  bond break very easily to release proton ( H⁺ ). Consequently, picric acid or 2,4,6-tri nitro phenol more acidic than phenol .

What is acidic order of  o-nitro phenol , m-nitro phenol and p-nitro phenol and why?

All the mono nitro phenols are more acidic than phenol. Because – NO₂ group is a strong electron withdrawing group . 

Due to – I effect and – R effect, – NO₂ group can attract electron from any position (ortho, meta or para). Hence, all the mono nitro phenols are more acidic than phenol.

The acidic order of mono nitro phenols with compare to phenol are,

Now, among the o-nitro phenol, m-nitro phenol and p-nitro phenol, only o-nitro phenol and p-nitro phenol can attract electron from - OH  group due to – R effect of –NO₂ group.

 Because , due to presence of – NO₂ group in ortho and para position , a conjugation takes place between the lone pair of oxygen atom of – OH group and – NO₂ group through benzene nucleus . 

But, in case of m-nitro phenol, - NO₂ group present in meta position . Hence in this case conjugation between lone pair of – OH group and – NO₂ group does not occur.

So ,for meta isomer, the density of electron of –OH  group decrease due to – I effect only .

Hence the density of electron on oxygen atom is lowest for p-nitro phenol and highest for m-nitro phenol. As a result , oxygen atom attract O – H bonded electron towards itself .

Consequently, the – OH bond for p-isomer becomes more polar and more weak . But the – OH bond of m-nitro phenol becomes less polar. 

So p-nitro phenol gives proton ( H+ ) more easily than the other two isomers .Hence the acidic order of  o-nitro phenol , m-nitro phenol and p-nitro phenol are as follows,

Why o-nitro phenol and p-nitro phenol are more acidic than phenol?

O-nitro phenol and p-nitro phenol  are more acidic than phenol. Because – NO₂ group is a strong electron withdrawing group . 

Due to – I effect  of– NO2 group, it can attract electron from any position ( ortho, meta or para ).

Besides this , for  – R effect of – NO₂ group , a conjugation takes place between the lone pair of oxygen atom of – OH group and – NO₂ group through pi bond of benzene nucleus . Hence, oxygen atom gets partially positive charge.

As a result , oxygen atom attract O – H bonded electron towards itself . Hence O – H  bond becomes more polar and more weak .

So, O – H  bond  of o-nitro phenol and p-nitro phenol break very easily to release proton ( H⁺ ) than phenol.

Consequently, o-nitro phenol and p-nitro phenol are more acidic than phenol.

Practice problem:

What is acidic order of  o-nitro phenol , m-nitro phenol and p-nitro phenol and why ?

Why is picric acid or 2,4,6-tri nitro phenol more acidic than phenol ?

Why o-nitro phenol and p-nitro phenol are more acidic than phenol ? 

Why pyridine does not exhibits resonance ? Click here.