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 .
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 ?
