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 ?
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