Acidity of Aromatic Acids
Acidity of carboxylic
acids depends on the stability of acid anion a conjugate base thus.
Electron withdrawing group
stabilises anion, hence increases acidity
Similarly , electron donating group destabilises
anion, hence decreases acidity.
The First member of aromatic acid is
a benzoic acid which dissociates as follows
Thus
acidity of benzoic acid
will depends on the stability of
benzoate anion . . Greater the stability of carboxylate ion ,it is a weaker conjugate base and tendency to react with H+ decreases resulting in the increase of acidic nature .
Acidity of substituted Acids: Ortho
substituted benzoic acid is always a strong acid than m- and p-derivative
due to the ortho effect.
Case I : When group is –M and –I group.
at ORTHO POSITION
Thus ,
anion is stabilised by – M
and – I
effect and – I power is maximum
Anion is stabilised only by –I
effect of NO2 group .
Anion
is stabilised by –M and –I effect of NO2 group
Thus decreasing order of the
stability of these anions is follows:
I > III
> II > IV
Ortho derivative is
the most acidic due to ortho effect therefore decreasing order of acidity of these acids is as follows :
Case
II : When group has +I effect.
Ortho derivative will be most acidic due to ortho effect. The anion is destabilised due to +I
effect.As distance between + I group and carboxylate increases +I influence decreases .Thus methyl at para is less donating compared to methyl at meta .At ortho position eventhough the distance between CH3 group and carboxylate is very less DUE TO ORTHO EFFECT IT IS MOST ACIDIC.
Case
III : When group has
+M and –I effect and - I dominates +M
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