Arrange the following compounds in order of decreasing acidity :

Arrange the following compounds in order of decreasing acidity :

1. $\text{III}>\text{I}>\text{II}>\text{IV}$
2. $\text{IV}>\text{III}>\text{I}>\text{II}$
3. $\text{II}>\text{IV}>\text{I}>\text{III}$
4. $\text{I}>\text{II}>\text{III}>\text{IV}$

Solution

Acidity is directly proportional to conjugated base stability.



Stability order of conjugate bases is III > I > II > IV

Conjugate base in I is stabilised by - I > + M

Conjugate base in II is destabilised by + I and hyperconjugation.

Conjugate  base in III is stabilised by strong withdrawing effect of $-{\mathrm{NO}}_2$ by - I & - M effect.

Conjugate base in IV is destabilised by + M effect of$-{\mathrm{OCH}}_3$

$\begin{array}{l}\mathrm{acidic}\mathrm{character}\propto -\mathrm{M},-\mathrm{I}\\ \propto \frac{1}{+\mathrm{M}},\frac{1}{+\mathrm{HC}},\frac{1}{+\mathrm{I}}\end{array}$

Electron releasing group (i.e +I effect, +M) decreases the acidic strength 
Electron withdrawing group (i.e -I effect,-M) increases acidic strength.


 

Asked in: JEE Main 2013 (07 Apr)

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