1. Tardigrade
  2. Question
  3. Chemistry
  4. The pKa value of (I) 2 - bromo propanoic acid (II) 2 -fluoro propanoic acid, (III) 3 - fluoro propanoic acid, (IV) 3 - bromo propanoic acid decreases as

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Q. The $ pK_{a} $ value of
$ (I) 2 $ - bromo propanoic acid
$ (II) 2 $ -fluoro propanoic acid,
(III) $ 3 $ - fluoro propanoic acid,
$ (IV) 3 $ - bromo propanoic acid decreases as

UPSEEUPSEE 2010

Solution:

Presence of electron withdrawing group (like $- Cl , Br$ ) increases the acidity of carboxylic acids. $Cl$ is more electron withdrawing as compared to Br. As the distance of electron withdrawing group from carboxylic group increases, acidic character decreases. Thus, the order of acidic character is $\underset{(IV)}{\text{3-bromopropanoic acid}} < \underset{(I)}{\text{2-bromopropanoic acid}} < \underset{(III)}{\text{3-fluoropropanoic acid}} < \underset{(II)}{\text{2-fluoropropanoic acid}}$

$p K_{a} \propto \frac{1}{K_{a}} \propto \frac{1}{\text { acidic strength }}$

Thus, the order of $p K_{a}$ value is

$IV > I > III > II$