Question

The $p{K}_a$ value of
$(I)2$ - bromo propanoic acid
$(II)2$ -fluoro propanoic acid,
(III) $3$ - fluoro propanoic acid,
$(IV)3$ - bromo propanoic acid decreases as

• A. $I>III>IV>II$
• B. $II>IV>III>I$
• C. $I>IV>III>II$
• D. $IV>I>III>II$

Answer 1

Answer: (D)

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

$IV>I>III>II$