Question

Which of the following acid has the lowest $P{K}_a$ value.

• A.
• B. $\mathrm{Cl}-\mathrm{CH}{}_2-\mathrm{CH}{}_2-\mathrm{COOH}$
• C. $\mathrm{CCl}{}_3\mathrm{COOH}$
• D. $\mathrm{CHCl}{}_2\mathrm{COOH}$

Answer 1

Answer: (C)

$-I$ groups ; if more in number and more close to $⎯COOH$ group, more is the value of $K_a$ while $+I$ group if are more in number and close to $⎯COOH$ group, decreases the value of $K_a$.
$p{K}_a\propto \frac{1}{K_a}$
where, $K_a=$ acidic ionisation constant.
(a) It has one $-(I)$ group (i.e. $Cl$) at $C-2$ position.
(b) It has also one $(-)I$ group (i.e. $Cl$) at $C-3$ position.
(c) It has three $(-)I$ groups (i.e. $Cl$) at $C-2$ position.
(d) It has one $(-)I$ group (i.e. $Cl$) at $C-2$ position.
Thus, option $CC{l}_{3}COOH$ is the correct answer.