Question

Which of the following has the lowest $p{K}_a$ value?

• A. $C{l}_{3}CCOOH$
• B. $CHC{l}_{2}COOH$
• C. $ClC{H}_{2}COOH$
• D. $C{H}_3-COOH$

Answer 1

The correct option is A $C{l}_{3}CCOOH$

Stronger acid has lower pKa value and strong acids are those which have weaker (stable) conjugate base.
In all the options given, stability of conjugate base are decided by the number of attached $Cl$ group at the $\alpha$ carbon.
Since $Cl$ is electronegative atom ann it stabilise the charge by $-I$ effect. Hence more the $Cl$ atom, more is the stabilisation.

$\therefore$ $C{l}_{3}CCOOH$ is the strongest acid having lowest pK value.