Question

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

• A. ${CH}_{3}CHFCOOH$
• B. $F{CH}_2{CH}_{2}COOH$
• C. $Br{CH}_2{CH}_{2}COOH$
• D. ${CH}_{3}CHBrCOOH$

Answer 1

The correct option is A ${CH}_{3}CHFCOOH$

Highest $p{K}_a$ value means weakest acid and also weakest dissociation constant

(i) The inductive effect decreases with increase in distance of halogen atom from the carboxylic group and hence the strength of acid proportionally decreases.

(ii) The acidity increases with the increase in electronegativity of the halogen present.

Smallest dissociation constant means weakest acid, which is $BrC{H}_{2}C{H}_{2}OOH$ because here $Br$ (less electronegative than $F$) is two carbon atoms away from $–COOH$.