Question

Based on pKa values, the order of decreasing acidity of carboxylic acid molecules containing EWG must be ?
$C{F}_{3}COOH$, $CC{l}_{3}COOH$, $HCC{l}_{2}COOH$, $N{O}_{2}C{H}_{2}COOH$,$F$, $ClC{H}_{2}COOH$, $BrC{H}_{2}COOH$, $HCOOH$, $ClC{H}_{2}C{H}_{2}COOH$, $C_6{H}_{5}C{H}_{2}COOH$ , $C{H}_{3}COOH$, $C{H}_{3}C{H}_{2}COOH$.

Answer 1

Hint: The acid is a molecule or a compound that can donate a proton i.e. $H^{+}$ ion. The Ka value is directly proportional to the acidity of a molecule. Stronger is the acid, greater is its Ka value. The pKa is the negative logarithm of Ka. Thus, the higher the Ka value, the lower will be the pKa value. So, if acid is stronger, higher will be its Ka value and lower will be its pKa value.

Complete Solution :
First, let us understand some basic terms related to this.
- The acid is a molecule or a compound that can donate a proton i.e. $H^{+}$ ion and base is the molecule that can donate a hydroxyl group. The acidity of a compound can be given in terms of Ka value which is the acid dissociation constant. The Ka value is directly proportional to the acidity of a molecule.
Stronger is the acid, greater is its Ka value.
- The pKa is the negative logarithm of Ka. Thus, the higher the Ka value, the lower will be the pKa value. So, if acid is stronger, higher will be its Ka value and lower will be its pKa value.
- Further, we know that the electron-withdrawing groups increase the acidity of the molecule and the electron-donating groups decrease the acidity of the molecule. Based on the above factors, we can arrange them in the order of decreasing acidity of carboxylic acid molecules as -

$C{F}_{3}COOH$ > $CC{l}_{3}COOH$ > $N{O}_{2}C{H}_{2}COOH$ > $HCC{l}_{2}COOH$ > $ClC{H}_{2}COOH$> $BrC{H}_{2}COOH$> $ClC{H}_{2}C{H}_{2}COOH$> $C_6{H}_{5}C{H}_{2}COOH$ > $HCOOH$ > $C{H}_{3}COOH$ > $C{H}_{3}C{H}_{2}COOH$ > $F$.

Note: It must be noted that the Fluorine is an electron withdrawing group. It withdraws electrons towards itself and thus, the O-H bond gets weakened and as a result breaks to release protons. Further, the F alone could not be acidic because it does not have protons to release and if we talk in terms of electrons then it is very electronegative and can not donate electrons so easily.