# Of the following which has the lowest pKa value? Why?

## $C {H}_{3} C {O}_{2} H$ $C l C {H}_{2} C {O}_{2} H$ $C {l}_{2} C H C {O}_{2} H$ $C {l}_{3} C C {O}_{2} H$

Jan 29, 2016

$C {l}_{3} C - C {O}_{2} H$

#### Explanation:

As physical scientists we should seek out numerical data:

${H}_{3} C - C {O}_{2} H$ ; pK_a=4.76

$C l {H}_{2} C - C {O}_{2} H$ ; pK_a=2.86

$C {l}_{2} H C - C {O}_{2} H$ ; pK_a=1.35

$C {l}_{3} C - C {O}_{2} H$ ; pK_a=0.66

So of course, we have to rationalize the enhanced acidity of the halogenated acids. This is a very old problem in inorganic chemistry. It turns out that this is an entropy rather than an enthalpy phenomenon. Trichloroacetate ion, $C {l}_{3} C - C {O}_{2}^{-}$, is less charge dense, due to the inductive effect of halogen substitution, and therefore is favoured entropically. A rationale based on the strength of the $O - H$ bond is known to be inadequate.