Question

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

`CH_3CO_2H`

`ClCH_2CO_2H`

`Cl_2CHCO_2H`

`Cl_3C CO_2H`

Answer 1

`Cl_3C-CO_2H`

Explanation:

As physical scientists we should seek out numerical data:

`H_3C-CO_2H` `; pK_a=4.76`

`ClH_2C-CO_2H` `; pK_a=2.86`

`Cl_2HC-CO_2H` `; pK_a=1.35`

`Cl_3C-CO_2H` `; 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, `Cl_3C-CO_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.