Question

How would you arrange the following compounds in order of decreasing pka: ClCH2CH2OH, CH3CH2OH, and Cl2CHCH2OH?

Answer 1

`Cl_2CHCH_2OH < ClCH_2CH_2OH < CH_3CH_2OH`

Explanation:

In general the higher the acid, the higher the value of `K_a` and the lower the `pK_a=-logK_a`.

The increasing order of acidity will reflect the decreasing order of `PK_a` then.

The increasing order of acidity is the following:

`CH_3CH_2OH < ClCH_2CH_2OH < Cl_2CHCH_2OH`

This order is based on the stability of the corresponding conjugate bases of the mentioned acids. The presence of the chlorine atom(s) in the structure of the acid will stabilize the negative charge on the oxygen of the conjugate base by inductive effect since the chlorine atom is an electronegative element. The more stable the conjugate base the stronger the acid.

The increasing order of stability of the conjugate bases is the following:

`CH_3CH_2O^- < ClCH_2CH_2O^- < Cl_2CHCH_2O^-`

And therefore, the decreasing order of `PK_a` is the following:

`Cl_2CHCH_2OH < ClCH_2CH_2OH < CH_3CH_2OH`