Question: What do the following pK_a values tell you about the electron-withdrawing abilities of nitro, cyano, chloro, and hydroxy groups?

An electron-withdrawing group withdraws electrons present in the reaction centre, and the electron density of a conjugated pi system decreases.Examples of electron-withdrawing groups are nitro (-NO₂), cyano (-CN), carboxy (-COOH) groups etc.

The acetic acid derivatives are often used to test the electronic effects of substituents series. The synthesis of these compounds is easy, and pK_avalues can easily be measured by the titration method.

The substituents present on carbon 2 of acetic acid can have only an inductive effect, and no resonance effect is possible. This is because theCH₂ group is sp³ hybridized, and there is no possibility of pi overlapping.

So, the conclusions can be drawn by the givenpK_a values. Firstly, all the four different substituents present on CH₂ group are electron-withdrawing groups because all are stronger than acetic acid. Secondly, the magnitude of the electron-withdrawing effect in the order of increasing manner is, OH < Cl < CN < NO₂. Thus, the nitro group is the strongest electron-withdrawing CH₂ group of all.

The magnitude of the electron-withdrawing effect in the order of increasing manner is, OH < Cl < CN <NO₂ . Thus, the nitro group is the strongest electron-withdrawing group of all.