Question

A scientist proposes a nucleophilic substitution reaction in which the reactant, anion nucleophile has a pK_abH of -8.5 and the product anion (which was the leaving group) has a pK_abH of -7.2. The proposed reaction represents a _____ pK_abH unit climb. Report your answer to the nearest tenth of a unit.

Answer 1

In nucleophilic substitution reaction , the relative basicity of the nucleophile and the leaving group can be used to determine whether the reaction will proceed in the forward or in the backward direction. Generally, a reaction follows that direction in which a weaker base will formed. But sometimes, a slightly stronger base can also form. In such cases the pKa rule comes into the picture, which states that a product or nucleophile formed can not be more than 10 units more basic than the incoming nucleophile.

As mentioned in the question, the nucleophile has a pKabH of -8.5 and leaving group has a pKabH of -7.2 which is slighty basic. So .the proposed reaction represents a -8.5+7.2=-1.3 pKa unit cilmb , which is highly favorable.