Answer to Question #61056 in Organic Chemistry for Amit Sharma

Question

How do the pKa, basicity and nucleophilicity vary in the following moieties? Explain taking into consideration their conjugate acids. Cl- , CN- , RS- , R3N, R2NH, OH-

Answer 1

Basicity is a subset of nucleophilicity. All nucleophiles are Lewis bases; they donate a lone pair of electrons. A “base” (or, “Bronsted base”) is just the name we give to a nucleophile when it’s forming a bond to a proton (H+). To summarize, when we’re talking about basicity and nucleophilicity, we’re talking about these two types of events.
The acidity as well as basicity can be measured in pKa, which is a reflection of the position of the equilibrium between an acid and its conjugate base.
In a row Cl- , OH-, RS-, CN-, R2NH, R3N the pKa grows from (-6)in Cl- to 25 in R3N. This means that the weakest base will be Cl, the strongest - R3N(basicity grows from left to right in a row as well as nuchleophility).