Nucleophilicity Versus Basicity

Base Strength versus Nucleophile Reactivity Basicity is measured as the pKa (pKa = - log10 Ka) of the base’s conjugate acid, where Ka is the acid dissociation constant, an equilibrium constant. Nucleophilicity is measured as the relative rate coefficients, k, for various nucleophiles with a particular electrophile.

The Arrhenius Equation The van ’t Hoff Equation K versus k Chemical Kinetics The Arrhenius Equation Chemical Equilibrium The van ’t Hoff Equation

Energy of Activation versus Energy of Reaction

K versus k Keq depends on the ratio kf/kr, while kf is independent of Keq, consequently . . Factors influencing Keq may not have the same effect on the rates, especially if . . We are comparing two different types of chemical reaction, nucleophilic substitution versus proton transfer.

Nucleophilicity versus Basicity No general trend between them!

Hybridization

Delocalization

Steric hindrance Basicity trend here depends on inductive electron donation, while the nucleophilicity trend is based on steric hindrance.

Electronegativity

Polarizability

Factors Influencing Nucleophilicity and Basicity Same element: higher p-character orbitals are more basic; localized orbitals are more basic and more nucleophilic; sterically hindered orbitals are less nucleophilic. Same period: lower electronegativity atoms, with higher energy orbitals, are more basic and more nucleophilic. Same group: larger, more polarizable atoms are substantially more nucleophilic while less basic.