= Basicity
<Basicity> measures a <chemical species>' tendency to act as a <base (chemistry)>. For proton transfer, it is the tendency to accept a <proton>; for a <Lewis base>, it concerns association with a <Lewis acid> through an <electron pair>. A comparison must specify the reaction partner and medium.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Basicity, entry B00611]{ref}
For a base $B$ in <water>,
$$
B+\mathrm H_2O\rightleftharpoons BH^++OH^-,\qquad K_b=\frac{a_{BH^+}a_{OH^-}}{a_B}.
$$
The $a_i$ are <thermodynamic activities>, with the solvent convention absorbed into $K_b$. At the same <temperature> and standard-state conventions, $K_a(BH^+)K_b(B)=K_w$. Thus a higher conjugate-acid $pK_a$ corresponds to stronger <basicity> in that medium. <Basicity> is an equilibrium property; a fast <nucleophilic substitution> need not identify the stronger equilibrium <base (chemistry)>.
Back to article page
