pKaBH

pKaBH is a term encountered in some chemical data resources to denote the acid dissociation constant (pKa) associated with boron–hydrogen containing species. It is not a universally standardized constant; its meaning depends on the particular boron compound being considered, as well as the solvent, temperature, and data source.

In practice, pKaBH most often appears in discussions of boronic acids and related boron compounds that can equilibrate with their boronate forms. For these species, the acidity reflects the deprotonation of a hydroxyl group to form a tetra-coordinate boronate anion. Reported pKa values for aryl boronic acids in water typically lie in the high single digits, around 7 to 9, though substituents and solvent strongly influence the value. Electron-withdrawing groups generally lower the pKa, while electron-donating groups raise it.

Other boron-containing systems, such as boric acid, have well-established acid–base behavior and associated pKa values under aqueous conditions (for boric acid, pKa about 9.2 for the equilibrium leading to the borate anion). In some datasets, pKaBH may be used to refer to analogous equilibria in boron hydride chemistry, but this usage is highly context-dependent and not universal.

Measurement and use: pKaBH values are determined by standard acid–base titration, spectrophotometric methods, or computational estimations under defined conditions (solvent, temperature). Because boron chemistry is diverse and several species are unstable in water, reported pKaBH values are often specific to the solvent and temperature of the measurement, and direct comparisons across compounds should be made with caution.