pKaHC

pKaHC is a term used in physical organic chemistry to denote the pKa value associated with the deprotonation of a carbon–hydrogen bond in hydrocarbons. It reflects the acidity of hydrocarbons as Brønsted acids, where removal of a proton from a C–H bond yields a carbanion. Because many C–H bonds are highly nonpolar and stabilized only modestly as anions, pKaHC values are among the largest typically tabulated acidities.

The concept is highly solvent-dependent and bond-specific. In aqueous solution, C–H bonds display a broad range of pKa values: acetylene (sp) C–H bonds are among the more acidic hydrocarbon C–H bonds, with pKa around 25; arenes and benzylic C–H bonds are typically around 40–43; alkanes (sp3) are markedly less acidic, often near 50 or higher. The exact numbers can shift significantly with different substituents and solvent environments, and nonaqueous solvents can alter the scale substantially.

In practice, pKaHC values are used to compare relative acidity of different C–H bonds and to assess reactivity in processes such as C–H bond activation, organometallic proton transfers, and radical or carbanion formation. They are typically determined by experimental acid–base measurements in a chosen solvent or estimated computationally via thermodynamic cycles and quantum chemical methods.

See also: pKa, carbanion, Brønsted acidity, C–H bond activation, solvent effects on acidity.