basicity

Basicity is the ability of a chemical species to accept a proton. In Brønsted–Lowry terms, a base B reacts with H+ to form BH+. In Lewis terms, a base is an electron-pair donor to a Lewis acid. Basicity is a thermodynamic concept that can be quantified in a given solvent by equilibrium constants or related scales such as pKa and pKb.

In aqueous solution, the common way to compare bases is via pKb or the pKa of the conjugate acid BH+. A higher pKa for BH+ (i.e., a weaker acid) corresponds to a stronger base B, because protonation becomes more favorable. Conversely, a lower pKa for BH+ indicates a weaker base. Substituents that donate electron density (for example, alkyl groups) generally increase basicity by stabilizing the conjugate acid less or by increasing electron density on the basic site; resonance or delocalization can reduce basicity, as in amide nitrogens.

Basicity is environment-dependent. Gas-phase basicity is often described by proton affinity and reflects intrinsic tendency, independent of solvent. In nonaqueous solvents, different scales apply (for example, donor numbers or conjugate-acid pKa values in that solvent). Steric hindrance and geometry also influence basicity: bulky bases may be less basic despite having many lone pairs because protonation is hindered. Examples in organic chemistry include amines (generally basic) vs amides (much weaker bases) and alcoholates or oxide ions as strong bases in appropriate media.

Basicity differs from nucleophilicity: a base may be thermodynamically strong yet kinetically hindered, and nucleophilicity depends on reaction dynamics and solvent, not just proton affinity.