HammettParametern

The Hammett parameter, named after Louis Plack Hammett, is a set of constants used in physical organic chemistry to quantify how substituents on an aromatic ring influence reaction rates and equilibria. The core idea is that electronic effects of substituents can be related to changes in reactivity through a linear free-energy relationship.

The primary quantity is the substituent constant sigma, derived from the influence of various substituents on the acidity or basicity of benzoic acids or related systems. Common forms are sigma_p for para substitutions and sigma_m for meta substitutions. Other related constants include sigma_p+ and sigma_p−, which account for specific resonance and polar effects in reactions with cationic or anionic transition states. The Hammett equation is typically written as log(k/k0) = rho * sigma or log(K/K0) = rho * sigma, where k or K are the rate or equilibrium constants for a substituted system, k0 or K0 are those for the reference system, and rho is a reaction constant that measures sensitivity to electronic effects.

Sigma values are determined empirically from measurements across a series of substituted benzoic acids or related compounds in a given solvent. Positive sigma indicates electron-withdrawing substituents, while negative sigma indicates electron-donating substituents. The rho value reflects how strongly a reaction responds to those electronic changes.

Limitations include dependence on solvent, the assumption of a single rate-determining step, and the potential influence of steric or intramolecular interactions, especially for ortho substitutions. Nevertheless, the Hammett parameters remain a foundational tool for understanding structure–reactivity relationships, enabling predictions in organic synthesis, catalysis, and medicinal chemistry.