The prehistory of the Hammett equation

The evolution of linear free energy relationships is traced from the Bronst ed equation for general acid-base catalysis (1924) to the Hammett rho-sigma equation for the effects of meta or para substituents on the rate or equil ibrium constants of side-chain reactions of benzene derivatives (1937). In addition to a natural emphasis on Louis P. Hammett's own contributions, tho se of G. N. Burkhardt and his colleagues are also described, along with the extent to which these two authors interacted.