Substituent effects on the Sandmeyer reaction. Quantitative evidence for rate-determining electron transfer

Abstract

The substituent effect in the Sandmeyer reaction of aromatic diazonium salts induced by several reducing cations (Sn²⁺, Cu⁺, Fe²⁺) has been determined in H₂O–dioxane (2:1). Electron-withdrawing substituents increase the reactivity of the substrate in the radical chlorodediazoniation process, while the reverse holds for electron donor ones. For the change of the reduction potential of the reductant from 0.15 to 0.77 V the Hammett ρ value varies from 0.6 to 1.0. Conventional spectrophotometry has been employed to measure the rate of the reaction of two diazonium ions with Fe²⁺: reasonable agreement is found with the rate constants for the corresponding electron transfer step as calculated according to Marcus theory. The conclusion is drawn that the electron transfer step is rate determining in the Sandmeyer reaction under these conditions.