Unlocking Interfacial Catalytic Halogen Atom Transfer at Ag Electrodes with Brønsted Acids

Halogen atom transfer enables mild and programmable access to carbon-centered radicals from accessible C(sp³) organic halides for synthetic chemistry. Unlocking this fundamental mechanistic step with reusable heterogeneous electrocatalysts requires strategies to identify and molecularly tune interfacial halogen atom transfer at complex electrified interfaces. Here, we reveal that Brønsted acids unlock interfacial halogen-atom transfer of unactivated alkyl bromides at catalytic Ag electrodes near the thermodynamic potential. In the absence of Brønsted acids, electrochemical and in situ spectroscopic data are consistent with the passivation of Ag surfaces by the halogen upon a single turnover, requiring an overpotential of nearly 1.1 V to activate the electrophile that would erode functional-group tolerance. The addition of acids promotes halogen atom transfer by 1.1 V, enabling access to primary carbon-centered radicals under mild conditions. This promotion effect is unique to catalytic Ag electrode surfaces and general across structurally diverse acids, provided that their pK_a falls below a specific threshold. By demonstrating C(sp³)-C(sp³) bond formation, we show that Brønsted acids enable interfacial halogen atom transfer at reusable catalytic electrodes for electroorganic synthesis.