Atomic-Scale Selective C-H Halogenation Driven by Tip Electric Field in Water.

Precise atomic-level control over chemical reactions is an ultimate goal across chemistry, physics, and materials science. Here, utilizing the nanoconfined electric field generated between a scanning tunneling microscope (STM) tip and a substrate, we achieve precise chlorination, bromination, and iodination of the α-C-H bonds in both aryl and alkyl carboxylic acids at room temperature in aqueous solutions containing halide ions. In situ STM break-junction technique revealed that the reaction follows pseudo-first-order kinetics, with an apparent rate that increases with both electric field strength and halide ion concentration. Notably, replacing a Na+ cation with Cs+ doubled the apparent reaction rate. Mechanistic investigations indicate that a strong confined electric field induces the dissociation of water molecules, generating hydroxyl radicals that abstract α-hydrogen atoms from carboxylic acids to form alkyl radicals, thereby facilitating halogenation. This work introduces a precise and environmentally benign strategy for C-H bond halogenation, offering potential for advancing green synthesis technologies and atomic-scale manufacturing.