Kinetics of primary mechanochemical covalent-bond-forming reactions

Abstract

Mechanical activation of reactions can reduce significantly the amounts of solvent and energy required to form covalent organic bonds. Despite growing interest in the field of mechanochemistry and increasing reports of mechanochemical synthesis of organic molecules, the fundamental question of how stresses activate covalent-bond-forming (CBF) reactions remains unresolved. This question remains unresolved because of the difficulties involved in measuring the applied forces and the reaction times in mechanochemical reactors, and the challenges related to deconvoluting microscopic (primary) and macroscopic (secondary) processes in the analysis of reaction kinetics. Here we discuss the use nanoscopic probe-microscope tips to explore the kinetics of CBF reactions. Because these experiments examine reactions on monolayers, surfaces, or nanoscopic particles, they circumvent secondary processes to isolate how stress affects the rates of the primary, CBF events. The major result of these studies is an emerging consensus that stress accelerates reactions by distorting organic molecules and in doing so, lowers reaction activation energies and alters reaction trajectories. This new understanding of how stresses activate reactions can be used to predict the outcomes of CBF mechanochemical reactions, which will lead to the wider adoption of sustainable mechanochemical processes by the synthetic community.