Reactiespecies

Reactiespecies is a term used in chemistry to denote species that participate in a reaction step. The term encompasses transient intermediates, radicals, ions, and short‑lived molecular entities formed during the reaction mechanism, as well as sometimes transition states that describe the highest-energy points along a reaction coordinate. The term is not tied to a single formal definition and can be used differently across subfields, but it generally refers to species that are not spectator reactants or final products.

Common examples include radicals such as Cl• or OH•, carbocations and carbanions, reaction complexes formed by association of reactants, and molecules in electronically excited states. In many mechanisms, these species are highly reactive and their concentrations are low, so they are often treated with the steady-state approximation or considered in rate equations as intermediate steps rather than as isolated participants.

Detection and study of reactiespecies rely on time-resolved spectroscopic techniques such as flash photolysis and transient absorption, electron paramagnetic resonance for radicals, fast mixing methods, and mass spectrometry for trapped intermediates. Computational chemistry complements experiments by locating potential energy minima for intermediates and estimating activation barriers and lifetimes. Understanding which species appear and how they interconvert helps explain reaction rates and selectivity and supports catalyst design and mechanism elucidation.