elementärsteg

Elementärsteg, or elementary step, is a concept in chemical kinetics describing a single molecular event that occurs during a reaction mechanism. An elementärsteg corresponds to a transition state and, in principle, to one collision event or rearrangement that cannot be broken into simpler steps. In a sequence of such steps, intermediates may appear between steps, but each step itself is elementary.

In an elementary step, the rate law is determined directly by its molecularity: unimolecular steps have rate laws of the form rate = k[species], bimolecular steps rate = k[reactant1][reactant2], and, less commonly, termolecular steps rate = k[species1][species2][species3] (or 3A). Because true termolecular encounters are rare, most observed complex reactions that seem termolecular are combinations of rapid bimolecular steps or are effective rate laws from fast equilibria.

In a multi-step mechanism, the observed rate law for the overall reaction is not the rate law of the entire sequence, but is determined by the rate-determining step or the interplay of steps via steady-state or pre-equilibrium approximations. The overall stoichiometry is the sum of the elementary steps, while intermediates are consumed and produced in different steps.

Examples: The SN2 reaction (nucleophilic substitution) is often treated as an elementary bimolecular step with rate = k[A][B]. SN1 is not an elementary step; it involves a fast dissociation to form a carbocation, followed by a slower capture by a nucleophile; the overall rate law is first order in substrate, not in nucleophile. A -> products represents a unimolecular elementary step.

In practice, identifying elementary steps helps interpret kinetic data and guide mechanistic models. Transition-state theory, Arrhenius analysis, and computational chemistry are used to evaluate proposed steps and their rate constants.