reactiemechanisme

A reactiemechanisme, or reaction mechanism, in chemistry, is the step-by-step sequence of elementary reactions that converts reactants into products. It specifies the individual molecular events, including reactive intermediates and transition states, that connect the overall transformation. The mechanism explains how the observed rate law arises and why the reaction proceeds under certain conditions.

Core concepts involve listing the elementary steps with defined stoichiometry, identifying intermediates that do not appear in the overall equation, and locating transition states along the pathway. The rate-determining step, or slowest step, often governs the overall reaction rate and the required energy barrier. Energy profile diagrams illustrate relative activation energies and the stability of intermediates.

Determination of a mechanism relies on experimental evidence and, increasingly, computational methods. Kinetic data, such as reaction orders and rate constants, help distinguish possible steps. Isotope-labeling and kinetic isotope effects reveal which bonds are involved in rate-determining steps. Spectroscopic detection of intermediates, trapping experiments, and temperature variation provide supporting evidence. Computational chemistry can model transition states and propose feasible pathways that complement experimental findings.

Common archetypes include SN1 and SN2 mechanisms for nucleophilic substitution, and E1 and E2 for elimination reactions. Radical and chain reactions feature initiation, propagation, and termination steps. Some reactions can proceed via concerted pathways or through multiple competing routes depending on solvent, temperature, and catalysts.

Limitations include the possibility of multiple mechanisms contributing under different conditions. A mechanistic understanding aids in optimizing yield and selectivity, guiding catalyst design, solvent choice, and reaction conditions, and it underpins predictive control in synthesis.