intermediats

Intermediats, often written intermediates in modern chemical literature, are transient species that appear during a multistep reaction mechanism. They are not present among the initial reactants or the final products, but arise and are consumed as the sequence of elementary steps proceeds. Intermediats are typically short-lived and accumulate only at low concentrations because they quickly react to form the next species in the pathway.

Characteristics and study methods

Intermediats can be neutral, charged, radical, or covalently bound complexes. Their fleeting nature makes direct observation difficult; scientists often infer their existence from kinetic data, trapping experiments, or spectroscopic evidence obtained at low temperatures or under rapid-mixing conditions. Isotopic labeling and computational modeling also help delineate their structures and roles within a mechanism.

Common examples

Typical intermediats include tetrahedral intermediates in nucleophilic acyl substitution, carbocations formed in unimolecular ionizations (such as E1 mechanisms), enolate species in aldol-type condensations, and arenium (sigma) complexes in electrophilic aromatic substitution. In catalysis, metal–alkyl or other transient metal-bound species frequently serve as intermediats within catalytic cycles.

Distinction from related concepts

Intermediats are distinct from transition states, which are high-energy points along a reaction coordinate that cannot be isolated. They are also different from stable products or long-lived catalysts, though some intermediats can be stabilized by resonance or specific substituents.

Significance

Understanding intermediats provides insight into reaction rates, selectivity, and possible routes to optimize or redesign synthetic pathways and catalytic systems.