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Why does finding a temporary substance in a reaction mixture not always prove that it is an intermediate?

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dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

Finding a temporary substance in a reaction mixture does not always prove it is an intermediate because the substance may simply be a temporary repository for a portion of the reactants. In such cases, the material does not progress toward the final product; instead, it reverts to the starting material.1

An intermediate is defined as a chemical substance produced during the conversion of reactants to products, representing a distinct stage in the reaction mechanism. While identifying transient species is crucial for understanding reaction pathways, the presence of a substance does not automatically confirm its role as a necessary step in the transformation. Distinguishing between true intermediates and temporary side-products requires careful analysis, often utilizing sensitive instrumental techniques like mass spectrometry or magnetic resonance to observe the reaction's progression.21