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How can isotope labeling reveal where an atom goes during a chemical reaction?

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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).

Isotopic labeling is a powerful technique used to track the movement of specific atoms during a chemical reaction. Because isotopes behave chemically like the ordinary atoms they replace, they can be incorporated into a reactant molecule to serve as a "marker" without significantly altering the reaction's course.1

By using compounds containing these isotopes, chemists can determine the origin of atoms in the final products. For example, in the hydrolysis of ethyl acetate, researchers can label water with oxygen-18. By analyzing the resulting ethyl alcohol, they can establish whether the oxygen atom in the product originated from the water or the ethyl acetate.1

This method is particularly valuable for:
* Identifying reaction pathways: It reveals which specific atoms are involved in a reaction and whether a process is intramolecular or intermolecular, such as in rearrangement reactions.2
* Studying exchange: It can demonstrate that specific atoms, such as hydrogen, are exchanging between reacting species.2
* Kinetic analysis: When combined with mass spectrometry, labeling allows for the identification of intermediate species and the resolution of detailed reaction kinetics.23