intermediates

Intermediates are chemical species that appear temporarily during a reaction or metabolic pathway before the final product is formed. They are not reactants or end products in the overall equation, but lie along the sequence of steps that converts starting materials into the end product. Their lifetimes are often short.

In chemical reaction mechanisms, intermediates may be carbocations, carbanions, radicals, or metal-oxo species. They are formed in one step and consumed in a subsequent step. The stability and reactivity of an intermediate influence the rate and outcome of the reaction. Many intermediates are highly reactive and difficult to isolate; their existence is inferred from kinetics, trapping experiments, and spectroscopic observations.

In industrial synthesis, intermediates are compounds produced in early steps that are used as inputs for later steps to reach the final product. Some intermediates are isolated and purified, while others are kept in situ to minimize handling or degradation and to improve overall yield and process efficiency.

Biological systems also feature metabolic intermediates, which link successive enzymatic reactions in pathways such as glycolysis and the citric acid cycle. Examples include glucose-6-phosphate, pyruvate, acetyl-CoA, and oxaloacetate. These intermediates participate in energy production and biosynthesis, and their concentrations are often tightly regulated.

Understanding intermediates supports mechanism elucidation, optimization of chemical yields, and the design of new synthetic routes or drugs. Their transient nature presents experimental challenges, but advances in spectroscopy, rapid-mixing techniques, and computational methods aid identification and characterization.