Chemical intermediates are the chemical substances produced during the conversion of some reactant to a product. Most synthetic processes involve transformation of some readily available and often inexpensive substance to some desired product through a succession of steps. All the substances generated by one step and used for the succeeding step are considered intermediates. Apart from substances that can be recovered as products if the reaction is stopped at the point of generation of the intermediate, unstable molecules, some chemical substances are either known or hypothesized to be intermediate, even if they have not yet been isolated. Among the classes of generally unstable intermediates that are well studied are free radicals, carbenes, carbonium ions, and carbanions. These intermediates are highly reactive fragments of molecules that ordinarily remain uncombined for only very short periods of time.
Chemical intermediates are often times used as building blocks in drug discovery. Building blocks refers to organic compounds that are mainly used in the field of new drug discovery and development, as well as in the construction of other organic molecular structures, metal organic frameworks, and supramolecular composites. High quality building blocks are critical not only to the biological activities of different molecules but also to ADMET profiles, which significantly impacts the success rate of drug discovery projects.