In chemistry, chemical synthesis is a purposeful execution of chemical reactions to obtain a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions. In modern laboratory usage, this tends to imply that the process is reproducible, reliable, and established to work in multiple laboratories. A chemical synthesis begins by selection of compounds that are known as reagents or reactants. Various reaction types can be applied to these to synthesize the product, or an intermediate product. This requires mixing the compounds in a reaction vessel such as a chemical reactor or a simple round-bottom flask. Many reactions require some form of work-up procedure before the final product is isolated. Synthesis can start with very simple compounds (such as various petroleum distillates) and work toward a complex one (total synthesis), or it can start with natural products from plants in which various portions of the desired end-product molecules (such as particular functional groups) are already conveniently assembled (semisynthesis). (Semisynthesis is often more cost-effective than total synthesis for industrial chemical products.) The amount of product in a chemical synthesis is the reaction yield. Typically, chemical yields are expressed as a weight in grams or as a percentage of the total theoretical quantity of product that could be produced. A side reaction is an unwanted chemical reaction taking place that diminishes the yield of the desired product. The word synthesis in the present day meaning was first used by the chemist Hermann Kolbe.