Selectivity describes the ability of a chemical reaction to preferentially select a particular Product (value product) while minimizing the formation of undesirable by-products. A distinction is made Nebenproducte of coupling products that may be desired in certain cases. Selectivity is often expressed as a percentage and represents the proportion of the desired product in relation to the total of all products formed. High selectivity is particularly important in the chemical industry, as it enables more efficient use of raw materials, lower waste quantities and thus lower production costs.
An example of the formation of by-products is the production of chlorobenzene, which is obtained catalytically by the reaction of benzene with chlorine. In a subsequent reaction, dichlorobenzenes (eg 1,2-dichlorobenzene) can be formed, which are undesirable in many applications.
Factors that influence selectivity include the reaction conditions used in the process, such as temperature, pressure, solvent and reaction time. For example, increasing the temperature to reduce the reaction time can promote the formation of by-products. Furthermore, catalysts can increase selectivity in a variety of ways. For example, the choice of pore size in the catalyst pellet can influence the reaction of certain reactants (reactant selectivity) or the formation of certain products (product selectivity), due to a selection regarding the size of the molecules. In addition, catalysts can promote the formation of a product by forming molecular interactions with the reactants stereoselective control.
For example, ethanol can be dehydrated to diethyl ether using a copper catalyst at 250–350 °C, while a nickel catalyst splits ethanol at 400–450 °C into methane and synthesis gas. The choice of suitable catalysts and reaction conditions is therefore crucial for industrial processes where the formation of by-products is to be completely suppressed.
In summary, selectivity is a key concept that enables the targeted control of chemical processes and makes a decisive contribution to the development of efficient and environmentally friendly processes.
