Selectivity describes the ability of a chemical reaction to preferentially form a specific product (value product) while minimizing the formation of undesired byproducts. A distinction is made between byproducts and coupling products, which may be desirable in certain cases. Selectivity is often expressed as a percentage and represents the proportion of the desired product relative to the total number of products formed. High selectivity is particularly crucial in the chemical industry, as it enables more efficient use of raw materials, reduces waste, and thus lowers production costs.
An example of the formation of byproducts is the production of chlorobenzene, which is obtained catalytically through the reaction of benzene with chlorine. In a subsequent reaction, dichlorobenzenes (e.g., 1,2-dichlorobenzene) can be formed, which are undesirable in many applications.
Factors influencing selectivity include reaction conditions such as temperature, pressure, solvent, and reaction time. For example, increasing the temperature to reduce reaction time can promote the formation of byproducts. Furthermore, catalysts can enhance selectivity in various ways. The choice of pore size in the catalyst pellet can determine the reaction of specific reactants (reactant selectivity) or the formation of specific products (product selectivity) based on molecular size. Additionally, catalysts can stereoselectively control product formation by forming molecular interactions with the reactants.
For example, ethanol can be dehydrated to diethyl ether at 250–350 °C using a copper catalyst, while a nickel catalyst splits ethanol into methane and synthesis gas at 400–450 °C. The choice of suitable catalysts and reaction conditions is therefore crucial for industrial processes where the formation of byproducts must be completely suppressed.
Zusammengefasst ist die Selektivität ein Schlüsselkonzept, das die gezielte Steuerung chemischer Prozesse ermöglicht und entscheidend zur Entwicklung effizienter und umweltfreundlicher Verfahren beiträgt.
