The mass action law describes the ratio of the activities (or simplified, the concentrations) of the substances involved in a chemical reaction in the state of equilibrium. This ratio is constant in the state of equilibrium.

The state of chemical equilibrium can be thought of as a situation in which the rates of the outward and backward reactions are equal in each case, without the chemical reaction having come to a “standstill”.

The law of mass action applies to all reversible reactions that are in equilibrium. The derived equilibrium constant K has a fixed value for the reaction under consideration, but it can be influenced by external influences such as temperature or (for gas reactions) pressure.

With the possibility of influencing the position of a chemical equilibrium and calculating the concentration of one reaction partner (e.g. a reaction product) from those of the others, the law of mass action assumes fundamental importance in the quantitative consideration of chemical reactions and in the optimization of yields in industrial processes.

The reactions of organic substances with each other can be classified according to different criteria: according to the reaction type, according to the type of reactive intermediates, according to the structure of the substrates or according to synthesis tasks of the reaction. In most cases, however, categorization is either by reaction type (substitution, addition, elimination) or by substrates (aromatic reactions, carbonyl reactions).

Substitution reactions are those in which one molecular fragment is exchanged for another. The breaking of the old bond and the formation of the new one can occur either synchronously or sequentially. Hybridization does not change in such reactions.

Addition reactions are those in which two molecular fragments are added to the substrate. Here, in contrast to substitution, new bonds are formed. Addition reactions usually occur with alkenes or alkynes and the hybridization of the carbon atoms involved in the substrate changes.

Elimination reactions can be considered as “reversals” of additions. Two molecular fragments are removed in the process.

In rearrangement reactions, unlike all other reaction types, neither molecule fragments are added nor removed. Only a change of the substituents in comparison to the substrate takes place.

Heterocycles are cyclic chemical compounds with ring-forming atoms of at least two different chemical elements. In most cases, one of these chemical elements is carbon. Other common elements contained in heterocycles are nitrogen, sulfur, or oxygen. Heterocycles can have aromatic character, in which case they are also referred to as heteroaromatics.

Due to the diversity of heterocyclic compounds and their structural diversity, they can be divided into different subclasses. For the three- to seven-membered ring systems, subgroups with their own basic bodies are distinguished in each case.

Heterocyclic compounds are of great importance for many basic building blocks of bioorganic and bioinorganic chemistry (DNA, cytochromes, cobalamins, hemoglobin, proteinogenic amino acids such as histidine or proline, etc.). In particular, nitrogen-containing compounds play an important role.

In preparative chemistry, heterocyclic substances often play an important role as solvents or auxiliary bases for initiating organic or inorganic reactions.