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.