What are substitution reactions

Types of reactions in organic chemistry

Depending on the topological changes In organic chemistry, one differentiates between different reactions Reaction types.

Substitution reactions (S)


In the case of a substitution reaction (exchange reaction), a fragment (substituent) Y is exchanged for another fragment X. The incoming reactive particle X can be a nucleophile, an electrophile or a radical. The leaving particle Y is called the leaving group. The reverse of a substitution reaction is in turn a substitution reaction, i.e., back and forth reactions are topologically equivalent.

Nucleophilic substitution:

With aliphatic substrates are nucleophilic Substitutions most important. The attacking nucleophile X must necessarily have a free electron pair feature; it can be negatively charged (e.g. iodide I.- or hydroxide OH-), but it can also be uncharged (e.g. ammonia NH3 or water H2O).

In the case of a substitution reaction, one bond is broken (substrate-Y) and another is tied (substrate-X), whereby there are various possibilities for the chronological sequence. In the case of a monomolecular nucleophilic substitution reaction (pN1) the bond to the leaving group Y is first loosened, whereby a carbenium ion is formed as an intermediate, which then binds to the nucleophile X. In a bimolecular nucleophilic substitution reaction (pN2) bond cleavage and bond formation take place concerted, so in a sense simultaneously (synchronously); this reaction proceeds single stage via a trigonal-bipyramidal transition state. OneN2-reaction can be thought of as folding an umbrella; an inversion of the configuration occurs on asymmetrical substrates (Waldensche reversal).

Addition reactions (A)


In an addition reaction, a particle (X-Y, sometimes just a two-bonded atom such as O) is added to a substrate, i.e. "added". The substrate usually has one Multiple binding (i.e. a double or triple bond, e.g. C = C or C = O); During the reaction, a π bond is broken in the substrate, but two σ bonds are formed (to X and to Y). (In principle, another bond cleavage takes place in the attacking particle X-Y.) The reverse of an addition reaction is an elimination reaction (see below).

Particularly noteworthy special addition reactions are the Hydrogenation (Addition of hydrogen H2) and the Hydration (Addition of water H2O). Depending on the nature of the attacking particle (in the decisive step), a distinction is made between electrophilic, nucleophilic and radical addition reactions. Addition reactions also take place in certain ring-opening reactions, especially on three-membered rings.

Elimination Reactions (E)


In an elimination reaction, a particle (X-Y, sometimes just a double-bonded atom such as O) is split off from a substrate, i.e. "eliminated". As a rule, a double bond is formed in the substrate (or a triple bond if the elimination takes place on a double bond). The elimination reaction is the reverse of the addition reaction.

Particularly noteworthy special elimination reactions are the Dehydration (Elimination of hydrogen H2) and the Dehydration (Splitting off of water H.2O). In certain elimination reactions, rings can form instead of multiple bonds (e.g. three-membered rings in γ-eliminations); with α-eliminations (e.g.) carbenes (CR2).

Condensation reactions


In a condensation reaction, two fragments (R.1 and R2) linked together, whereby a small molecule (e.g. water H2O or an alcohol R-OH) is split off. In the case of a condensation reaction, an addition takes place first, which is followed by an elimination. The reversal of the condensation reaction is solvolysis, the most important special case of which is hydrolysis is (i.e. the implementation with water).


A typical example of a condensation reaction is (formally) the formation of a dipeptide from two amino acid molecules. Further examples are the (acid-catalyzed) esterification and the ester condensation (Claisen condensation).

Rearrangement reactions (R)


In a rearrangement reaction, a fragment R migrates with a rearrangement of the binding structure, i.e. a change in connectivity. So it finds one Isomerization instead of. A Tautomerization (e.g. the keto-enol tautomerization) is one reversible Rearrangement reaction in which a hydrogen atom (or proton) usually migrates.


The Cope rearrangement is a fine example of a rearrangement reaction. she is a pericyclic reaction, in which three π bonds, so to speak, fold in a concerted cyclic manner (with a six-membered ring as a transition state).

Redox reactions (oxidation and reduction)


In a redox reaction - at least formally - electrons are transferred: in one oxidation electrons are given off at one reduction electrons are absorbed. Oxidation and reduction are always coupled with one another; one therefore always refers to the substrate of interest. The required oxidizing or reducing agent can be an inorganic reagent, but also an organic one. Whether the substrate is oxidized or reduced is determined by the change in the Oxidation numbers at the reaction center. (Oxidation = increase in the oxidation number, reduction = decrease.) Redox reactions fall out of the systematic classification of the reaction types insofar as they are not always "orthogonal" to it. This means that redox processes are often side effects of reactions classified elsewhere; for example, a hydrogenation is also a reduction and conversely a dehydrogenation is an oxidation.


A typical example of a (also biologically relevant) reversible redox reaction can be found in the quinone / hydroquinone system. It should be noted that in this system the redox potential is pH-dependent (Nernst equation).