Substitution reaction

In chemistry , substitution (late Latin : substitute : replace) describes a chemical reaction in which atoms or groups of atoms in a molecule are replaced by another atom or group of atoms.

Reaction types

Basically, a distinction is made between substitution reactions according to the attacking group (" phil " means inclined):

• Electrophilic reagents look for locations in molecules with an excess of electrons (= locations with negative charges / partial charges or free electron pairs ). → S _E reaction.
• Nucleophilic reagents look for locations in molecules with an electron deficit (= locations with positive charges / partial charges or electron gaps ). → S _N reaction.
• Reagents with an unpaired electron ( radicals ) have a large excess of energy. As a rule, they do not react very selectively. → S _R reaction.

One speaks of electrophilic , nucleophilic or radical substitutions .

Affected molecules can be:

• Molecules with an excess of electrons ( alkenes , alkynes , appropriately substituted aromatics , anions )
• Molecules with an electron deficit (correspondingly substituted alkanes , aromatics, carbonyl compounds , cations )

Molecules of these two categories can usually be selectively substituted with the appropriate reagent. The attack against radicals is less selective.

strategy

The attacking group tries to force the group to be replaced out of its connection. This can be done by the attacking group gradually approaching the binding partner of the group to be replaced (and forming the new bond), while at the same time the bond of the displaced group is increasingly weakened.

In this type of reaction, two molecules are always involved at the same time.

${\displaystyle \mathrm {angreifende\ {Gruppe}+R{-}Abgangsgruppe\rightleftharpoons angreifende\ {Gruppe{-}R}+Abgangsgruppe} }$

Since two molecules are involved in the decisive (rate-determining) step, one speaks of a bimolecular substitution, or S _{2 for} short .

Alternatively, it is conceivable that the leaving group , possibly supported by an auxiliary reagent, leaves the molecular assembly and leaves a gap in the molecule that is quickly filled by the attacking group. The rate-determining step here is the disintegration of the starting compound:

${\displaystyle \mathrm {R{-}Abgangsgruppe\rightleftharpoons R+Abgangsgruppe} }$

Only one molecule is involved in this; one speaks of a monomolecular reaction, or S _{1 for} short .

Nucleophilic substitution

The nucleophilic substitution can occur on compounds of the type RX, with R = aryl, alkyl. The grouping X is replaced.

In the case of R = aryl there are the following options

• Aromatic electron-poor due to (–M) -substituents; S _N Ar ( nucleophilic aromatic substitution ).
• Aromatic not electron-poor due to (–M) -substituents; Arin mechanism .

In the case of R = alkyl there are the following options

• monomolecular substitution; S _N 1
• bimolecular substitution; S _N 2
• intra-molecular substitution; S _N i

See nucleophilic substitution .

In the case of R = R'-CO with R '= aryl, alkyl, there is a substitution on a carbonyl compound. The transesterification is mentioned as an example.

Electrophilic substitution

Electrophilic substitution is typical for aromatics, especially electron-rich aromatics, abbreviated as S _E Ar

• Electrophilic aromatic substitution

Radical substitution

The radical substitution is less suitable, because they usually little proceeds selectively for the laboratory scale, abbreviated to S _R .

Competitive reaction

In the case of substitutions, the elimination often occurs as a competitive reaction .

History of the substitution reaction

At the beginning of the 19th century, various hypotheses and theories were formulated about the structure and formation of organic bodies .

Overview

From the beginning to the middle of the 19th century, there were essentially two competing theories explaining the chemical compound in organic chemistry : the dualistic-additional theory around Jöns Jakob Berzelius and the radical theory or core theory of Justus von Liebig , Friedrich Wöhler and Auguste Laurent , on the one hand, and the conception, which Jean Baptiste Dumas and the discovery of substitution referred to as a unitarist type theory , on the other.

Dualistic-addition theory

Jöns Jakob Berzelius first used the term organic chemistry in 1808 and published the first independent treatise on organic compounds in 1827. The consideration of chemical proportions in the compounds and the development of stoichiometry were important to him . So Berzelius set himself the task of precisely determining the relationships according to which the organic bodies connect. In the salts, for example, he found that the quantity of oxygen in the acid and the base are in a simple integer ratio to one another, as Joseph-Louis Proust had already formulated in his law of constant proportions in 1807. For Berzelius, the results of his stoichiometric analyzes were at the same time a confirmation of the atomic hypothesis of John Dalton from 1803. He coined the terms electronegative and electropositive and established an electronegativity scale . The affinity of the chemical body Berzelius explained by the intensity of their polarity , in the union of two elements, the atoms laid with their opposite poles together. In another way, the electrical current can break a connection back to its original polarity or its atoms.

With his hypothesis , important questions of the chemistry of his time could be explained. This system was based on the assumption that every chemical body is composed of two parts, an electropositive and an electronegative part. Berzelius built up the dualistic-addition theory supported by electrochemical experiments around Humphry Davy , according to which every chemical compound is to be understood as the union of an electronegative acid and an electropositive base .

Radical theory

When examining many compounds, it turned out that they very often only consist of three or four elements, namely carbon, hydrogen, oxygen and, less often, nitrogen. The molecular formula of the chemical bodies could be determined, but very many substances were known that had completely different properties with the same composition (molecular formula), i.e. were different substances. The chemical properties also depended on the connection between the elements, but the structure of a compound could not yet be determined with the earlier methods. It was also assumed that in a chemical reaction, all substances involved would be broken down into their components and reassembled. Liebig and Wöhler therefore developed a model in which they assumed that a substance would consist of several smaller groups of elements, the so-called radicals, which would be adopted unchanged in a reaction.

Compounds are made up of radicals that can exchange each other.

The term radical i. The radical theory is not to be equated with the modern concept of the radical (e.g. as used above under 1.4 radical substitution ) in the current theoretical chemistry concepts. In the radical theory, statements are made about the composition of a chemical body that consists of several small groups of elements, namely the radicals. The radical theory was based on the immutability of radicals, in the sense of small groups of elements.

However, this contradicted the fact that the molecular formula of the substances often only changes slightly during a reaction and the findings from the inorganic chemistry, which has already been further developed, could therefore not be applied.

Unitarian theory

According to Berzelius, carbon was the electronegative particle and hydrogen was the electropositive particle in an organic compound . He imagined that it was impossible for an electropositive particle like hydrogen to be displaced by an electronegative particle like chlorine. On the basis of his dualistic views, Berzelius took the view that one of the electropositive hydrogen atoms in a compound such as methylene chloride could not be replaced by another, third electronegative chlorine to form chloroform .

Type and core theory

Jean-Baptiste Dumas founded the substitution theory (substitutability of the hydrogen of organic compounds by e.g. halogens ) in 1834 and set up a type theory of chemical bonds in Paris in 1839 . The following anecdote is described: In the first third of the 19th century, King Charles X (France) (1757–1836) gave an evening reception, soirée de gala . A lot of white candles were burning in the castle: But this candle wax burned down with a lot of soot. In addition, there was severe irritation of the respiratory tract with sometimes extreme coughing attacks in the Gala guests. The king instructed the court's chemical affairs expert, the director of the Royal Porcelain Manufactory Alexandre Brongniart , to investigate the causes of this incident. Brongniart, in turn, passed the investigation on to his son-in-law, Jean-Baptiste Dumas. This came to the following conclusion: The asphyxiating vapors were released hydrogen chloride gas (HCl) that developed when the candle wax was burned and irritated the mucous membranes and the respiratory tract when inhaled. He was also able to prove that the wax manufacturer had bleached the raw material with chlorine . Some of the chlorine was chemically bound to the wax, which released hydrogen chloride when it burned. In addition to the practical use for further candle production, the analysis of the incident led to a new theoretical understanding within organic chemistry. However, Jean-Baptiste Dumas demonstrated the substitution of hydrogen by chlorine in the action of chlorine on candle wax and thus contradicted Berzelius' view that the electropositive hydrogen could not be replaced by the electronegative chlorine.

Charles Frédéric Gerhardt developed a modified type theory . According to his considerations, all organic compounds could be derived from four types: hydrogen, hydrogen chloride, water and ammonia. The concept of chemical valency was derived from the theory of types . The theory of homologous series already suggested by his predecessors took on firm form with him (1845); the concept of heterology also comes from him. The most important conclusion from the type theory and from the homologues was that they demand a unitary system - in contrast to Berzelius' dualistic system.

In 1836 Auguste Laurent formulated the core theory , according to which there are so-called parent nuclei (or, in other words, radicals ioS), such as carbon and hydrogen, and derived nuclei that emerged from the parent nuclei by replacing (substituting) the hydrogen with other atoms or groups. In organic molecules, unlike in inorganic chemistry, there are stem nuclei of carbon atoms. The carbon parent nuclei are spatially ordered and can accommodate secondary nuclei of hydrogen, chlorine or oxygen atoms according to geometrical and stoichiometric laws. Only a few specific secondary nuclei in the molecule can split off and form a radical.

Laurent suspected that stem nuclei could be located in the center of a pyramid , on the edges of the pyramid there are secondary nuclei such as hydrogen, oxygen and halogen atoms which can be exchanged under certain reaction conditions. This is the core theory (chemistry) .

literature

• Peter Sykes: Reaction Mechanisms in Organic Chemistry. 7th edition. Verlag Chemie, 1979, ISBN 3-527-21047-4 .
• Author collective: Organikum. 22nd edition. Wiley-VCH, 2004, ISBN 3-527-31148-3 .
• Ralf M. Cahn: Historical and philosophical aspects of the periodic table of the chemical elements. (PDF; 574 kB). Hyle 2002, pp. 76-82.

Web links

• Joachim Schummer: Lecture in the summer semester 2000: Introduction to the history and philosophy of chemistry, University of Karlsruhe

Individual evidence

1. ↑ Heinz Kaufmann, Alfons Hädener: Fundamentals of organic chemistry. 10th edition. Birkhäuser Verlag, 1996, ISBN 3-7643-5232-9 .
2. ↑ Entry on substitution reaction . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.S06078 .
3. ↑ William. H. Brock: The Norton History of Chemistry. Norton, New York 1992, pp. 210-269.
4. ^ Wilhelm Strube: History of chemistry. Volume 2, Klett, Düsseldorf 2007, pp. 27-34.
5. ^ Wilhelm Strube: Jöns Jacob Berzelius. (PDF; 15 kB). Chemistry didactics University of Oldenburg.
6. ^ Horst Remane: Friedrich Wöhler. (PDF; 221 kB). Chemistry didactics University of Oldenburg.
7. ↑ Hjelt Edvard: History of organic chemistry from the earliest times to the present. (PDF; 4.4 MB). Vieweg & Son, Braunschweig 1916.
8. ↑ Horst Remane: Jean-Baptiste André Dumas. (PDF; 253 kB). Chemistry didactics University of Oldenburg.
9. ↑ August Kekulé among others: Textbook of organic chemistry. Volume 1, p. 66 ff. (Google Books) .