Substitution reaction

In chemistry, the substitution referred to ( late latin substituere: replace ) a chemical reaction, are replaced with the atoms or groups of atoms in a molecule by another atom or another functional group (or substituent).

• 4.1 Duolithic - additionelle theory 4.1.1 Radical theory

Reaction types

In principle, substitution reactions are distinguished by the attacking group ( " phil " means inclined ):

• Electrophilic reagents Search For Jobs in molecules with an excess of electrons ( = sites with negative charges / partial charges or lone pairs ). → SE reaction.
• Nucleophilic reagents Search For Jobs in molecules with an electron deficit ( = points with positive charges / partial charges or electron holes ). → SN reaction.
• Reagents with an unpaired electron ( radical ) have a high energy excess. They respond very selective in the rule. → SR reaction.

One thus speaks of electrophilic, nucleophilic or radical substitutions.

Assaulted molecules can be:

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

Molecules of these two categories can usually be selectively substituted by the corresponding reagent. Less selective is the attack on radicals.

Strategy

The attacking group to be replaced group tries to oust from their connection. This can be done by the fact that the attacking group approaches the binding partners of the group to be replaced gradually ( and the new bond is formed ), while simultaneously the binding of the displaced group is being undermined.

In this type of reaction, two molecules are always simultaneously involved.

As step two molecules are involved in the crucial ( rate-limiting ), it is called a bimolecular substitution, S2 abbreviated.

Alternatively, it is conceivable that the leaving group, possibly assisted by an auxiliary reagent, leaving the molecular structure and the molecule leaves a void that is quickly filled by the attacking group. The rate-determining step here is the decay of the parent compound:

In this only one molecule is involved, it is called a monomolecular reaction, abbreviated S1.

Nucleophilic substitution

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

In the case of R = aryl, there are the following possibilities

• Aromatic electron- through ( M ) substituents; SNAr ( nucleophilic aromatic substitution).
• Aromatic not electron- through ( M ) substituents; Arinmechanismus.

In the case of R = alkyl following options are available

• Monomolecular substitution; SN1
• Bimolecular substitution; SN2
• Intramolecular substitution; SNi

See nucleophilic substitution.

In the case of R = R ' CO with R ' = aryl, alkyl is a substitution of a carbonyl compound before. The transesterification was exemplified.

Electrophilic Substitution

The electrophilic substitution is typical of aromatic compounds, particularly electron-rich aromatic compounds, abbreviated as SEAr

• Electrophilic Aromatic Substitution

Radical Substitution

The free-radical substitution is less suitable, as they are mostly little proceeds selectively, abbreviated SR for the laboratory scale.

Competing reaction

When substitutions often occurs on the elimination as a competing reaction.

History of the substitution reaction

At the beginning of 19.ten century, various hypotheses and theories formulated on the structure and formation of organic bodies.

Survey

Start competing until mid 19th century, two main theories to explain the chemical compound in organic chemistry, it was the dualistic theory additionelle to Jöns Jakob Berzelius and as radical or core theory of Justus von Liebig, Friedrich Wöhler and Auguste Laurent derived therefrom concepts on the one hand, and designated by Jean Baptiste Dumas and the discovery of the substitution type theory as a Unitary Approach to the other side.

Dualistic - additionelle theory

Jöns Jakob Berzelius used in 1808 for the first time the term organic chemistry and published in 1827 the first independent treatise on Organic Compounds. It was important chemical consideration in the proportions of the compounds and the development of the stoichiometry. Thus Berzelius introduced the task, the conditions under which combine the organic body to determine most accurate. He set about firmly into the salts that the quantity of oxygen to each other the acid and the base in a simple integer ratio, as it had already formulated his law of definite proportions in 1807 Joseph -Louis Proust. For Berzelius the results of its stoichiometric analyzes were also an acknowledgment of the atomic theory of John Dalton from the year 1803. He coined the terms electronegative and electropositive and founded a electronegativity. The affinity of the chemical body Berzelius explained by the intensity of their polarity, in the union of two elements, the atoms deposited with their opposite poles together. Otherwise it could the original polarity or their atoms disassemble a by the electric current to connect again.

With his hypothesis, important questions of chemistry of his time could be explained. Basis of this system was the assumption that each chemical body composed of two parts, an electropositive and an electronegative part. Berzelius built on the supported by electrochemical experiments to Humphry Davy dualistic additionelle theory that any chemical compound is to be regarded as a union of an acid and an electronegative electropositive base.

Radical theory

In the investigation of many compounds, it turned out that this very often consist of only three or four elements, namely carbon, hydrogen, oxygen, nitrogen and rare. The empirical formula of the chemical body could be determined, however, many substances were known, the same composition ( molecular formula ) had completely different properties, so were other substances. Thus, the chemical properties also hung on the link between the elements, the structure of a compound with the previous methods could not be determined, however. Furthermore, it was assumed that in a chemical reaction, all substances involved would be dismantled into their components and newly added together. Liebig and Wohler therefore developed a model in which they assumed that a substance of several smaller groups of elements, the so-called radicals, would, would have been assumed unchanged in the reaction.

The term radical individual section the free radical theory is incompatible with the modern concept of the radical (such as above 1.4 Radical substitution used ) in the current theoretical chemistry concepts equated. In the radical theory statements about the composition of a chemical the body are made ​​of these consists of several small groups of elements, just the radicals. The free radical theory was based on the stability of the radicals, iS small groups of elements, from.

But this contradicted the fact that the empirical formula of the substances in a reaction often changed only slightly and the findings from the already advanced inorganic chemistry could therefore not be applied.

Unitary theory

In an organic compound by Berzelius said electronegative particle of the carbon and the hydrogen was electropositive particle. After his presentation, it was impossible that an electropositive particles such as hydrogen could be displaced by an electronegative particles such as chlorine. Berzelius represented on the basis of his dualistic views of the opinion that could be replaced by another, third electronegative chlorine to chloroform about in a compound such as methylene chloride, one of the electropositive hydrogen atoms.

Types and nuclear theory

Jean -Baptiste Dumas established in 1834 the theory of substitution ( substitution of hydrogen by organic compounds such as halogens ) and introduced in 1839 in Paris, a type theory of the chemical bond. The following anecdote is portrayed on this: In the first third of the 19 th century, King Charles X ( France) ( 1757-1836 ) gave an evening reception, soirée de gala. The castle burned many white candles: But this candle wax burned down under very strong soot. In addition, there were strong irritation of the respiratory tract with sometimes extreme coughing at the gala guests. The king instructed the experts of the court for chemical Affairs, the Director of the Royal Porcelain Manufactory, Alexandre Brongniart, the clarification of the causes of this incident. Brongniart, in turn, forwarded the investigation to his son, Jean -Baptiste Dumas. This came to the following conclusion: The suffocating acting vapors were released hydrogen chloride gas ( HCl) which evolved during the combustion of candle wax and inhalation irritation to the mucous membranes and respiratory tract led. He was able to demonstrate, moreover, that the wax manufacturer had bleached the raw material with chlorine. Chlorine was partially chemically bonded to the wax, which was released during the combustion of hydrogen chloride. In addition to the practical benefits for more candle industry, the analysis of the incident led to a new theoretical understanding within the organic chemistry. However, Jean -Baptiste Dumas dismissed the substitution of hydrogen by chlorine by the action of chlorine on candle wax and after Disagreed so berzeliusschen the view that the electropositive hydrogen can not be replaced by the electronegative chlorine.

A modified type theory developed Charles Frédéric Gerhardt. After his reflections could all organic compounds of four types: derived hydrogen, hydrogen chloride, water, and ammonia. From the typology of the concept of chemical valence was concluded. The already hinted of predecessors theory of homologous series took with him solid shape (1845 ); also comes the notion of heterology from him. The most important implication of the theory of types and of the homologues was that they unitares a system - as opposed to Berzelius ' dualistic - demand.

Auguste Laurent formulated in 1836, the core theory according to which it called stem nuclei ( or in other words radicals IOS), such as carbon and hydrogen, exist, and emerged derived cores from the master cores by replacement (substitution) of hydrogen by other atoms or groups. For organic molecules, there are different than in inorganic chemistry stem nuclei of carbon atoms. The carbon stem nuclei are arranged in space and can have side - nuclei of hydrogen, chlorine or oxygen atoms take on geometrical and stoichiometric laws. Only a select few accessory nuclei in the molecule can secede and form a radical.

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