Replacement reactions: description, equation, examples

Many substitution reactions open the way to obtaining a variety of compounds that have economic application. An enormous role in chemical science and industry is assigned to electrophilic and nucleophilic substitution. In organic synthesis, these processes have a number of features that should be addressed.

Diversity of chemical phenomena. Substitution reactions

Chemical changes associated with the transformation of substances, have a number of features. Different can be the final results, thermal effects; Some processes go to the end, in others there is a chemical equilibrium. The change in substances is often accompanied by an increase or decrease in the degree of oxidation. When classifying chemical phenomena by their final result, attention is drawn to the qualitative and quantitative differences between the reagents and the products. On these grounds, we can distinguish 7 types of chemical transformations, including substitution proceeding according to the scheme: A-B + C A-C + B. Simplified recording of a whole class of chemical phenomena gives an idea that among the initial substances there is a so-called "attacking "A particle that replaces in the reagent an atom, an ion, a functional group. Reaction of substitution is characteristic for limiting and aromatic hydrocarbons.

Substitution reactions can occur as a double exchange: A-B + C-E A-C + B-E. One of the subspecies is the displacement, for example, of copper by iron from a solution of copper sulfate: CuSO ₄ + Fe = FeSO ₄ + Cu. As an "attacking" particle can act as atoms, ions or functional groups

Replacement homolytic (radical, SR)

Under the radical mechanism of breaking covalent bonds, the electron pair, common for different elements, is distributed proportionally between the "fragments" of the molecule. Free radicals are formed. These are unstable particles, the stabilization of which occurs as a result of subsequent transformations. For example, when obtaining ethane from methane, free radicals appear that participate in the substitution reaction: CH ₄ CH ₃ • + • H; CH ₃ • + • CH ₃ → C2H ₅ ; H • + • H → H2. The homolytic bond breaking according to the above replacement mechanism is characteristic of alkanes, the reaction is of a chain nature. In methane, the H atoms can be successively replaced by chlorine. Similarly, the reaction with bromine, but iodine is unable to directly replace hydrogen in alkanes, fluorine reacts too energetically with them.

Heterolytic method of breaking communication

Under the ionic mechanism of the course of substitution reactions, the electrons are distributed unevenly between newly formed particles. The binding pair of electrons goes completely to one of the "fragments", most often, to that communication partner, toward which the negative density in the polar molecule was shifted. Reactions of substitution include the formation of methyl alcohol CH ₃ OH. In bromomethane CH3Br, the disruption of the molecule is heterolytic, charged particles are stable. Methyl acquires a positive charge, and bromine - a negative: CH ₃ Br → CH ₃ ⁺ + Br ^- ; NaOH → Na ⁺ + OH ^- ; CH ₃ ⁺ + OH ^- → CH ₃ OH; Na ⁺ Br + NaBr.

Electrophiles and nucleophiles

Particles that lack electrons and can accept them are called "electrophiles". These include carbon atoms connected to halogens in haloalkanes. Nucleophiles have an increased electron density, they "sacrifice" a pair of electrons when creating a covalent bond. In substitution reactions, nucleophiles rich in negative charges are attacked by electrophiles experiencing a shortage of electrons. This phenomenon is associated with the displacement of an atom or another particle-the outgoing group. Another type of substitution reaction is the electrophile attack by the nucleophile. Sometimes it is difficult to distinguish between two processes, refer the substitution to one or another type, since it is difficult to specify exactly which of the molecules is a substrate and which is a reagent. Usually in such cases the following factors are taken into account:

• The nature of the outgoing group;
• The reactivity of the nucleophile;
• The nature of the solvent;
• Structure of the alkyl part.

Substitution nucleophilic (SN)

In the process of interaction, an increase in polarization is observed in the organic molecule. In equations, a partial positive or negative charge is marked with the letter of the Greek alphabet. The polarization of the bond makes it possible to judge the nature of its discontinuity and the subsequent behavior of the "fragments" of the molecule. For example, the carbon atom in iodomethane has a partial positive charge, is an electrophilic center. It attracts that part of the water dipole, where oxygen is located, which has an excess of electrons. When the electrophile interacts with a nucleophilic reagent, methanol is formed: CH ₃ I + H ₂ O → CH ₃ OH + HI. Reactions of nucleophilic substitution take place with the participation of a negatively charged ion or a molecule possessing a free electron pair that does not participate in chemical bonding. The active participation of iodomethane in SN ₂ reactions is explained by its openness to the nucleophilic attack and the mobility of iodine.

Electrophilic substitution (SE)

An organic molecule can have a nucleophilic center, for which an excess of electron density is characteristic. It reacts with a lack of negative charges with an electrophilic reagent. Such particles include atoms that have free orbitals, molecules with sites of reduced electron density. In sodium formate, carbon having a charge of "-" interacts with the positive part of the water dipole with hydrogen: CH ₃ Na + H ₂ O → CH ₄ + NaOH. The product of this electrophilic substitution reaction is methane. In heterolytic reactions, the oppositely charged centers of organic molecules interact, which gives them similarity to ions in the chemistry of inorganic substances. It should not be overlooked that the conversion of organic compounds is rarely accompanied by the formation of real cations and anions.

Monomolecular and bimolecular reactions

Nucleophilic substitution is monomolecular (SN1). By this mechanism hydrolysis of an important product of organic synthesis - tertiary butyl chloride - takes place. The first stage is slow, it is associated with a gradual dissociation of the carbonium cation and the chloride anion. The second stage proceeds more quickly, the reaction of the carbonium ion with water proceeds. The equation for the replacement of a halogen in an alkane by a hydroxy group and the preparation of a primary alcohol: (CH ₃ ) ₃ C-Cl → (CH ₃ ) ₃ C ⁺ + Cl ^- ; (CH ₃ ) ₃ C ⁺⁺ H ₂ O → (CH ₃ ) ₃ C-OH + H ⁺ . The one-step hydrolysis of primary and secondary alkyl halides is characterized by the simultaneous destruction of the carbon-halogen bond and the formation of a C-OH pair. This is the mechanism of nucleophilic bimolecular substitution (SN2).

The mechanism of heterolytic substitution

The mechanism of substitution is associated with the transfer of an electron, the creation of intermediate complexes. The reaction proceeds the faster, the more easily intermediate products appear that are characteristic of it. Often the process proceeds simultaneously in several directions. Advantage usually gets the way in which particles are used that require the least energy costs for their education. For example, the presence of a double bond increases the probability of the occurrence of an allylic cation CH2 = CH-CH2 ⁺ , in comparison with the ion CH3 ⁺ . The reason lies in the electron density of the multiple bond, which affects the delocalization of the positive charge, distributed throughout the molecule.

Substitution reactions of benzene

A group of organic compounds, which are characterized by electrophilic substitution, are arenas. Benzene ring - a convenient object for electrophilic attack. The process begins with the polarization of the bond in the second reagent, resulting in the formation of an electrophile adjacent to the electronic cloud of the benzene ring. As a result, a transition complex appears. A complete connection of the electrophilic particle with one of the carbon atoms is not yet available, it is attracted to the entire negative charge of the "aromatic six" of the electrons. In the third stage of the process, the electrophile and one carbon ring atom are bound by a common electron pair (covalent bond). But in this case, the "aromatic six" is destroyed, which is unprofitable from the point of view of achieving a stable stable energy state. There is a phenomenon that can be called a "proton emission". There is a splitting off of H ⁺ , a stable communication system, typical of arenas, is restored. The by-product contains a hydrogen cation from the benzene ring and an anion from the second reagent.

Examples of substitution reactions from organic chemistry

For alkanes, the substitution reaction is especially characteristic. Examples of electrophilic and nucleophilic transformations can be cited for cycloalkanes and arenes. Similar reactions in molecules of organic substances occur under ordinary conditions, but more often - with heating and in the presence of catalysts. The widespread and well-studied processes include electrophilic substitution in the aromatic nucleus. The most important reactions of this type are:

1. Nitration of benzene with nitric acid in the presence of H ₂ SO ₄ proceeds according to the scheme: C ₆ H ₆ → C ₆ H ₅ -NO ₂ .
2. Catalytic halogenation of benzene, in particular chlorination, according to the equation: C ₆ H ₆ + Cl ₂ → C ₆ H ₅ Cl + HCl.
3. Aromatic sulfonation of benzene proceeds with "fuming" sulfuric acid, benzenesulfonic acids are formed.
4. Alkylation is the replacement of the hydrogen atom from the composition of the benzene ring by alkyl.
5. Acylation is the formation of ketones.
6. Formation - the replacement of hydrogen by the CHO group and the formation of aldehydes.

Replacement reactions include a reaction in alkanes and cycloalkanes, in which halogens attack the available C-H bond. Derivatization may be due to the substitution of one, two or all hydrogen atoms in the ultimate hydrocarbons and cycloparaffins. Many of the halogenoalkanes with a small molecular weight find application in the production of more complex substances belonging to different classes. The successes achieved in studying the mechanisms of substitution reactions gave a powerful impetus to the development of syntheses based on alkanes, cycloparaffins, arenes and halogenated hydrocarbons.