Many substitution reactions open the way to obtaining a variety of compounds that have economic applications. A huge role in chemical science and industry is given to electrophilic and nucleophilic substitution. In organic synthesis, these processes have a number of features that should be noted.
Variety of chemical phenomena. Substitution reactions
Chemical changes associated with the transformations of substances are distinguished by a number of features. The final results, thermal effects may be different; some processes go to the end, in others chemical equilibrium occurs. A change in substances is often accompanied by an increase or decrease in the degree of oxidation. When classifying chemical phenomena according to their end result, attention is paid to the qualitative and quantitative differences between the reactants and the products. According to these features, 7 types of chemical transformations can be distinguished, including substitution, following the scheme: A-B + C A-C + B. A simplified record of a whole class of chemical phenomena gives an idea that among the starting substances there arecalled "attacking" particle, replacing an atom, ion, functional group in the reagent. The substitution reaction is typical for saturated and aromatic hydrocarbons.
Substitution reactions can occur in the form of a double exchange: A-B + C-E A-C + B-E. One of the subspecies is the displacement, for example, of copper with iron from a solution of copper sulfate: CuSO4 + Fe=FeSO4 + Cu. Atoms, ions or functional groups can act as an “attacking” particle
Homolytic substitution (radical, SR)
With the radical mechanism of breaking covalent bonds, an electron pair common to different elements is proportionally distributed among 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 ethane is obtained from methane, free radicals appear that participate in the substitution reaction: CH4 CH3• + •Н; CH3• + •CH3 → С2Н5; H• + •H → H2. Homolytic bond breaking according to the given substitution mechanism is characteristic of alkanes, the reaction is chain. In methane, H atoms can be successively replaced by chlorine. The reaction with bromine proceeds similarly, but iodine is unable to directly replace hydrogen in alkanes, fluorine reacts too vigorously with them.
Heterolytic way to break the bond
With the ionic mechanism of substitution reactionselectrons are unevenly distributed among the newly formed particles. The binding pair of electrons goes completely to one of the "fragments", most often, to that bond partner, towards which the negative density in the polar molecule was shifted. Substitution reactions include the formation of methyl alcohol CH3OH. In bromomethane CH3Br, the cleavage of the molecule is heterolytic, and the charged particles are stable. Methyl acquires a positive charge, and bromine becomes negative: CH3Br → CH3+ + Br -; NaOH → Na+ + OH-; CH3+ + OH- → CH3OH; Na+ + Br- ↔ NaBr.
Electrophiles and nucleophiles
Particles that lack electrons and can accept them are called "electrophiles". These include carbon atoms bonded to halogens in haloalkanes. Nucleophiles have an increased electron density, they "donate" a pair of electrons when creating a covalent bond. In substitution reactions, nucleophiles rich in negative charges are attacked by electron-starved electrophiles. This phenomenon is associated with the displacement of an atom or other particle - the leaving group. Another type of substitution reaction is the attack of an electrophile by a nucleophile. It is sometimes difficult to distinguish between two processes, to attribute substitution to one type or another, since it is difficult to specify exactly which of the molecules is the substrate and which is the reagent. Usually, in such cases,the following factors:
- nature of the leaving group;
- nucleophile reactivity;
- nature of the solvent;
- structure of the alkyl part.
Nucleophilic substitution (SN)
In the process of interaction in an organic molecule, an increase in polarization is observed. In equations, a partial positive or negative charge is marked with a letter of the Greek alphabet. The polarization of the bond makes it possible to judge the nature of its rupture and the further behavior of the "fragments" of the molecule. For example, the carbon atom in iodomethane has a partial positive charge and is an electrophilic center. It attracts that part of the water dipole where oxygen, which has an excess of electrons, is located. When an electrophile interacts with a nucleophilic reagent, methanol is formed: CH3I + H2O → CH3OH +HI. Nucleophilic substitution reactions take place with the participation of a negatively charged ion or a molecule that has a free electron pair that is not involved in the creation of a chemical bond. The active participation of iodomethane in SN2-reactions is explained by its openness to nucleophilic attack and the mobility of iodine.
Substitution electrophilic (SE)
An organic molecule may contain a nucleophilic center, which is characterized by an excess of electron density. It reacts with an electrophilic reagent that lacks negative charges. Such particles include atoms with free orbitals, molecules with areas of low electron density. ATIn sodium formate, carbon with a “–” charge interacts with the positive part of the water dipole - with hydrogen: CH3Na + H2O → CH 4 + NaOH. The product of this electrophilic substitution reaction is methane. In heterolytic reactions, oppositely charged centers of organic molecules interact, which makes them similar to ions in the chemistry of inorganic substances. It should not be overlooked that the transformation of organic compounds is rarely accompanied by the formation of true cations and anions.
Monomolecular and bimolecular reactions
Nucleophilic substitution is monomolecular (SN1). According to this mechanism, the hydrolysis of an important product of organic synthesis, tertiary butyl chloride, proceeds. The first stage is slow, it is associated with gradual dissociation into carbonium cation and chloride anion. The second stage is faster, the carbonium ion reacts with water. The equation for the reaction of replacing a halogen in an alkane with an hydroxy group and obtaining a primary alcohol: (CH3)3C-Cl → (CH3)3C+ + Cl-; (CH3)3C+ + H2O → (CH3)3C-OH + H+. The single-stage 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).
Heterolytic substitution mechanism
The substitution mechanism is associated with the transfer of an electron, the creationintermediate complexes. The reaction proceeds the faster, the easier it is to form the intermediate products characteristic of it. Often the process goes in several directions at the same time. The advantage is usually obtained by the way in which the particles that require the least energy costs for their formation are used. For example, the presence of a double bond increases the probability of the appearance of an allyl cation CH2=CH-CH2+, compared to the ion CH3 +. The reason lies in the electron density of the multiple bond, which affects the delocalization of the positive charge dispersed throughout the molecule.
Benzene substitution reactions
Group of organic compounds, which are characterized by electrophilic substitution - arenas. The benzene ring is a convenient target for electrophilic attack. The process begins with the polarization of the bond in the second reactant, resulting in the formation of an electrophile adjacent to the electron cloud of the benzene ring. The result is a transitional complex. There is still no full-fledged connection of an electrophilic particle with one of the carbon atoms, it is attracted to the entire negative charge of the “aromatic six” of electrons. At the third stage of the process, the electrophile and one carbon atom of the ring are connected by a common pair of electrons (covalent bond). But in this case, the “aromatic six” is destroyed, which is unfavorable from the point of view of achieving a stable sustainable energy state. There is a phenomenon that can be called "proton ejection". H+ is split off, stablea communication system specific to arenas. The by-product contains a hydrogen cation from the benzene ring and an anion from the composition of 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 given for cycloalkanes and arenes. Similar reactions in the molecules of organic substances occur under normal conditions, but more often - when heated and in the presence of catalysts. Electrophilic substitution in the aromatic nucleus is one of the widespread and well-studied processes. The most important reactions of this type are:
- Nitration of benzene with nitric acid in the presence of H2SO4 - goes according to the scheme: C6 H6 → C6H5-NO2.
- Catalytic halogenation of benzene, in particular chlorination, according to the equation: C6H6 + Cl2→ C6H5Cl + HCl.
- Aromatic sulfonation of benzene proceeds with "fuming" sulfuric acid, benzene sulfonic acids are formed.
- Alkylation is the replacement of a hydrogen atom from the benzene ring with an alkyl.
- Acylation - the formation of ketones.
- Formylation - replacement of hydrogen with a CHO group and the formation of aldehydes.
Substitution reactions include reactions in alkanes and cycloalkanes, in which halogens attack the available C-H bond. The preparation of derivatives may be associated with the substitution of one, two or all hydrogen atoms in saturated hydrocarbons andcycloparaffins. Many of the low molecular weight haloalkanes are used in the production of more complex substances belonging to different classes. The progress achieved in the study of the mechanisms of substitution reactions gave a powerful impetus to the development of syntheses based on alkanes, cycloparaffins, arenes and halogen derivatives of hydrocarbons.
