Many substitution reaction opened the way to get a variety of compounds having economic use.A huge role in chemical science and industry is given electrophilic and nucleophilic substitution.In organic synthesis, these processes have a number of features that need to be addressed.
variety of chemical phenomena.Substitution reactions
chemical changes associated with the transformation of matter, a number of different features.May be different outcomes, thermal effects;Some processes are to the end, in other chemical equilibrium occurs.Change agents are often accompanied by an increase or decrease in the degree of oxidation.In the classification of chemical phenomena in their final results draw attention to the qualitative and quantitative differences between the reactants from products.These features can be identified 7 types of chemical reactions, including substitution, coming under the scheme: A + C + A to C B. Simplified record an entire class of chemical events gives the idea that among the starting materials are so-called "attack"particle reagent replacement atom, ion, a functional group.The substitution reaction is characteristic of saturated and aromatic hydrocarbons.
Substitution reactions may occur in the form of a double exchange: A-B + C to E A C + B-E.One subspecies - the displacement of, for example, copper, iron from a solution of copper sulfate: CuSO4 + Fe = FeSO4 + Cu.As an "attack" particles can act atoms, ions or functional groups
homolytic substitution (radical, SR)
When radical mechanism rupture covalent bonds the electron pair, common to different elements, proportionally distributed between the "fragments" of the molecule.Free radicals are formed.This unstable particle stabilization which occurs as a result of the subsequent transformations.For example, in the preparation of ethane from methane produce free radicals involved in the substitution reaction: CH4 CH3 H • + •;• + • CH3 CH3 → C2H5;H • + • H → H2.Homolytic bond cleavage as described in the replacement mechanism is characteristic of alkanes is a reaction chain character.The methane H atoms can be successively replaced by chlorine.Similarly, a reaction with bromine, iodine but is unable to directly replace hydrogen alkanes, fluorine too vigorously reacts with them.
heterolytically way to break ties
When the ionic mechanism of substitution reactions, electrons are unevenly distributed between the newly emerged particles.The binding pair of electrons moves all the way to one of "fragments", most often, to the communication partner, the side of which was offset negative density in the polar molecule.By substitution reactions include reaction of methyl alcohol CH3OH.In brommetane CH3Br heterolytic rupture of the molecule is the character, the charged particles are stable.Methyl acquires a positive charge, and bromine - negative: CH3Br → CH3 + + Br-;NaOH → Na + + OH-;CH3 + + OH- → CH3OH;Na + + Br- ↔ NaBr.
electrophiles and nucleophiles
particles that lack of electrons and can accept them, are called "electrophiles."These include the carbon atoms coupled to halogens haloalkanes.Nucleophiles have high electron density, they are "victims" of a pair of electrons to create a covalent bond.The substitution reactions rich negative charges are attacked by nucleophiles electrophiles that lack electrons.This phenomenon is associated with the movement of atoms or other particles - the leaving group.Another type of substitution reaction - an attack the electrophile nucleophile.Sometimes it is difficult to distinguish between the two processes, referred to the substitution of one or the other type, because it is difficult to pinpoint which of the molecule - substrate, and which - reagent.Usually in such cases the following factors:
- nature of the leaving group;
- nucleophilic reactivity;
- nature of the solvent;
- structure of the alkyl moiety.
nucleophilic substitution (SN)
During the interaction of the organic molecule has been increasing polarization.The equations of a partial positive or negative charge noted the letter of the Greek alphabet.Polarization communication gives an indication of the nature of its rupture and the future behavior of "fragments" of the molecule.For example, the carbon atom at iodomethane has a partial positive charge is the electrophilic center.It attracts the part of the dipole water, where the oxygen has an excess of electrons.The interaction of the electrophile with a nucleophile reagent formed methanol: CH3I + H2O → CH3OH + HI.Nucleophilic substitution reactions take place with the participation of a negatively charged ion or molecule having a free electron pair, is not participating in the creation of a chemical bond.Active participation of iodomethane in SN2-reactions due to its openness to nucleophilic attack and mobility of iodine.
electrophilic substitution (SE)
in organic molecules may be present nucleophilic center, which is characterized by an excess of electron density.It reacts with the lack of negative charges electrophilic reagent.Such particles are atoms having free orbital molecule with portions of reduced electron density.The sodium formate carbon having charge "-", is reacted with the positive part of the dipole water - hydrogen: CH3Na + H2O → CH4 + NaOH.The product of this reaction, an electrophilic substitution - methane.When heterolytic reaction of oppositely charged centers of organic molecules, which makes them similar to the ions in the chemistry of inorganic substances.Not to be overlooked, that the conversion of organic compounds are rarely accompanied by the formation of these cations and anions.
Monomolecular and bimolecular reactions
is unimolecular nucleophilic substitution (SN1).By this mechanism occurs hydrolysis important product of organic synthesis - tertiary butyl chloride.The first stage is slow, it is associated with progressive dissociation carbonium cation and chloride anion.The second step is faster the reaction proceeds carbonium ion with water.Equation substitution reaction on the halogen in the alkane and produce primary hydroxy alcohol: (CH3) 3C-Cl → (CH3) 3C + + Cl-;(CH3) 3C + + H2O → (CH3) 3C-OH + H +.For a one-step hydrolysis of primary and secondary alkyl halides is characteristic simultaneous destruction of the carbon halogen and pairing C-OH.This mechanism of bimolecular nucleophilic substitution (SN2).
mechanism of heterolytic substitution
replacement mechanism associated with electron transfer, the creation of intermediate complexes.The reaction takes place more quickly than lighter there are characteristic for her intermediate products.Often the process is going in several directions simultaneously.The advantage usually gets the way in which the particles are used, requiring the least expenditure of energy for their education.For example, the presence of a double bond increases the likelihood of an allyl cation CH2 = CH-CH2 +, compared to the CH3 + ions.The reason lies in the electron density of the double bond, which affects the delocalization of the positive charge, dispersed over the entire molecule.
substitution reactions of benzene
group of organic compounds, which are characterized by electrophilic substitution - the arena.The benzene ring - convenient object for electrophilic attack.The process begins with the polarization of the second reagent thereby forming electrophile electron cloud adjacent the benzene ring.The result is a complex transition.Full communication electrophilic particles with one of the carbon atoms not yet, it is attracted to the negative charge around the "aromatic six" electrons.At the third stage of the electrophile and one carbon atom rings share a common pair of electrons (covalent bond).But in this case is the destruction of the "aromatic six", which is disadvantageous in terms of achieving sustainable stable energy state.There is a phenomenon that can be called 'release of a proton. "Cleaves H +, stable communication system is restored, typical scene.Side cation substance comprises hydrogen from the benzene ring and the anion from the second reagent.
Examples of substitution reactions of organic chemistry to
alkanes especially typical substitution reaction.Examples of electrophilic and nucleophilic reactions can lead to cycloalkanes and arenes.Similar reactions in the molecules of organic substances are under normal conditions, but usually - and by heating in the presence of catalysts.By common and well-studied processes include electrophilic substitution in the aromatic ring.The most important reaction of this type:
- nitration of benzene with nitric acid in the presence of H2SO4 - follows the scheme: C6H6 → C6H5-NO2.
- catalytic halogenation of benzene, especially chlorination, by the equation: C6H6 + Cl2 → C6H5Cl + HCl.
- Aromatic sulfonation of benzene occurs with "fuming" sulfuric acid, benzene sulfonic acid form.
- Alkylation - replacement of a hydrogen atom from the benzene ring in the alkyl.
- acylation - the formation of ketones.
- Formylation - replacing a hydrogen on a group of CHO and the formation of aldehydes.
By substitution reactions include the reaction in alkanes and cycloalkanes in which halogens attack available C-H bond.Derivatization may be associated with replacement of one, two or all of the hydrogen atoms in saturated hydrocarbons and naphthenes.Many galogenoalkanov low molecular weight are used in the production of more complex substances belonging to different classes.Advances in the study of reaction mechanisms of substitution, gave a powerful impetus to the development of the synthesis based on alkanes, cycloparaffins, arenas and Halogenated hydrocarbons.
