Reactions of addition of a hydrogen molecule to an organic substance. Addition reactions
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Hydrogen addition reactions are reversible.
The reaction of adding hydrogen to a double or triple carbon-carbon bond is catalyzed by many substances. The beginning of the industrial application of this catalytic process was laid in France by the work of Sabatier, who studied vapor-phase hydrogenation at moderate pressures, and in Russia by Ipatiev, who worked on the hydrogenation of liquids at high pressures.
The reaction of hydrogen addition to unsaturated compounds is called hydrogenation or hydrogenation.
The reactions of hydrogen addition, polymerization and condensation, accompanied by the formation of one molecule of a larger molecular weight from two or more molecules, proceed with the release of heat. The negative thermal effect of decomposition reactions indicates that they are favored high temperatures; the depth of exothermic reactions increases with decreasing temperature. Thus, the more selectively the process proceeds, the higher its total thermal effect, which in this case is not affected by other reactions that proceed in parallel and sometimes have a thermal effect of the opposite sign.
The reaction of hydrogen addition to the double bond of ethylene in the presence of various metal catalysts, of which nickel, platinum, rhodium, and palladium are the most active, is one of the most studied experimentally. Apparently, for this reason, scientists cannot come to a consensus regarding the intimate mechanism of it on the surface of the catalyst. Is simultaneous activation of the hydrogen molecule necessary, and if so, how does this happen? Do all catalyzing surfaces work according to the same principle, or does each of the metals choose the mechanism that suits it best to its liking. As always in such cases, academia is divided into several groups according to the number of alternative mechanisms and a protracted discussion begins.
Hydrogen addition reactions are called hydrogenation reactions.
The hydrogen addition reaction in the presence of metal catalysts in this sense has significant advantages over conventional addition reactions. The metal catalyst is selective with respect to different types molecules and even various parts the same molecule. It sorts them and therefore streamlines the process, causing the reactions that take place simultaneously in the absence of a catalyst to proceed sequentially. And this makes it possible in the study of the escape of hydrogenation curves to have a precious auxiliary method for studying the process of hydrogenation and the properties of conjugated systems.
The hydrogen addition reaction occurs not only for the specified type of acids, but in general for the entire series of acids under consideration and their derivatives, for example, esters (fats); the presence of catalysts (palladium black or finely divided nickel) greatly facilitates the process. This reaction is currently used in technology for the conversion of liquid vegetable oils, as well as fish oil and whale blubber, rich in glycerol esters of oleic and other unsaturated acids, into solid crystalline masses similar to lard. In this case, liquid glycerol esters of unsaturated acids are converted into solid esters of saturated acids.
Many reactions of hydrogen addition, isotope exchange, dehydrogenation, selective or complete oxidation, addition of carbon monoxide, and polymerization of hydrocarbons can best be explained by assuming the existence of radical-like neutral intermediates linked to the active sites of the catalyst by homopolar bonds.
temperature limits hydrogen addition reactions are largely determined by the conditions of the fuel hydrogenation process in technology. In fact, as is clear from the above, at temperatures close to 500, the processes of hydrogen addition, even at high pressures, are sharply reduced, especially for heavy ethylene hydrocarbons, which already appear among the initial cracking products of heavy gas oils and oil residues.
However, in the reactions of addition of hydrogen to the double bond in olefins, the considered metals exhibit extremely low activity. So, in it is indicated that hexene-1 does not undergo transformation on metallic titanium at 100 - 400 C in a stream of hydrogen.
Thus, the reaction of hydrogen addition to ethylene to form ethane must be exothermic; the molar heat of hydrogenation of ethylene is 316 kcal.
This reaction is similar to the hydrogen addition reaction, with the only difference that it does not require the presence of a catalyst. It flows instantly room temperature. The solvent used to dilute bromine is usually carbon tetrachloride.
Addition reactions.
1.1. Accession
CH 2 \u003d CH 2 + H 2 ® CH 3 -CH 3
The reaction proceeds in the presence of catalysts (Pd, Pt, Ni).
1.2. Addition of halogens:
CH 2 \u003d CH 2 + Br 2 ® CH 2 Br-CH 2 Br
1.3. Addition of hydrogen halides:
CH 2 \u003d CH 2 + HC1 ® CH 3 -CH 2 C1
The addition of hydrogen halides to ethylene homologues occurs according to the rule of V.V. Markovnikov: the hydrogen atom becomes the most hydrogenated carbon atom, and the halogen atom becomes the least hydrogenated, for example:
CH 3 -CH \u003d CH 2 + HBr-\u003e CH 3 - CH Br -CH3
1.4. Accession of water (hydration reaction). The reaction proceeds in the presence of a catalyst - sulfuric acid:
CH 2 \u003d CH 2 + H 2 O ® CH 3 - CH 2 OH
This is the overall reaction equation. In reality, the reaction proceeds in two stages. First, sulfuric acid is added to ethylene at the double bond break to form ethylsulfuric acid:
CH 2 \u003d CH 2 + H-O-SO 2 - OH ® CH3-CH 2 - O-SO 2 -OH
Then ethylsulfuric acid, interacting with water, forms alcohol and acid:
CH 3 - CH 2 - O-SO 2 - OH + H - OH ® CH 3 - CH 2 OH + HO-SO 2 - OH
At present, the addition reaction of water to ethylene in the presence of solid catalysts is used for the industrial production of ethyl alcohol from unsaturated hydrocarbons contained in oil cracking gases (associated gases), as well as in coke oven gases.
2. An important chemical property of ethylene and its homologues is the ability to easily oxidize even at ordinary temperatures. In this case, both carbon atoms connected by a double bond undergo oxidation. If ethylene is passed into an aqueous solution of potassium permanganate KMpO 4, then the characteristic violet color of the latter disappears - ethylene is oxidized with potassium permanganate:
ZSN 2 \u003d CH 2 + 2KMp0 4 + 4H 2 O ® ZNON 2 C - CH 2 OH + 2MnO 2 + 2KOH
ethylene glycol
This reaction is used to establish the unsaturation of organic matter - the presence of double or triple bonds in it.
2.2. Ethylene burns with a luminous flame to form carbon(IV) oxide and water:
CH 2 \u003d CH 2 + 4 O 2 ® 2CO 2 + 4H 2 O
3. Polymerization reactions.
polymerization is serial connection identical molecules into larger ones.
Polymerization reactions are especially characteristic of unsaturated compounds. So, for example, a high molecular weight substance, polyethylene, is formed from ethylene. Connection of ethylene molecules
takes place at the site where the double bond is broken. In short, the equation for this reaction is written as follows: nCH 2 \u003d CH 2 ® (- CH 2 - CH 2 - )n
Some free atoms or radicals (for example, hydrogen atoms from ethylene) are attached to the ends of such molecules (macromolecules). The product of the polymerization reaction is called a polymer (from the Greek poly - many, meros - part), and the starting substance that enters into the polymerization reaction is called a monomer.
Polymer - a substance with a very large relative molecular weight, the molecule of which consists of a large number repeating groupings that have the same structure. These groupings are called elementary links or structural units. For example, the elementary link of polyethylene is a grouping of atoms - CH 2 - CH 2 -.
The number of elementary units repeating in a macromolecule is called the degree of polymerization (denoted by n). Depending on the degree of polymerization, substances with different properties can be obtained from the same monomers.
So, polyethylene with short chains (n=20) is a liquid with lubricating properties. Polyethylene with a chain length of 1500 - 2000 links is a hard but flexible plastic material from which films can be obtained, bottles and other utensils, flexible pipes, etc. Finally, polyethylene with a chain length of 5 - 6 thousand links is a solid substance from which cast products, rigid pipes, strong threads can be prepared.
If a small number of molecules take part in the polymerization reaction, then low-molecular substances are formed, for example, dimers, trimers, etc. The conditions for the occurrence of polymerization reactions are very different. Sometimes catalysts are needed and high pressure. But the main factor is the structure of the monomer molecule. Unsaturated (unsaturated) compounds enter the polymerization reaction due to the breaking of multiple bonds.
The structural formulas of polymers are briefly written as follows: the formula of the elementary unit is enclosed in brackets and the letter n is put at the bottom right. For example, structural formula polyethylene (- CH 2 - CH 2 - ) P. It is easy to conclude that the name of the polymer is made up of the name of the monomer and the prefix poly-, for example, polyethylene, polyvinyl chloride, polystyrene, etc.
With the help of polymerization reactions, high-molecular synthetic substances are obtained, for example, polyethylene, polytetrafluoroethylene (Teflon), polystyrene, synthetic rubbers, etc. They are of great economic importance.
Teflon is a product of the polymerization of tetrafluoroethylene:
nCF 2 = CF 2 ->-(-CF 2 - CF 2 -)
This is the most inert organic matter (only molten potassium and sodium affect it). It has high frost and heat resistance.
Application. Ethylene is used to produce ethyl alcohol, polyethylene. It accelerates the ripening of fruits (tomatoes, citrus fruits, etc.) with the introduction of small amounts of it into the air of greenhouses. Ethylene and its homologues are used as a chemical raw material for the synthesis of many organic substances.
What is the mechanism of addition reactions with alkenes?
1. Due to the electrons of the π-bond in the molecules of alkenes, there is a region of increased electron density (a cloud of π-electrons above and below the plane of the molecule):
Therefore, the double bond tends to be attacked by an electrophilic (electron-deficient) reagent. In this case, a heterolytic cleavage of the π-bond will occur and the reaction will go along ionic mechanism as an electrophilic addition.
2. On the other hand, the carbon-carbon π-bond, being non-polar, can be broken homolytically, and then the reaction will go according to radical mechanism.
The addition mechanism depends on the reaction conditions.
In addition, alkenes are characterized by reactions isomerizationandoxidation (including reaction burning, characteristic of all hydrocarbons).
Addition reactions to alkenes.
Hydrogenation (addition of hydrogen)
Alkenes interact with hydrogen when heated and at elevated pressure in the presence of catalysts (Pt, Pd, Ni, etc.) to form alkanes:
Alkene hydrogenation is the reverse reaction of alkane dehydrogenation. According to Le Chatelier's principle, hydrogenation is favored by increased pressure, since this reaction is accompanied by a decrease in the volume of the system.
The addition of hydrogen to carbon atoms in alkenes leads to a decrease in the degree of their oxidation:
Therefore, the hydrogenation of alkenes is referred to as reduction reactions. This reaction is used in industry to produce high-octane fuel.
Halogenation (addition of halogens)
The addition of halogens to the C=C double bond occurs easily under normal conditions (at room temperature, without a catalyst). For example, the rapid discoloration of the red-brown color of a solution of bromine in water (bromine water) serves as a qualitative reaction to the presence of a double bond:
The addition of chlorine is even easier:
These reactions proceed by the mechanism of electrophilic addition with heterolytic bond breaking in the halogen molecule.
When heated to 500 °C, a radical substitution of the hydrogen atom at the carbon atom adjacent to the double bond is possible:
Hydrohalogenation (addition of hydrogen halides)
The reaction proceeds according to the mechanism of electrophilic addition with heterolytic bond cleavage.
CH 2 \u003d CH 2 + HCl CH 3 -CH 2 Cl
The direction of the reaction of addition of hydrogen halides to alkenes of an asymmetric structure (for example, to propylene CH 2
=CH–CH 3
) is determined by the Markovnikov rule:
In addition reactions of polar HX-type molecules to unsymmetrical alkenes, hydrogen is added to the more hydrogenated carbon atom at the double bond (i.e., the carbon atom associated with the largest number of hydrogen atoms).
So, in the reaction of HCl with propylene, from two possible structural isomers of 1-chloropropane and 2-chloropropane, the latter is formed:
It should be noted that Markovnikov's rule in its classical formulation is observed only for electrophilic reactions of the alkenes themselves. In the case of some derivatives of alkenes or when changing the mechanism, the reactions go against Markovnikov's rule.
Hydration(water connection)
Hydration occurs in the presence of mineral acids by the mechanism of electrophilic addition:
In the reactions of unsymmetrical alkenes, Markovnikov's rule is observed.
Polymerization- the reaction of formation of a high molecular weight compound (polymer) by sequential addition of molecules of a low molecular weight substance (monomer) according to the scheme:
nM M n
Number n in the polymer formula ( M n) is called the degree of polymerization. The polymerization reactions of alkenes are due to the addition of multiple bonds:
Obtaining alkenes
In nature, alkenes occur to a much lesser extent than saturated hydrocarbons, apparently due to their high reactivity. Therefore, they are obtained using various reactions.
I. Cracking of alkanes:
For example:
II. Cleavage (elimination) of two atoms or groups of atoms from neighboring carbon atoms with the formation of a -bond between them.
Dehydrohalogenation of haloalkanes under the action of an alcoholic solution of alkali
Dehydration of alcohols at elevated temperatures (above 140 C) in the presence of water-removing agents
Elimination reactions proceed according to ruleZaitseva:
The elimination of a hydrogen atom in the reactions of dehydrohalogenation and dehydration occurs predominantly from the least hydrogenated carbon atom.
Modern formulation: cleavage reactions proceed with the formation of more substituted alkenes at the double bond.
Such alkenes have a lower energy.
Dehalogenation of dihaloalkanes having halogen atoms at neighboring carbon atoms under the action of active metals:
Dehydrogenation of alkanes at 500С:
Application of alkenes
Alkenes are used as raw materials in the production of polymeric materials (plastics, rubbers, films) and other organic substances.
Ethylene(ethene) H 2 C \u003d CH 2 is used to produce polyethylene, polytetrafluoroethylene (Teflon), ethyl alcohol, acetaldehyde, halogen derivatives and many other organic compounds.
It is used as a means for the accelerated ripening of fruits.
Propylene(propene) H 2 C \u003d CH 2 -CH 3 and butylenes(butene-1 and butene-2) are used to produce alcohols and polymers.
Isobutylene(2-methylpropene) H 2 C \u003d C (CH 3) 2 is used in the production of synthetic rubber.
What hydrocarbons are called alkenes?
What is the general formula for alkenes?
What type of hybridization do alkenes have?
What kind Chemical properties characteristic of alkenes?
Why are alkenes used as a starting material for the production of HMCs?
What is the essence of Markovnikov's rule?
What methods of obtaining alkenes do you know?
What is the mechanism for the addition reaction in alkenes?
How do they change physical properties in the homologous series of alkenes?
Where are alkenes used?
Lecture No. 17: Alkadienes. Structure. Properties. Rubber.
Alkadienes (dienes)- unsaturated aliphatic hydrocarbons, the molecules of which contain two double bonds.
General formula of alkadienes FROM n H 2n-2 .
The properties of alkadienes largely depend on the mutual arrangement of double bonds in their molecules. On this basis, three types of double bonds in dienes are distinguished.
1. Isolated double bonds are separated in a chain by two or more σ-bonds:
CH 2 =CH–CH 2 –CH=CH 2
Separated by sp 3 -carbon atoms, such double bonds do not mutually influence each other and enter into the same reactions as the double bond in alkenes. Thus, alkadienes of this type exhibit chemical properties characteristic of alkenes.
2. Cumulated double bonds are located at one carbon atom:
CH 2 =C=CH 2 (allen)
Such dienes (allenes) belong to a rather rare type of compounds.
3. Conjugated double bonds are separated by one σ-bond:
CH 2 =CH–CH=CH 2
Conjugated dienes are of the greatest interest. They differ in characteristic properties due to the electronic structure of the molecules, namely, a continuous sequence of 4 sp 2 carbon atoms.
Individual representatives of these dienes are widely used in the production of synthetic rubbers and various organic substances.
According to IUPAC rules, the main chain of an alkadiene molecule must contain both double bonds. The numbering of carbon atoms in the chain is carried out so that the double bonds receive the smallest numbers. The names of alkadienes are derived from the names of the corresponding alkanes (with the same number of carbon atoms), in which last letter is replaced by the ending –diene.
The location of double bonds is indicated at the end of the name, and substituents - at the beginning of the name.
For example:
The name "divinyl" comes from the name of the radical –CH=CH 2 "vinyl".
Isomerism of conjugated dienes
Structural isomerism
1. Isomerism of the position of conjugated double bonds:
2. Isomerism of the carbon skeleton:
3. Interclass isomerism with alkynes and cycloalkenes.
For example, the formula FROM 4 H 6 match the following connections:
Spatial isomerism
Dienes having different substituents at carbon atoms at double bonds, like alkenes, exhibit cis-trans isomerism.
In addition, rotation along the σ-bond separating double bonds is possible, leading to rotational isomers. Some chemical reactions conjugated dienes go selectively only with a certain rotational isomer.
Properties of Conjugated Alkadienes
Of greatest practical importance are divinyl or butadiene-1,3 (easily liquefying gas, bp = -4.5 C) and isoprene or 2-methylbutadiene-1,3 (liquid with bp = 34 C).
Diene hydrocarbons are chemically similar to alkenes. They are easily oxidized and enter into addition reactions. However, conjugated dienes differ in some features, which are due to the delocalization (dispersal) of π-electrons.
Molecule of butadiene-1,3 CH 2 =CH-CH=CH 2 contains four carbon atoms in the sp 2 hybridized state and has a flat structure.
π-electrons of double bonds form a single π-electron cloud (conjugated system) and are delocalized between all carbon atoms.
The bond order (the number of shared electron pairs) between carbon atoms is intermediate between 1 and 2, i.e. There are no purely single and purely double bonds. The structure of butadiene is more accurately reflected by the formula with delocalized bonds.
Isoprene molecules are similarly constructed:
Formation of a single π-electron cloud covering 4 carbon atoms:
leads to the possibility of attaching the reagent at the ends of this system, i.e. to C 1 and C 4 atoms. Therefore, divinyl and isoprene, along with the addition of 1 mol of the reagent at one of the double bonds (1,2- or 3,4-), enter into 1,4-addition reactions. The ratio of 1,2- and 1,4-addition products depends on the reaction conditions (with an increase in temperature, the probability of 1,4-addition usually increases).
Polymerization of conjugated dienes. Rubbers
Divinyl and isoprene enter into polymerization and copolymerization (ie joint polymerization) with other unsaturated compounds, forming rubbers. Rubbers are elastic high-molecular materials (elastomers), from which rubber is obtained by vulcanization (heating with sulfur).
natural rubber- natural high-molecular unsaturated hydrocarbon of composition (C 5 H 8) n, where n is 1000-3000 units. It has been established that this polymer consists of repeating units of 1,4-cis-isoprene and has a stereoregular structure:
Under natural conditions, natural rubber is formed not by the polymerization of isoprene, but by another, more complex method.
The polymerization of 1,3-dienes can proceed either by the 1,4-addition type or by a mixed 1,2- and 1,4-addition type. The direction of addition depends on the reaction conditions.
The first synthetic rubber obtained by the method of S.V. Lebedev during the polymerization of divinyl under the action of metallic sodium, was a polymer of an irregular structure with a mixed type of units of 1,2- and 1,4-addition:
In the presence of organic peroxides (radical polymerization), an irregular polymer with 1,2- and 1,4-addition units is also formed. Rubbers of irregular structure are characterized by low quality during operation. Selective 1,4-addition occurs when using organometallic catalysts (for example, butyllithium C 4 H 9 Li, which not only initiates polymerization, but also coordinates in a certain way in space the attached diene molecules):
In this way, stereoregular 1,4-cis-polyisoprene, a synthetic analog of natural rubber, was obtained. This process proceeds as ionic polymerization.
For practical use, rubbers are converted into rubber. Rubber - it is a vulcanized rubber with a filler (carbon black). The essence of the vulcanization process is that heating a mixture of rubber and sulfur leads to the formation of a three-dimensional network structure of linear rubber macromolecules, giving it increased strength. Sulfur atoms are attached to the double bonds of macromolecules and form cross-linking disulfide bridges between them:
The mesh polymer is more durable and exhibits increased elasticity - high elasticity (capacity for high reversible deformations).
Depending on the amount of the crosslinking agent (sulfur), it is possible to obtain networks with different crosslinking frequencies. Extremely cross-linked natural rubber - ebonite - does not have elasticity and is a solid material.
Obtaining alkadienes
General methods for obtaining dienes are similar to methods for obtaining alkenes.
1. Catalytic two-stage dehydrogenation of alkanes (through the stage of formation of alkenes). In this way, divinyl is obtained in industry from butane contained in oil refining gases and associated gases:
Isoprene is obtained by catalytic dehydrogenation of isopentane (2-methylbutane):
2. Synthesis of divinyl according to Lebedev:
3. Dehydration of glycols (dihydric alcohols, or alkanediols):
4. The action of an alcoholic solution of alkali on dihaloalkanes (dehydrohalogenation):
Questions to fix the topic:
What hydrocarbons are called dienes?
What types of isomerism are observed in alkadienes?
What are the chemical properties of diene hydrocarbons?
How can alkadienes be obtained?
What type of hybridization is typical for alkadienes?
What is rubber?
What is rubber?
What determines the physical properties of alkadienes?
What chemical properties of alkadienes are similar to what?
Lecture No. 18: Alkynes. Structure, properties, application.
Alkynes (acetylenic hydrocarbons)- unsaturated aliphatic hydrocarbons, the molecules of which contain a C≡C triple bond.
General formula for alkynes with one triple bond FROM n H 2n-2 .
The triple bond C≡C is carried out by 6 common electrons:
Carbon atoms are involved in the formation of such a bond in sp-hybridized state. Each of them has two sp- hybrid orbitals directed to each other at an angle of 180, and two non-hybrid R-orbitals located at an angle of 90° with respect to each other and to sp- hybrid orbitals:
The structure of the C≡C triple bond
A triple bond is a combination of one σ- and two π-bonds formed by two sp hybridized atoms. σ-bond occurs with axial overlap sp-hybrid orbitals of neighboring carbon atoms; one of the π bonds is formed by lateral overlap R y-orbitals, the other - with lateral overlap R z-orbitals. The formation of bonds using the example of an acetylene molecule H–C≡C–H can be represented as a diagram:
C≡C σ-bond (overlap 2 sp-2sp);
π bond (2 R y-2 R y);
π bond (2 R z-2 R z);
C–H σ-bond (overlap 2 sp-AO carbon and 1 s-AO of hydrogen).
π-bonds are located in mutually perpendicular planes:
σ-bonds formed sp-hybrid orbitals of carbon, are located on one straight line (at an angle of 180 to each other). Therefore, the acetylene molecule has a linear structure:
Alkyne nomenclature
According to the systematic nomenclature, the names of acetylenic hydrocarbons are derived from the names of the corresponding alkanes (with the same number of carbon atoms) by replacing the suffix –en on the –in :
2 C atoms → ethane → eth in ; 3 C atoms → propane → prop in etc.
The main chain is chosen so that it necessarily includes a triple bond (i.e., it may not be the longest).
The numbering of carbon atoms starts from the end of the chain closest to the triple bond. The number indicating the position of the triple bond is usually placed after the suffix –in . For example:
For the simplest alkenes, historically established names are also used: acetylene(ethin), allylene(propyne), crotonylene(butin-1), valerylene(pentin-1).
In the nomenclature of various classes of organic compounds, the following monovalent alkyne radicals are most often used:
Alkyne isomerism
Structural isomerism
Triple bond position isomerism (starting from C 4 H 6):
Isomerism of the carbon skeleton (starting from C 5 H 8):
Interclass isomerism with alkadienes and cycloalkenes, starting from C 4 H 6:
Spatial isomerism with respect to the triple bond does not appear in alkynes, because substituents can be located in only one way - along the communication line.
Properties of alkynes
physical properties. The boiling and melting points of acetylenic hydrocarbons increase with increasing molecular weight. Under normal conditions, alkynes C 2 H 2 -C 4 H 6 - gases, C 5 H 8 -C 16 H 30 - liquids, C 17 H 32 - solids. The boiling and melting points of alkynes are higher than those of the corresponding alkenes.
Physical properties of alkynes and alkenes
Alkynes are poorly soluble in water, better in organic solvents.
Chemical properties.
Addition reactions to alkynes
1. Hydrogenation
In the presence of metal catalysts (Pt, Ni), alkynes add hydrogen to form alkenes (the first π-bond is broken), and then alkanes (the second π-bond is broken):
When a less active catalyst is used, hydrogenation stops at the stage of alkene formation.
2. Halogenation
The electrophilic addition of halogens to alkynes proceeds more slowly than for alkenes (the first π-bond is more difficult to break than the second):
Alkynes decolorize bromine water (qualitative reaction).
Explanatory note (6)
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Explanatory note (7)
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The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). Obviously, the whole variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions already familiar to you that occur between inorganic substances.
As a rule, the main organic compound involved in the reaction is called substrate, and the other component of the reaction is conditionally considered as reagent.
Substitution reactions
Substitution reactions- these are reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms.
Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes. Let us give examples of such reactions.
Under the action of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
The equation for this reaction can be written differently:
With this form of recording, the reagents, catalyst, reaction conditions are written above the arrow, and the inorganic reaction products below it.
As a result of reactions substitutions in organic substances are formed not simple and complex substances, as in inorganic chemistry, and two complex substances.
Addition reactions
Addition reactions are reactions in which two or more molecules of reactants combine into one.
Unsaturated compounds, such as alkenes or alkynes, enter into addition reactions. Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration, and other addition reactions are distinguished. Each of them requires certain conditions.
1.Hydrogenation- the reaction of adding a hydrogen molecule to a multiple bond:
2. Hydrohalogenation- hydrogen halide addition reaction (hydrochlorination):
3. Halogenation- halogen addition reaction:
4.Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined with each other to form molecules of a substance with a very high molecular weight - macromolecules.
Polymerization reactions are the processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.
An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and radical polymerization initiator R.
The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common electrons can be different.
Types of reactive particles
An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. This is where education takes place covalent bond according to the exchange mechanism:
The exchange mechanism for the formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.
The process opposite to the formation of a covalent bond by the exchange mechanism is bond breaking, in which one electron () goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:
Such particles are called free radicals.
free radicals- atoms or groups of atoms having unpaired electrons.
Free radical reactions are reactions that occur under the action and with the participation of free radicals.
In the course of inorganic chemistry, these are reactions of interaction of hydrogen with oxygen, halogens, combustion reactions. Reactions of this type are characterized by high speed, release a large number heat.
A covalent bond can also be formed by the donor-acceptor mechanism. One of the orbitals of an atom (or anion), which contains an unshared electron pair, overlaps with an unfilled orbital of another atom (or cation) that has an unfilled orbital, and a covalent bond is formed, for example:
Breaking a covalent bond leads to the formation of positively and negatively charged particles (); since in this case both electrons from a common electron pair remain with one of the atoms, the other atom has an unfilled orbital:
Consider the electrolytic dissociation of acids:
One can easily guess that a particle having an unshared electron pair R: -, i.e., a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge.
Particles with unshared electron pairs are called nucleophilic agents (nucleus- "nucleus", the positively charged part of the atom), that is, the "friends" of the nucleus, a positive charge.
Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons, interacting with the regions of the molecules, on which the effective positive charge is concentrated.
Examples of nucleophiles: Cl - (chloride ion), OH - (hydroxide anion), CH 3 O - (methoxide anion), CH 3 COO - (acetate anion).
Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to the regions of the molecules that have an increased electron density, a negative charge, and an unshared electron pair. They are electrophiles, "friends" of an electron, a negative charge, or particles with an increased electron density.
electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.
Not every particle is an electrophile with an empty orbital. So, for example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity.
From this we can conclude that despite the presence of an unfilled orbital, such particles will not be electrophiles.
Main reaction mechanisms
There are three main types of reacting particles - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanism:
- free radical;
- electrophilic;
- nullophilic.
In addition to classifying reactions according to the type of reacting particles, organic chemistry distinguishes four types of reactions according to the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from the English. to eliminate- delete, split off) and regroup. Since addition and substitution can occur under the action of all three types of reactive species, several majorreaction mechanisms.
In addition, consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.
6. Elimination:
A distinctive feature of alkenes (unsaturated hydrocarbons) is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism of electrophilic addition.
Hydrohalogenation (addition of halogen hydrogen):
When a hydrogen halide is added to an alkene hydrogen is added to more hydrogenated carbon atom, i.e., the atom at which there are more atoms hydrogen, and halogen - to less hydrogenated.
Addition reactions
Addition reactions are the most typical reactions of alkenes. Hydrogen, halogens, hydrogen halides, water, acids and other reagents can be attached to the double bond. Many of these reactions are great importance in the chemistry of terpenoids and are widely used for practical purposes.
3.2.1.1 Addition of hydrogen. The addition of hydrogen (hydrogenation) converts unsaturated compounds into saturated ones. Attachment is associated with the breaking of the π-bond and the formation of two stronger σ-bonds instead. As a result, energy is released, i.e. hydrogenation is an exothermic reaction, ∆Н ≈-125 kJ/mol.
In the absence of a catalyst, hydrogenation proceeds extremely slowly even with strong heating. This means that the reaction is characterized by a high activation energy ∆Еа. The catalyst effectively reduces the value of ∆Еа, sorbing the reagents on the active centers of its developed surface with the weakening or destruction of π-bonds. The addition of hydrogen and desorption of the saturated molecule complete the process. The most active catalysts are platinum group metals. For practical purposes, more accessible nickel catalysts are more often used.
Hydrogenation is a reversible process. Catalysts simultaneously accelerate the reverse reaction - dehydrogenation (as is known, the catalyst does not affect the equilibrium of the reaction). To shift the equilibrium towards the hydrogenation products, the process is carried out at high pressure.
The hydrogenation reaction is sensitive to steric hindrance. Terminal double bonds are most easily saturated. Double bonds in the middle of the chain, especially in the presence of bulky substituents, are difficult to saturate. Therefore, in limonene, for example, it is easy to achieve high selectivity for saturation of only one double bond:
limonene P-menthen
Conjugated double bonds are hydrogenated stepwise:
myrcene linaloolen
Strained three- and four-member cycles of bicyclic terpenoids can also add hydrogen. However, hydrogenation through the cycle is more difficult than through the double bond, so the double bond is saturated first, and then the strained cycle opens:
3-karen karan P-mentan m-mentan
Many hydrogenation reactions are used in industrial processes. The conversion of α-pinene to pinane is the first step in the synthesis of geraniol, linalool, and other terpenoid aromatic compounds from this available hydrocarbon.
Hydrogenation of isopulegol, piperitol, piperitone produces menthol.
The transformation of citral into citronellal is the first step in the synthesis of one of the most valuable fragrant substances - hydroxydihydrocitronellal (hydroxycitronellal, GOC):
citral citronellal GOC
3.2.1.2 Zelinsky's irreversible catalysis. Interestingly studied by Academician N.D. Zelinsky, the behavior of mono- and bicyclic monoterpene hydrocarbons when heated with hydrogenation catalysts in an inert atmosphere in the absence of hydrogen. Such compounds, regardless of the position of double bonds and the nature of the cycles, irreversibly turn into a mixture consisting of aromatic hydrocarbons and saturated derivatives of cyclohexane:
limonene P-mentan P-cymen
α-pinene P-mentan P-cymen
These transformations clearly illustrate the relative thermodynamic instability of terpenoids. The processes of dehydrogenation and rehydrogenation occurring in the presence of a catalyst lead to disproportionation, i.e. to the formation of fully saturated and fully unsaturated (aromatic) cycles, which have a much lower energy reserve than partially unsaturated and strained cycles of terpenoids. This causes a complete shift of the equilibrium to the right and makes the reaction irreversible. The described process is known in chemistry as Zelinsky's irreversible catalysis.
3.2.1.3 Addition of oxygen. Terpenoids with conjugated bonds are able to add oxygen when interacting with air in the light. The energy of visible light quanta is sufficient to activate the conjugated system to a biradical that easily attaches an oxygen molecule to form cyclic peroxide:
This process is facilitated in the presence of dyes that intensely absorb visible light and are able to transfer the perceived energy to the molecules of unsaturated compounds. So, α-terpinene is easily oxidized to ascaridol by adding a small amount of methylene blue:
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hν, dye
α-terpinene ascaridol
It is believed that in a similar way ascaridole is formed in chenopodium essential oil. The role of the dye facilitating the process is performed by chlorophyll.
The obtained peroxides are unstable and easily split with the formation of biradicals:
Biradicals initiate the polymerization of unsaturated compounds. Polymerization leads to an increase in viscosity, and in thin films, to solidification of the substance. Similar processes occur during the "drying" of varnishes and paints, which contain unsaturated compounds with conjugated double bonds.
3.2.1.4 Addition of halogens. Unsaturated terpenoids, like other alkenes, easily react with chlorine and bromine.
Myrcene reacts with bromine to form tetrabromide:
The addition of two bromine atoms to the conjugated system passivates the remaining double bond and no further addition occurs.
Monocyclic terpenoids with conjugated systems react similarly. Thus, phellandrenes and α-terpinene, whose structures are characterized by two conjugated bonds, form only dibromides. Limonene, terpinolene, β-terpinene, which do not have conjugated bonds, form tetrabromides:
Tetrabromides are crystalline substances and can be used to identify parent compounds.
bicyclic terpenoids in mild conditions bromine is attached only at the double bond. The opening of strained cycles occurs at elevated temperatures and is accompanied by rearrangements.
Chlorination of double bonds proceeds in a special way at the points of branching of the carbon chain. The process proceeds according to the attachment-cleavage mechanism:
Monochloride 2 is prone to allyl rearrangement:
For example, chlorination of α-pinene proceeds along this pathway. At temperatures up to 70 ° C, a monochloride of the following structure is formed:
Heating causes a rearrangement and leads to myrtenyl chloride:
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myrtenyl chloride
3.2.1.5 Addition of nitrosyl chloride. The interaction of terpenoids with nitrosyl chloride proceeds according to the mechanism of electrophilic addition reactions (AE); while the electrophile is the nitrosyl cation NO + . The addition proceeds according to the Markovnikov rule:
limonene nitrosyl limonene chloride
Nitrosyl chlorides are not final products reactions if the carbon atom bonded to nitrogen has hydrogen. In this case, a rearrangement occurs, similar to the enolization of carbonyl compounds. The rearrangement leads to the formation of oxychloride, which is more stable than nitrosyl chloride:
oxymchloride
If the double bond is between fully substituted carbon atoms, then nitrosyl chloride is retained. It has an intense blue color. The appearance of this color serves as a qualitative reaction to tetrasubstituted ethylenes. For example, terpinolene reacts in a similar way:
terpinolene nitrosyl chloride
(painted bright blue)
The reaction with nitrosyl chloride is used industrially to produce carvone from limonene. Oximchloride is subjected to dehydrohalogenation in the presence of bases, and the resulting carvoxime is converted to carvone by hydrolysis in an acidic medium, binding the released hydroxylamine with acetone:
oxymchloride carvone oxime carvone
limonene (carvoxime)
3.2.1.6 Accession of water (hydration reaction). Water is capable of adding to the most reactive double bonds and strained cycles of terpenoids to form alcohols. A sufficient reaction rate can be achieved only in the presence of catalysts - acids. Hydration is a typical reaction of type A E and proceeds according to Markovnikov's rule.
Consider how myrcene hydration proceeds. The presence of several double bonds in the myrcene molecule leads to the formation of a mixture of various alcohols. The hydration product of myrcene, named by Barbier (1901) "myrceneol", actually consisted of many alcohols different structure. Their formation can be explained by considering the reaction mechanism. Hydration proceeds more easily through conjugated double bonds. The proton binds to one of the extreme atoms of the conjugated system:
Further interaction of the resulting intermediates with water, which is added to position 1 or 3 of the conjugated carbenium ions, leads to a mixture of four alcohols:
nerol linalool
Cations 1 and 2 can isomerize into cyclic structures before adding water. Thus, cation 1 easily forms a structure P- menthane (see 3.1.2.1); interaction with water leads to α-terpineol:
1 3 α-terpineol
If we take into account the possibility of adding water to the isolated double bond of myrcene, as well as the isomerization transformations of the initially formed alcohols, we can get an idea of the complexity of the composition of the product of myrcene hydration.
Currently, myrcenol is called an alcohol of the following structure:
myrcenol
It is obtained by hydration of myrcene in the presence of Lewis acids and catalysts with a developed surface, such as active carbon.
The addition of water to limonene (or dipentene) leads under normal conditions to a two-tertiary glycol terpine, since the reactivity of both double bonds is approximately the same:
Terpine is also formed during the hydration of 3-carene, α- and β-pinenes, where the reaction proceeds both due to a double bond and due to a tense three- or four-link cycle:
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Terpine crystallizes from aqueous solutions with one water molecule, forming terpinhydrate. Terpinhydrate is used in medicine as a mild cough remedy.
By partial dehydration of terpine one can obtain monohydric alcohols - isomers of terpineol:
α-terpineol β-terpineol γ-terpineol
The possibility of partial dehydration is explained by the fact that terpineols, unlike terpine, are relatively volatile and are removed from the reaction zone by steam distillation. Terpineols (with a predominance of the α-isomer) are widely used as lilac fragrances and are commercially produced on a large scale.
There are conditions when the hydration of limonene, α-pinene, 3-carene directly leads to the formation of terpineol. Domestic industry uses, for example, the hydration of α- and β-pinenes with an aqueous acetone solution of sulfuric acid and obtains terpineol in one stage:
H 2 O, acetone, H 2 SO 4
α-pinene α-terpineol
Hydration proceeds with the formation of carbenium ions as intermediates, which, as noted earlier, are distinguished by a high tendency to various kinds of rearrangements. This results in the formation, along with the supposed products of hydration, of compounds of an unexpected structure. Thus, during the hydration of α-pinene, together with terpineol, impurities of borneol, isoborneol and fenchols, bicyclic alcohols of the camphane and fennhan series, are formed.
The mechanism of their formation can be represented as follows. The first step is the addition of a proton to the double bond, which usually begins hydration:
1
In cation 1, before a water molecule joins it, a rearrangement can occur, associated with the movement of an electron pair from one of the neighboring carbon atoms to the carbon atom that carries the charge.
Previously (see 3.1.1.3) the displacement of an electron pair of hydrogen (hydride shift) was considered. AT this example we are meeting with "alkyl shift", i.e. with the movement of an electron pair that binds a neighboring carbon atom to another carbon atom, i.e. with an alkyl radical.
The hydride shift in cation 1 is energetically unfavorable, since the displacement of hydrogen from atoms 3 or 8 leads to the formation of less stable carbenium ions - secondary and primary. For the same reason, there is no alkyl shift from atom 3, i.e., the movement of the electron pair that binds atom 4 to atom 3 to a charged atom 2.
The displacement of hydrogen from atom 1 can lead to the formation of a tertiary and seemingly stable carbenium ion. In fact, the resulting cation is extremely unstable due to stresses caused by the impossibility of locating bonds in a given cation in one plane, and is not actually formed (Bredt's rule).
In practice, there is a shift of one of the electron pairs that bind atom 1 to other carbon atoms - an alkyl shift, which proceeds especially easily in bicyclic structures.
An electron pair can move from atom 1, connecting it with atoms 6 or 7. Let's consider both options. The shift of the electron pair of atoms 1-6 leads to the fenhan structure:
1 2 2 fenhol
The transformation of the tertiary cation 1 into the less stable secondary 2 turns out to be thermodynamically favorable due to the opening of the strained four-link ring and the formation of an unstressed bicyclic structure.
The shift of the electron pair of atoms 1-7 forms the camphane structure:
borneol + isoborneol
It is possible to choose the conditions under which borneols are the main products of hydration.
These transformations were first described by the Russian scientist E.E. Wagner; in organic chemistry they are known as Wagner-Meyerwein rearrangements.
Wagner-Meyerwein rearrangements are especially characteristic of camphene. Hydration of camphene only under mild conditions makes it possible to obtain a normal reaction product, the tertiary alcohol camphenhydrate:
camphene camphenhydrate
The more thermodynamically stable borneol and isoborneol, which are formed as a result of rearrangements of carbenium ions, turn out to be the usual products of hydration:
camphene 1 2 isoborneol borneol
This rearrangement is called camphene rearrangement of the first kind. It was discovered by Wagner, who, assuming its course, first pointed out correct structure camphene. Camphene rearrangement of the 1st kind, however, did not explain all the features of the behavior of camphene. In particular, it remained unclear why the hydration of one of the camphene enantiomers results in the formation of a racemic mixture; (±)-borneol and (±)-isoborneol. The type 1 rearrangement affects only one of the two camphene chiral centers and racemization should not occur. The explanation of racemization during the transformations of camphene was given by Academician S.S. Nametkin (1925). He discovered a different sequence of transformations of camphene, called camphene rearrangement of the second kind.
The camphene rearrangement of the 2nd kind differs in that a double alkyl shift occurs in it: first, the electron pair of the methyl group moves, and then a rearrangement occurs similar to the rearrangement of the 1st kind:
1 1 1 2 1
Rearrangement of the 2nd kind affects the second chiral atom in the camphene molecule. The simultaneous occurrence of both rearrangements leads to racemization.
Another practical example of a hydration reaction is the addition of water to citronellal.
Hydration of citronellal is carried out in order to obtain hydroxydihydrocitronellal (GOC) - one of the widely known fragrant substances. A feature of the process is the impossibility of direct hydration of citronellal, since this aldehyde in the presence of acids, i.e. under hydration conditions, readily cyclizes to isopulegol (see 3.1.3.1). Hydration is carried out by previously inactivating (protecting) the carbonyl group, for example, by reaction with NaHSO 3:
citronellal GOC
HOC has a strong and pleasant smell of linden and lily of the valley with a note of greenery and large quantities used in perfumery.
Hydration of pulegone is interesting in that it leads to the splitting of the molecule at the double bond. This process proceeds especially easily in the presence of formic acid:
pulegone acetone 3-methylcyclohexanone
The reaction begins with the protonation of the conjugated system at the oxygen atom. The addition of water and the rearrangement of the resulting enol complete the actual hydration process:
The addition of a proton to the carbonyl group of the ketoalcohol causes a strong bond tension between the isopropyl radical and the ring, which breaks under the influence of nearby acceptors - oxygen and a positive charge:
3.2.1.7 Addition of hydrogen halides. Unsaturated terpenoids add hydrogen halides at a high rate. The reaction does not require a catalyst because the hydrogen halides themselves are strong acids. The addition follows the Markovnikov rule. For example, when limonene is treated with hydrochloric acid, dihydrochloride is formed:
Hydrochlorination, like hydration, is often accompanied by isomerization transformations. The addition of HCl to α-pinene only at temperatures up to minus 10 ° C gives a normal addition product:
t -10 o C
An increase in temperature is accompanied by isomerization (see 3.2.1.6) with the formation of bornyl and phenyl chlorides:
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bornyl chloride, isobornyl chloride
phenyl chloride
The addition of HCl to camphene also causes rearrangements (of the 1st and 2nd kind; see 3.2.1.6) and leads to bornyl and isobornyl chloride.
In the case of interaction with HBr, terpenoids, like other alkenes, can give addition products against the Markovnikov rule, which is explained by the well-known Harasz peroxide effect.
3.2.1.8 Addition of carboxylic acids. By a mechanism similar to hydration, carboxylic acids react with some unsaturated terpenoids to form esters. The reaction of camphene with acetic acid is widely known, leading to the production of isobornyl acetate, a common fragrant substance with a coniferous smell:
isobornyl acetate
The process is catalyzed by sulfuric acid and is accompanied by rearrangements (see 3.2.1.6):
The interaction of acetic acid with cation 2 proceeds stereospecifically. A new bond with a carbon atom bearing a positive charge is known to be formed along a line perpendicular to the plane in which the bonds of the carbenium ion lie. Since the access of the acetic acid molecule along this line is free only from one side of the plane (on the other side, the plane of the second five-membered cycle interferes with the large molecule of acetic acid), one diastereomer is formed - isobornyl acetate:
isobornyl acetate
3.2.1.9 Polymerization. Unsaturated terpenoids, like other alkenes, can undergo polymerization. Polymerization is especially easy in the presence of conjugated bonds. The process is catalyzed by acids (cationic polymerization) or free radicals. Polymerization leads to an increase in viscosity, loss of odor and is an undesirable process in fragrance chemistry.
3.2.1.10 Addition of formaldehyde (Prince's reaction). The Prince reaction is the interaction of alkenes with formaldehyde in an acetic acid medium in the presence of mineral acids.
The reaction proceeds by the addition-cleavage mechanism. A feature of the Prins reaction mechanism is the initial addition of a proton not at the double bond of the alkene, but at the carbonyl group of formaldehyde:
The resulting cation is added at the double bond in accordance with Markovnikov's rule. The process ends with the elimination of a proton. The reaction is often carried out in the presence of acetic acid to esterify the resulting alcohol.
The Prince reaction is used in industry to obtain some aromatic substances from terpene hydrocarbons, for example:
valteryl acetate
Valteryl acetate has a woody-herbaceous odor and is used in perfumes and soap fragrances.
3.2.1.11 Hydroxylation of double bonds. Hydroxylation of alkenes refers to the addition of hydroxyl groups to form glycols. Hydroxylation is carried out by the action of oxidizing agents such as H 2 O 2 , OsO 4 , KMnO 4 . The use of a solution of KMnO 4 at room temperature in a weakly alkaline medium for the hydroxylation of double bonds introduced into organic chemistry HER. Wagner. With this reaction, he established the structures of many known terpenoids.
Oxidation of limonene under such conditions leads to the formation of a tetrahydric alcohol with a melting point of 191.5-192 ˚С.
The interaction of potassium permanganate with α-pinene, along with the hydroxylation of the double bond, causes the opening of a strained ring and the reaction product also turns out to be a tetrahydric alcohol:
