General formula of alkenes. Properties and characteristics of alkenes
4. Chemical properties of alkenes
The energy of the double carbon-carbon bond in ethylene (146 kcal/mol) is significantly lower than the doubled energy of a single C-C bond in ethane (288=176 kcal/mol). The -C-C bond in ethylene is stronger than the -bond, therefore, the reactions of alkenes, accompanied by the breaking of the -bond with the formation of two new simple -bonds, are a thermodynamically favorable process. For example, in the gas phase, according to the calculated data, all the reactions below are exothermic with a significant negative enthalpy, regardless of their real mechanism.
From the point of view of the theory of molecular orbitals, one can also conclude that the -bond is more reactive than the -bond. Consider the molecular orbitals of ethylene (Fig. 2).
Indeed, the bonding -orbital of ethylene has a higher energy than the bonding -orbital, and vice versa, the antibonding *-orbital of ethylene lies below the antibonding *-orbital of the C=C bond. Under normal conditions, *- and *-orbitals of ethylene are vacant. Therefore, the boundary orbitals of ethylene and other alkenes, which determine their reactivity, will be -orbitals.
4.1. Catalytic hydrogenation of alkenes
Despite the fact that the hydrogenation of ethylene and other alkenes to alkanes is accompanied by the release of heat, this reaction proceeds at a noticeable rate only in the presence of certain catalysts. The catalyst, by definition, does not affect the thermal effect of the reaction, and its role is reduced to lowering the activation energy. One should distinguish between heterogeneous and homogeneous catalytic hydrogenation of alkenes. In heterogeneous hydrogenation, finely divided metal catalysts are used - platinum, palladium, ruthenium, rhodium, osmium and nickel, either in pure form or supported on inert carriers - BaSO 4 , CaCO 3 , activated carbon, Al 2 O 3, etc. All of them insoluble in organic media and act as heterogeneous catalysts. Ruthenium and rhodium are the most active among them, but platinum and nickel are the most widespread. Platinum is usually used in the form of black dioxide PtO 2 , commonly known as "Adams catalyst". Platinum dioxide is obtained by fusing chloroplatinic acid H 2 PtCl 6 . 6H 2 O or ammonium hexachloroplatinate (NH 4) 2 PtCl 6 with sodium nitrate. Hydrogenation of alkenes with an Adams catalyst is usually carried out at normal pressure and a temperature of 20-50 0 C in alcohol, acetic acid, ethyl acetate. When hydrogen is passed through, platinum dioxide is reduced directly in the reaction vessel to platinum black, which catalyzes the hydrogenation. Other more active platinum group metals are used on inert supports, such as Pd/C or Pd/BaSO 4 , Ru/Al 2 O 3 ; Rh/C, etc. Palladium deposited on coal catalyzes the hydrogenation of alkenes to alkanes in an alcohol solution at 0-20 0 C and normal pressure. Nickel is usually used in the form of the so-called "Raney nickel". To obtain this catalyst, the nickel-aluminum alloy is treated with hot aqueous alkali to remove almost all of the aluminum and then with water until neutral. The catalyst has a porous structure and is therefore also called a skeletal nickel catalyst. Typical conditions for the hydrogenation of alkenes over Raney nickel require a pressure of about 5-10 atm and a temperature of 50-100 0 C, i.e. this catalyst is much less active than platinum group metals, but it is whiter and cheaper. The following are some typical examples of heterogeneous catalytic hydrogenation of acyclic and cyclic alkenes:
Since both hydrogen atoms are attached to the carbon atoms of the double bond from the surface of the catalyst metal, the addition usually occurs on one side of the double bond. This type of attachment is called syn- connection. In those cases when two fragments of the reagent are attached from different sides of the multiple bond (double or triple) anti- accession. Terms syn- and anti- are equivalent in meaning to terms cis- and trance-. To avoid confusion and misunderstandings, the terms syn- and anti- refer to the type of connection, and the terms cis- and trance- to the structure of the substrate.
The double bond in alkenes is hydrogenated at a faster rate than many other functional groups (C=O, COOR, CN, etc.) and therefore the hydrogenation of the C=C double bond is often a selective process if the hydrogenation is carried out under mild conditions (0- 20 0 С and at atmospheric pressure). Below are some typical examples:
The benzene ring is not reduced under these conditions.
A great and fundamentally important achievement in catalytic hydrogenation is the discovery of soluble metal complexes that catalyze hydrogenation in a homogeneous solution. Heterogeneous hydrogenation on the surface of metal catalysts has a number of significant disadvantages, such as isomerization of alkenes and splitting of single carbon-carbon bonds (hydrogenolysis). Homogeneous hydrogenation is devoid of these disadvantages. In recent years, a large group of homogeneous hydrogenation catalysts has been obtained - transition metal complexes containing various ligands. The best catalysts for homogeneous hydrogenation are complexes of rhodium (I) and ruthenium (III) chlorides with triphenylphosphine - tris (triphenylphosphine) rhodium chloride (Ph 3 P) 3 RhCl (Wilkinson's catalyst) and tris (triphenylphosphine) ruthenium hydrochloride (Ph 3 P) 3 RuHCl. The most accessible rhodium complex, which is obtained by the interaction of rhodium (III) chloride with triphenylphosphine. Wilkinson's rhodium complex is used to hydrogenate the double bond under normal conditions.
An important advantage of homogeneous catalysts is the possibility of selective reduction of a mono- or disubstituted double bond in the presence of a tri- and tetrasubstituted double bond due to large differences in their hydrogenation rates.
In the case of homogeneous catalysts, hydrogen addition also occurs as syn- accession. So recovery cis-butene-2 deuterium under these conditions leads to meso-2,3-dideuterobutane.
4.2. Double bond reduction with diimide
The reduction of alkenes to the corresponding alkanes can be successfully carried out using diimide NH=NH.
Diimide is obtained by two main methods: oxidation of hydrazine with hydrogen peroxide in the presence of Cu 2+ ions or by the interaction of hydrazine with Ni-Raney (hydrazine dehydrogenation). If an alkene is present in the reaction mixture, its double bond undergoes hydrogenation under the action of a very unstable diimide. A distinctive feature of this method is the strict syn-stereospecificity of the recovery process. It is believed that this reaction proceeds through a cyclic activated complex with a strict orientation of both reacting molecules in space.
4.3. Electrophilic addition reactions at the double bond of alkenes
The boundary orbitals of the HOMO and LUMO of alkenes are the occupied - and empty *-orbitals. Consequently, in reactions with electrophiles (E +), the -orbital will participate, and in reactions with nucleophiles (Nu -), the *-orbital of the C=C bond will participate (see Fig. 3). In most cases, simple alkenes easily react with electrophiles, and react with nucleophiles with great difficulty. This is explained by the fact that usually the LUMO of most electrophiles is close in energy to the energy of -HOMO of alkenes, while the HOMO of most nucleophiles lies much lower than *-LUMO.
Simple alkenes react only with very strong nucleophilic agents (carbanions) under harsh conditions, however, the introduction of electron-withdrawing groups into alkenes, for example, NO 2 , COR, etc., leads to a decrease in the *-level, due to which the alkene acquires the ability to react with medium-strength nucleophiles (ammonia, RO - , NєC - , enolate anion, etc.).
As a result of the interaction of the electrophilic agent E + with an alkene, a highly reactive carbocation is formed. The carbocation is further stabilized by the rapid addition of the nucleophilic agent Nu - :
Since the addition of an electrophile is a slow step, the process of addition of any polar agent E + Nu - should be considered precisely as an electrophilic addition to the multiple bond of the alkene. A large number of reactions of this type are known, where the role of the electrophilic agent is played by halogens, hydrogen halides, water, divalent mercury salts, and other polar reagents. Electrophilic addition to a double bond in the classification of organic reaction mechanisms has the symbol Ad E ( Addition Electrophilic) and, depending on the number of reacting molecules, is designated as Ad E 2 (bimolecular reaction) or Ad E 3 (trimolecular reaction).
4.3.a. Addition of halogens
Alkenes react with bromine and chlorine to form addition products at the double bond of one halogen molecule in a yield close to quantitative. Fluorine is too active and causes the destruction of alkenes. The addition of iodine to alkenes in most cases is a reversible reaction, the equilibrium of which is shifted towards the starting reagents.
The rapid discoloration of a solution of bromine in CCl 4 is one of the simplest tests for unsaturation, since alkenes, alkynes, and dienes react rapidly with bromine.
The addition of bromine and chlorine to alkenes occurs by an ionic rather than a radical mechanism. This conclusion follows from the fact that the rate of halogen addition does not depend on irradiation, the presence of oxygen, and other reagents that initiate or inhibit radical processes. Based on a large number of experimental data, a mechanism was proposed for this reaction, which includes several successive stages. At the first stage, the polarization of the halogen molecule occurs under the action of bond electrons. The halogen atom, acquiring some fractional positive charge, forms an unstable intermediate with electrons - bonds, called - complex or charge transfer complex. It should be noted that in the -complex, the halogen does not form a directional bond with any particular carbon atom; in this complex, the donor-acceptor interaction of the electron pair -bond as a donor and halogen as an acceptor is simply realized.
Further, the -complex is converted into a cyclic bromonium ion. In the process of formation of this cyclic cation, a heterolytic cleavage of the Br-Br bond occurs and an empty R-orbital sp 2 -hybridized carbon atom overlaps with R-orbital of the "lone pair" of electrons of the halogen atom, forming a cyclic bromonium ion.
At the last, third stage, the bromine anion, as a nucleophilic agent, attacks one of the carbon atoms of the bromonium ion. Nucleophilic attack by the bromide ion leads to the opening of the three-membered ring and the formation of a vicinal dibromide ( vic-beside). This step can be formally considered as a nucleophilic substitution of S N 2 at the carbon atom, where the leaving group is Br+ .
The addition of halogens to the double bond of alkenes is one of the formally simple model reactions, by the example of which one can consider the influence of the main factors that allow one to draw reasonable conclusions about the detailed mechanism of the process. For reasonable conclusions about the mechanism of any reaction, one should have data on: 1) reaction kinetics; 2) stereochemistry (stereochemical result of the reaction); 3) the presence or absence of an associated, competing process; 4) the effect of substituents in the initial substrate on the reaction rate; 5) the use of labeled substrates and (or) reagents; 6) the possibility of rearrangements during the reaction; 7) the influence of the solvent on the reaction rate.
Let us consider these factors using the example of halogenation of alkenes. Kinetic data make it possible to establish the order of the reaction for each component and, on this basis, to draw a conclusion about the overall molecularity of the reaction, i.e., about the number of reacting molecules.
For the bromination of alkenes, the reaction rate is generally described by the following equation:
v = k`[alkene] + k``[alkene] 2 ,
which in rare cases is simplified to
v = k`[alkene].
Based on the kinetic data, it can be concluded that one or two bromine molecules are involved in the rate-determining step. The second order in terms of bromine means that it is not the bromide ion Br - that reacts with the bromonium ion, but the tribromide ion formed during the interaction of bromine and the bromide ion:
This balance is shifted to the right. The kinetic data do not allow any other conclusions to be drawn about the structure of the transition state and the nature of the electrophilic species in the double bond halogen addition reaction. The most valuable information about the mechanism of this reaction is provided by data on the stereochemistry of addition. The addition of a halogen to a double bond is a stereospecific process (a process in which only one of the possible stereoisomers is formed; in a stereoselective process, one stereomer is predominantly formed) anti-additions for alkenes and cycloalkenes, in which the double bond is not conjugated to the benzene ring. For cis- and trance-isomers of butene-2, pentene-2, hexene-3, cyclohexene, cyclopentene and other alkenes, the addition of bromine occurs exclusively as anti- accession. In the case of cyclohexene, only trance-1,2-dibromocyclohexane (mixture of enantiomers).
The trans arrangement of bromine atoms in 1,2-dibromocyclohexane can be depicted in a simplified way with respect to the average plane of the cyclohexane ring (without taking into account conformations):
When bromine is added to cyclohexene, it initially forms trance-1,2-dibromocyclohexane in a, a-conformation, which then immediately passes into an energetically more favorable her-conformation. Anti- addition of halogens to the double bond makes it possible to reject the mechanism of one-step synchronous addition of one halogen molecule to the double bond, which can only be carried out as syn- accession. Anti- addition of a halogen is also inconsistent with the formation of an open carbocation RCH + -CH 2 Hal as an intermediate. In an open carbocation, free rotation around the C-C bond is possible, which should lead after the attack of the Br anion - to form a mixture of products anti- and so syn- connections. stereospecific anti- addition of halogens was the main reason for the creation of the concept of bromonium or chloronium ions as discrete intermediate particles. This concept perfectly satisfies the rule anti-addition, since the nucleophilic attack of the halide ion is possible with anti-sides on any of the two carbon atoms of the halonium ion by the S N 2 mechanism.
In the case of unsymmetrically substituted alkenes, this should result in two enantiomers treo-form upon addition of bromine to cis-isomer or to enantiomers erythro-forms during halogenation trance-isomer. This is indeed observed when bromine is added, for example, to cis- and trance-pentene-2 isomers.
In the case of bromination of symmetrical alkenes, for example, cis- or trance-hexenes-3 should form or a racemate ( D,L-form), or meso-form of the final dibromide, which is actually observed.
There is independent, direct evidence for the existence of halonium ions in a non-nucleophilic, indifferent medium at low temperatures. Using NMR spectroscopy, the formation of bromonium ions was recorded during the ionization of 3-bromo-2-methyl-2-fluorobutane under the action of a very strong Lewis acid antimony pentafluoride in a solution of liquid sulfur dioxide at -80 0 C.
This cation is quite stable at -80 0 C in a non-nucleophilic medium, but is instantly destroyed by the action of any nucleophilic agents or by heating.
Cyclic bromonium ions can sometimes be isolated in pure form if spatial obstacles prevent their opening under the action of nucleophiles:
It is clear that the possibility of the existence of bromonium ions, which are quite stable under special conditions, cannot serve as direct evidence of their formation in the reaction of the addition of bromine to the double bond of an alkene in alcohol, acetic acid, and other electron-donating solvents. Such data should be considered only as an independent confirmation of the fundamental possibility of the formation of halonium ions in the process of electrophilic addition to the double bond.
The concept of the halonium ion provides a rational explanation for the reversibility of the addition of iodine to the double bond. The halonium cation has three electrophilic centers accessible to nucleophilic attack by the halide anion: two carbon atoms and a halogen atom. In the case of chloronium ions, the Cl - anion appears to predominantly or even exclusively attack the carbon centers of the cation. For the bromonium cation, both directions of opening of the halonium ion are equally probable, both due to the attack of the bromide ion on both carbon atoms and on the bromine atom. Nucleophilic attack on the bromine atom of the bromonium ion leads to the initial reagents bromine and alkene:
The iodonium ion opens mainly as a result of the attack of the iodide ion on the iodine atom, and therefore the equilibrium between the initial reagents and the iodonium ion is shifted to the left.
In addition, the end product of the addition, vicinal diiodide, can undergo a nucleophilic attack on the iodine atom by the triiodide anion present in the solution, which also leads to the formation of the initial reagents alkene and iodine. In other words, under the conditions of the addition reaction, the resulting vicinal diiodide is deiodinated under the action of the triiodide anion. The vicinal dichlorides and dibromides are not dehalogenated under the conditions of the addition reaction of chlorine or bromine, respectively, to alkenes.
Anti-addition of chlorine or bromine is characteristic of alkenes, in which the double bond is not conjugated with the -electrons of the benzene ring. For styrene, stilbene and their derivatives along with anti-addition takes place and syn- addition of a halogen, which in a polar environment can even become dominant.
In cases where the addition of a halogen to a double bond is carried out in a medium of nucleophilic solvents, the solvent effectively competes with the halide ion in opening the three-membered ring of the halonium ion:
The formation of addition products with the participation of a solvent or some other "external" nucleophilic agent is called a conjugated addition reaction. When bromine and styrene react in methanol, two products are formed: vicinal dibromide and bromoether, the ratio of which depends on the concentration of bromine in methanol
In a highly dilute solution, the product of conjugated addition dominates, while in a concentrated solution, on the contrary, the predominant vicinal dibromide. In an aqueous solution, halohydrin (an alcohol containing a halogen at the -carbon atom) always predominates - the product of conjugated addition.
her-conformer trance-2-chlorocyclohexanol additionally stabilized by O-H hydrogen bond . . . Cl. In the case of unsymmetrical alkenes, in conjugated addition reactions, the halogen always adds to the carbon atom containing the largest number of hydrogen atoms, and the nucleophilic agent to the carbon with fewer hydrogen atoms. An isomeric product with a different arrangement of the joining groups is not formed. This means that the cyclic halonium ion formed as an intermediate should have an asymmetric structure with two C 1 -Hal and C 2 -Hal bonds differing in energy and strength and a large positive charge on the internal C 2 carbon atom, which can be graphically expressed in two ways:
Therefore, the carbon atom C 2 of the halonium ion is subjected to nucleophilic attack by the solvent, despite the fact that it is more substituted and sterically less accessible.
One of the best preparative methods for the synthesis of bromhydrins is the hydroxybromination of alkenes with N-bromosuccinimide ( NBS) in a binary mixture of dimethyl sulfoxide ( DMSO) and water.
This reaction can be carried out in water and without DMSO, however, the yields of bromhydrins in this case are somewhat lower.
The formation of conjugated addition products in the alkene halogenation reaction also makes it possible to reject the synchronous mechanism of addition of one halogen molecule. The conjugated addition to the double bond is in good agreement with the two-step mechanism involving the halonium cation as an intermediate.
For the reaction of electrophilic addition to the double bond, one should expect an increase in the reaction rate in the presence of electron-donating alkyl substituents and its decrease in the presence of electron-withdrawing substituents at the double bond. Indeed, the rate of addition of chlorine and bromine to the double bond increases sharply upon passing from ethylene to its methyl-substituted derivatives. For example, the rate of addition of bromine to tetramethylethylene is 10 5 times higher than the rate of its addition to butene-1. Such a huge acceleration definitely indicates a high polarity of the transition state and a high degree of charge separation in the transition state and is consistent with an electrophilic addition mechanism.
In some cases, the addition of chlorine to alkenes containing electron-donating substituents is accompanied by the elimination of a proton from the intermediate instead of the addition of a chloride ion. Elimination of a proton leads to the formation of a chlorine-substituted alkene, which can formally be considered as a direct substitution with double bond migration. However, experiments with isotopic labeling indicate a more complex nature of the transformations occurring here. When isobutylene is chlorinated at 0 0 C, 2-methyl-3-chloropropene (metallyl chloride) is formed instead of the expected dichloride - the product of addition to the double bond.
Formally, it seems that there is a substitution, not an addition. The study of this reaction using isobutylene labeled at position 1 with the 14 C isotope showed that direct replacement of hydrogen by chlorine does not occur, since the label is in the 14 CH 2 Cl group in the formed metallyl chloride. This result can be explained by the following sequence of transformations:
In some cases, 1,2-migration of the alkyl group can also occur
In CCl 4 (non-polar solvent) this reaction gives almost 100% dichloride B- the product of a conventional double bond addition (without rearrangement).
Skeletal rearrangements of this type are most characteristic of processes involving open carbocations as intermediate particles. It is possible that the addition of chlorine in these cases does not occur through the chloronium ion, but through a cationic particle close to an open carbocation. At the same time, it should be noted that skeletal rearrangements are a rather rare phenomenon in the addition of halogens and mixed halogens to the double bond: they are more often observed in the addition of chlorine and much less frequently in the addition of bromine. The probability of such rearrangements increases upon passing from nonpolar solvents (СCl 4) to polar ones (nitromethane, acetonitrile).
Summarizing the above data on stereochemistry, conjugated addition, the effect of substituents in the alkene, and rearrangements in the addition reactions of halogens at the double bond, it should be noted that they are in good agreement with the electrophilic addition mechanism involving the cyclic halonium ion. The data on the addition of mixed halogens to alkenes, for which the addition steps are determined by the polarity of the bond of two halogen atoms, can be interpreted in the same way.
Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the cycle also have an unsaturated character ( cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the cycle (three or four atoms). The property of "unsaturation" is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated or saturated hydrocarbons - alkanes.
The structure of alkenes
Acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula СnН2n. Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils.
Carbon atoms between which there is a double bond are in a state of sp 2 hybridization. This means that one s- and two p-orbitals participate in hybridization, while one p-orbital remains unhybridized. The overlap of hybrid orbitals leads to the formation of a σ-bond, and due to unhybridized p-orbitals
neighboring carbon atoms, a second, π-bond is formed. Thus, a double bond consists of one σ- and one π-bond. The hybrid orbitals of the atoms that form a double bond are in the same plane, and the orbitals that form a π bond are located perpendicular to the plane of the molecule. A double bond (0.132 im) is shorter than a single bond, and its energy is greater, since it is more durable. However, the presence of a mobile, easily polarizable π-bond leads to the fact that alkenes are chemically more active than alkanes and are able to enter into addition reactions.
The structure of ethylene
Double bond formation in alkenes
Homologous series of ethene
Unbranched alkenes form the homologous series of ethene ( ethylene): C 2 H 4 - ethene, C 3 H 6 - propene, C 4 H 8 - butene, C 5 H 10 - pentene, C 6 H 12 - hexene, C 7 H 14 - heptene, etc.
Isomerism of alkenes
Alkenes are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:
A special type of structural isomerism is the double bond position isomerism:
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis and transisomerism.
Cis isomers differ from trans isomers the spatial arrangement of fragments of the molecule (in this case, methyl groups) relative to the plane of the π-bond, and, consequently, the properties.
Alkene nomenclature
1. Selecting the main circuit. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.
2. Numbering of atoms of the main chain. The numbering of the atoms of the main chain starts from the end to which the double bond is closest.
For example, the correct connection name is:
If the position of the double bond cannot determine the beginning of the numbering of atoms in the chain, then it determines the position of the substituents in the same way as for saturated hydrocarbons.
3. Name formation. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix -en, denoting that the compound belongs to the class of alkenes. For example:
Physical properties of alkenes
The first three representatives of the homologous series of alkenes are gases; substances of the composition C5H10 - C16H32 - liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.
Chemical properties of alkenes
Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism electrophilic addition.
1. Hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:
This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, since it is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (CC14) leads to a rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihaloalkanes.
3. Hydrohalogenation (addition of hydrogen halide).
This reaction is subject to
When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom at which there are more hydrogen atoms, and a halogen to a less hydrogenated one.
4. Hydration (water addition). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol.
Note that a primary alcohol (with a hydroxo group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols.
This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is added to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
This addition reaction proceeds by a free radical mechanism.
Oxidation reactions.
1. Combustion. Like any organic compounds, alkenes burn in oxygen to form CO2 and H2O:
2. Oxidation in solutions. Unlike alkanes, alkenes are easily oxidized by the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:
ALKENES
Hydrocarbons, in the molecule of which, in addition to simple carbon-carbon and carbon-hydrogen σ-bonds, there are carbon-carbon π-bonds, are called unlimited. Since the formation of a π bond is formally equivalent to the loss of two hydrogen atoms by a molecule, unsaturated hydrocarbons contain 2p fewer hydrogen atoms than the limit, where P - number of π-bonds:
A series whose members differ from each other by (2H) n is called isological side. So, in the above scheme, the isologues are hexanes, hexenes, hexadienes, hexines, hexatrienes, etc.
Hydrocarbons containing one π-bond (i.e. double bond) are called alkenes (olefins) or, according to the first member of the series - ethylene, ethylene hydrocarbons. The general formula for their homologous series C p H 2n.
1. Nomenclature
In accordance with the rules of IUPAC, when constructing the names of alkenes, the longest carbon chain containing a double bond receives the name of the corresponding alkane, in which the ending -an changed to -en. This chain is numbered in such a way that the carbon atoms involved in the formation of a double bond receive the smallest number possible:
Radicals are named and numbered as in the case of alkanes.
For alkenes of relatively simple structure, simpler names are allowed. So, some of the most common alkenes are called by adding the suffix -en to the name of a hydrocarbon radical with the same carbon skeleton:
Hydrocarbon radicals formed from alkenes receive the suffix -enyl. The numbering in the radical starts from the carbon atom that has a free valence. However, for the simplest alkenyl radicals, instead of systematic names, it is allowed to use trivial ones:
Hydrogen atoms directly bonded to unsaturated carbon atoms forming a double bond are often referred to as vinyl hydrogen atoms,
2. Isomerism
In addition to the isomerism of the carbon skeleton, in the series of alkenes there is also the isomerism of the position of the double bond. In general, isomerism of this type - substituent position isomerism (functions)- is observed in all cases when there are any functional groups in the molecule. For alkane C 4 H 10, two structural isomers are possible:
For alkene C 4 H 8 (butene), three isomers are possible:
Butene-1 and butene-2 are isomers of the position of the function (in this case, its role is played by a double bond).
Spatial isomers differ in the spatial arrangement of substituents relative to each other and are called cis isomers, if the substituents are on the same side of the double bond, and trans isomers, if on opposite sides:
3. Double bond structure
The breaking energy of a molecule at the C=C double bond is 611 kJ/mol; since the energy of the σ-bond C-C is 339 kJ / mol, the energy of breaking the π bond is only 611-339 = 272 kJ / mol. π-electrons are much easier than σ-electrons to be influenced, for example, by polarizing solvents or by any attacking reagents. This is explained by the difference in the symmetry of the distribution of the electron cloud of σ- and π-electrons. The maximum overlap of p-orbitals and, consequently, the minimum free energy of the molecule are realized only with a planar structure of the vinyl fragment and with a shortened C-C distance equal to 0.134 nm, i.e. much smaller than the distance between carbon atoms connected by a single bond (0.154 nm). With the rotation of the "halves" of the molecule relative to each other along the axis of the double bond, the degree of overlapping of the orbitals decreases, which is associated with the expenditure of energy. The consequence of this is the absence of free rotation along the axis of the double bond and the existence of geometric isomers with the corresponding substitution at carbon atoms.
Continuation. For the beginning, see № 15, 16, 17, 18, 19/2004
Lesson 9
Chemical properties of alkenes
The chemical properties of alkenes (ethylene and its homologues) are largely determined by the presence of d ... bonds in their molecules. Alkenes enter into reactions of all three types, and the most characteristic of them are reactions p .... Consider them using propylene C 3 H 6 as an example.
All addition reactions proceed through a double bond and consist in the splitting of the α-bond of the alkene and the formation of two new α-bonds at the site of the break.
Addition of halogens:
Addition of hydrogen(hydrogenation reaction):
Water connection(hydration reaction):
Addition of hydrogen halides (HHal) and water to unsymmetrical alkenes according to the rule of V.V. Markovnikov (1869). Hydrogen acid Hhal attaches to the most hydrogenated carbon atom at the double bond. Accordingly, the Hal residue binds to the C atom, which has a smaller number of hydrogen atoms.
Combustion of alkenes in air.
When ignited, alkenes burn in air:
2CH 2 \u003d CHCH 3 + 9O 2 6CO 2 + 6H 2 O.
Gaseous alkenes form explosive mixtures with atmospheric oxygen.
Alkenes are oxidized by potassium permanganate in an aqueous medium, which is accompanied by discoloration of the KMnO 4 solution and the formation of glycols (compounds with two hydroxyl groups at adjacent C atoms). This process - hydroxylation of alkenes:
Alkenes are oxidized by atmospheric oxygen to epoxides. when heated in the presence of silver catalysts:
Polymerization of alkenes- the binding of many alkene molecules to each other. Reaction conditions: heating, presence of catalysts. The connection of molecules occurs by splitting intramolecular-bonds and the formation of new intermolecular-bonds:
In this reaction, the range of values n = 10 3 –10 4 .
Exercises.
1. Write the reaction equations for butene-1 with: a) Br2; b) HBr; in) H2O; G) H2. Name the reaction products.
2.
Conditions are known under which the addition of water and hydrogen halides to the double bond of alkenes proceeds against the Markovnikov rule. Write reaction equations
3-bromopropylene according to anti-Markovnikov with: a) water; b) hydrogen bromide.
3.
Write the equations for polymerization reactions: a) butene-1; b) vinyl chloride CH 2 =CHCl;
c) 1,2-difluoroethylene.
4. Make the equations for the reactions of ethylene with oxygen for the following processes: a) combustion in air; b) hydroxylation with water KMnO 4 ; c) epoxidation (250 °C, Ag ).
5. Write the structural formula of an alkene, knowing that 0.21 g of this compound can add 0.8 g of bromine.
6. When burning 1 liter of gaseous hydrocarbon, which decolorizes the raspberry solution of potassium permanganate, 4.5 liters of oxygen are consumed, and 3 liters are obtained CO2. Write the structural formula for this hydrocarbon.
Lesson 10
Obtaining and using alkenes
Reactions for obtaining alkenes are reduced to reversing the reactions representing the chemical properties of alkenes (their flow from right to left, see lesson 9). You just need to find the right conditions.
Elimination of two halogen atoms from dihaloalkanes containing halogens at neighboring C atoms. The reaction proceeds under the action of metals (Zn, etc.):
Cracking of saturated hydrocarbons. So, during cracking (see lesson 7) of ethane, a mixture of ethylene and hydrogen is formed:
Dehydration of alcohols. When alcohols are treated with water-removing agents (concentrated sulfuric acid) or when heated to 350 ° C in the presence of catalysts, water is split off and alkenes are formed:
In this way, ethylene is obtained in the laboratory.
An industrial method for producing propylene, along with cracking, is the dehydration of propanol over alumina:
Dehydrochlorination of chloroalkanes is carried out under the action of an alkali solution in alcohol, because In water, the reaction products are not alkenes, but alcohols.
The use of ethylene and its homologues based on their chemical properties, i.e., the ability to turn into various useful substances.
Motor fuels, with high octane numbers, are obtained by hydrogenation of branched alkenes:
Discoloration of a yellow solution of bromine in an inert solvent (CCl 4) occurs when a drop of alkene is added or a gaseous alkene is passed through the solution. Interaction with bromine - characteristic qualitative reaction to the double bond:
The product of ethylene hydrochlorination, chloroethane, is used in chemical synthesis to introduce the C 2 H 5 group into the molecule:
Chloroethane also has a local anesthetic (pain relieving) effect, which is used in surgical operations.
Alcohols are obtained by hydration of alkenes, for example, ethanol:
Alcohol C 2 H 5 OH is used as a solvent, for disinfection, in the synthesis of new substances.
Hydration of ethylene in the presence of an oxidizing agent [O] leads to ethylene glycol - antifreeze and intermediate product of chemical synthesis:
Ethylene is oxidized to produce ethylene oxide and acetaldehyde. raw materials in the chemical industry:
Polymers and plastics- products of polymerization of alkenes, for example, polytetrafluoroethylene (Teflon):
Exercises.
1. Complete the equations for the reactions of elimination (cleavage), name the resulting alkenes:
2.
Make the equations for hydrogenation reactions: a) 3,3-dimethylbutene-1;
b) 2,3,3-trimethylbutene-1. These reactions produce alkanes used as motor fuels, give them names.
3. 100 g of ethyl alcohol was passed through a tube filled with heated alumina. C 2 H 5 OH. This resulted in 33.6 liters of hydrocarbon (n.o.s.). How much alcohol (in%) reacted?
4. How many grams of bromine will react with 2.8 liters (n.o.s.) of ethylene?
5. Write an equation for the polymerization of trifluorochloroethylene. (The resulting plastic is resistant to hot sulfuric acid, metallic sodium, etc.)
Answers to exercises for topic 1
Lesson 9
5. Reaction of alkene C n H2 n with bromine in general:
Molar mass of alkene M(FROM n H2 n) = 0.21 160/0.8 = 42 g/mol.
This is propylene.
Answer.
The alkene formula is CH 2 \u003d CHCH 3 (propylene).
6. Since all the substances involved in the reaction are gases, the stoichiometric coefficients in the reaction equation are proportional to their volume ratios. Let's write the reaction equation:
FROM a H in+ 4.5O 2 3CO 2 + 3H 2 O.
The number of water molecules is determined by the reaction equation: 4.5 2 = 9 O atoms reacted, 6 O atoms are bound in CO 2, the remaining 3 O atoms are part of three H 2 O molecules. Therefore, the indices are equal: a = 3, in\u003d 6. The desired hydrocarbon is propylene C 3 H 6.
Answer.
The structural formula of propylene is CH 2 = CHCH 3.
Lesson 10
1. Elimination (cleavage) reaction equations - synthesis of alkenes:
2. Hydrogenation reactions of alkenes when heated under pressure in the presence of a catalyst:
3. The reaction of dehydration of ethyl alcohol has the form:
Here through X the mass of alcohol converted to ethylene is indicated.
Let's find the value X: X\u003d 46 33.6 / 22.4 \u003d 69 g.
The proportion of reacted alcohol was: 69/100 = 0.69, or 69%.
Answer.
69% alcohol reacted.
4.
Since the stoichiometric coefficients in front of the formulas of the reactants (C 2 H 4 and Br 2) are equal to one, the relation is valid:
2,8/22,4 = X/160. From here X= 20 g Br 2 .
Answer.
20 g Br 2 .
Alkenes are unsaturated hydrocarbons, which have one double bond between atoms. Their other name is olefins, it is associated with the history of the discovery of this class of compounds. Basically, these substances do not occur in nature, but are synthesized by man for practical purposes. In the IUPAC nomenclature, the name of these compounds is formed according to the same principle as for alkanes, only the suffix “an” is replaced by “ene”.
In contact with
The structure of alkenes
The two carbon atoms involved in the formation of a double bond are always in sp2 hybridization, and the angle between them is 120 degrees. The double bond is formed by overlapping π -π orbitals, and it is not very strong, so this bond is quite easy to break, which is used in the chemical properties of substances.
isomerism
Compared with the limiting ones, in these hydrocarbons it is possible more kinds, both spatial and structural. Structural isomerism can also be divided into several types.
The first also exists for alkanes, and consists in a different order of connection of carbon atoms. So pentene-2 and 2-methylbutene-2 can be isomers. And the second is a change in the position of the double bond.
Spatial isomerism in these compounds is possible due to the appearance of a double bond. It is of two types - geometric and optical.
Geometric isomerism is one of the most common types in nature, and almost always geometric isomers will have radically different physical and chemical properties. Distinguish cis and trans isomers. In the former, the substituents are located on one side of the multiple bond, while in the trans isomers they are in different planes.
Obtaining alkenes
They were first obtained, like many other substances, quite by accident.
The German chemist and researcher Becher at the end of the 17th century studied the effect of sulfuric acid on ethyl alcohol and realized that got unknown gas, which is more reactive than methane.
Later, several more scientists conducted similar studies, and they also learned that this gas, when interacting with chlorine, forms an oily substance.
Therefore, initially this class of compounds was named olefins, which translates as oily. But still, scientists could not determine the composition and structure of this compound. This happened only almost two centuries later, at the end of the nineteenth century.
Currently, there are many ways to obtain alkenes.
industrial methods
Receipt industrial methods:
- Dehydrogenation of saturated hydrocarbons. This reaction is possible only under the action of high temperatures (about 400 degrees) and catalysts - either chromium oxide 3 or aluminum-platinum catalysts.
- Dehalogenation of dihaloalkanes. Occurs only in the presence of zinc or magnesium, and at high temperatures.
- Dehydrohalogenation of haloalkanes. It is carried out using sodium or potassium salts of organic acids at elevated temperatures.
Important! These methods for obtaining alkenes do not give a pure product; the result of the reaction will be a mixture of unsaturated hydrocarbons. The compound prevailing among them is determined using the Zaitsev rule. It states that hydrogen is most likely to be split off from the carbon atom that has the fewest bonds to hydrogens.
Dehydration of alcohols. It can only be carried out when heated and in the presence of solutions of strong mineral acids that have a water-removing property.
hydrogenation of alkynes. Possible only in the presence of paladium catalysts.
Chemical properties of alkenes
Alkenes are very chemically active substances. This is largely due to the presence of a double bond. The most characteristic reactions for this class of compounds are electrophilic and radical addition.
- Alkene halogenation is a classic electrophilic addition reaction. It occurs only in the presence of inert organic solvents, most often carbon tetrachloride.
- Hydrohalogenation. Attachment of this type is carried out according to the Markovnikov rule. The ion attaches to the more hydrogenated carbon atom near the double bond, and accordingly, the halide ion attaches to the second carbon atom. This rule is violated in the presence of peroxide compounds - the Harrosh effect. The addition of hydrogen halide occurs completely opposite to the Markovnikov rule.
- Hydroboration. This reaction is of considerable practical importance. Therefore, the scientist who discovered and studied it even received the Nobel Prize. This reaction is carried out in several steps, while the addition of the boron ion does not occur according to the Markovnikov rule.
- Alkene hydration or addition. This reaction also proceeds according to Markovnikov's rule. The hydroxide ion adds to the least hydrogenated carbon atom at the double bond.
- Alkylation is another reaction often used in industry. It consists in the addition of saturated hydrocarbons to unsaturated ones under the influence of low temperatures and catalysts in order to increase the atomic mass of the compounds. The most common catalysts are strong mineral acids. Also, this reaction can proceed according to the free radical mechanism.
- The polymerization of alkenes is another uncharacteristic reaction for saturated hydrocarbons. It involves the connection of numerous molecules with each other in order to form a strong connection that differs in its physical properties.
n in this reaction is the number of molecules that have entered into a bond. A prerequisite for implementation is an acidic environment, elevated temperature and increased pressure.
Also, alkenes are characterized by other electrophilic addition reactions, which have not been widely used in practice.
For example, the addition reaction of alcohols, with the formation of ethers.
Or the addition of acid chlorides, with the production of unsaturated ketones - the Kondakov reaction.
Note! This reaction is only possible in the presence of a zinc chloride catalyst.
The next major class of reactions characteristic of alkenes is radical addition reactions. These reactions are possible only with the formation of free radicals under the influence of high temperatures, radiation and other actions. The most typical radical addition reaction is hydrogenation with the formation of saturated hydrocarbons. It occurs exclusively under the influence of temperatures and in the presence of a platinum catalyst.
Due to the presence of a double bond, alkenes are very characteristic of various oxidation reactions.
- Combustion is a classic oxidation reaction. It goes well without catalysts. Depending on the amount of oxygen, various end products are possible: from carbon dioxide to carbon.
- Oxidation with potassium permanganate in a neutral medium. The products are polyhydric alcohols and a brown precipitate of manganese dioxide. This reaction is considered qualitative for alkenes.
- Also, mild oxidation can be carried out with hydrogen peroxide, osmium oxide 8, and other oxidizing agents in a neutral environment. For the mild oxidation of alkenes, only one bond is broken; the reaction product, as a rule, is polyhydric alcohols.
- Hard oxidation is also possible, in which both bonds are broken and acids or ketones are formed. An acidic environment is a prerequisite, most often sulfuric acid is used, since other acids can also undergo oxidation with the formation of by-products.
Physical properties of alkenes
The only gases under normal conditions are ethylene, propene and butene.
From pentene to heptodecene, all alkenes are in a liquid state.
And all the rest are solids.
Melting and boiling points increase proportionally with molecular weight, but may change for isomers.
All alkenes do not dissolve in water, but readily soluble in inert organic solvents.
Application of alkenes
Alkenes are quite widely used in industry and are used for the synthesis of a large number of substances. For example, with the help of ethylene, polyvinyl chloride (PVC), styrene, ethylene glycol, ethanol, polyethylene, rubbers and many other substances are synthesized. The largest volume of propylene is used to produce polypropylene.
Alkenes - structure, properties, applications
We study chemistry - properties of alkenes, application in industry
Conclusion
In general, we can say for sure that alkenes, due to their chemical properties, are very popular in industry. They are involved in the production of a wide variety of plastics, rubbers and many other substances.
