4. Chemical properties of alkenes

The energy of a double carbon-carbon bond in ethylene (146 kcal/mol) turns out to be significantly lower than twice the energy of a single C-C bond in ethane (2 88 = 176 kcal/mol). -S-S connection In ethylene, the -bonds are stronger, therefore reactions of alkenes, accompanied by the cleavage of the -bond with the formation of two new simple -bonds, are a thermodynamically favorable process. For example, in the gas phase, according to calculated data, all the reactions below are exothermic with a significant negative enthalpy, regardless of their actual mechanism.

From the point of view of the theory of molecular orbitals, it can also be concluded that the -bond is more reactive than the -bond. Let's consider the molecular orbitals of ethylene (Fig. 2).

Indeed, the bonding orbital of ethylene has more high 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, the *- and *-orbitals of ethylene are vacant. Consequently, 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 occurs 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 reducing the activation energy. It is necessary to distinguish between heterogeneous and homogeneous catalytic hydrogenation of alkenes. In heterogeneous hydrogenation, finely ground 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 are insoluble in organic media and act as heterogeneous catalysts. The most active among them are ruthenium and rhodium, but platinum and nickel are 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. The 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 hydrogenation. Other more active platinum group metals are used on inert supports, for example, Pd/C or Pd/BaSO 4, Ru/Al 2 O 3; Rh/C, etc. Palladium supported 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 so-called "Raney nickel". To obtain this catalyst, a nickel-aluminum alloy is treated with hot aqueous alkali to remove almost all aluminum and then with water until a neutral reaction. 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 the use of a pressure of the order of 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 cheaper. Below are some typical examples of heterogeneous catalytic hydrogenation of acyclic and cyclic alkenes:

Since both hydrogen atoms are added 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 connection is called syn- accession. In cases where two reagent fragments are added together various sides multiple bond (double or triple) occurs anti- accession. Terms syn- And anti- are equivalent in meaning to the terms cis- And trance-. To avoid confusion and misunderstanding 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 higher rate compared to many other functional groups (C=O, COOR, CN, etc.) and therefore hydrogenation of the C=C double bond is often a selective process if the hydrogenation is carried out under mild conditions (0- 20 0 C and at atmospheric pressure). Below are some typical examples:

The benzene ring is not reduced under these conditions.

Big and important 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 cleavage of single carbon-carbon bonds (hydrogenolysis). Homogeneous hydrogenation does not have these disadvantages. For recent years A large group of homogeneous hydrogenation catalysts - transition metal complexes containing various ligands - has been obtained. 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 is obtained by reacting 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 ability to selectively reduce a mono- or disubstituted double bond in the presence of a tri- and tetra-substituted double bond due to the large differences in their hydrogenation rates.

In the case of homogeneous catalysts, hydrogen addition also occurs as syn- accession. So recovery cis-butene-2 ​​with deuterium under these conditions leads to meso-2,3-dideuterobutane.

4.2. Reduction of a double bond using diimide

The reduction of alkenes to the corresponding alkanes can be successfully accomplished using diimide NH=NH.

Diimide is obtained by two main methods: the oxidation of hydrazine with hydrogen peroxide in the presence of Cu 2+ ions or the reaction of hydrazine with Ni-Raney (hydrazine dehydrogenation). If an alkene is present in the reaction mixture, its double bond is hydrogenated by the very unstable diimide. A distinctive feature of this method is the strict syn-stereospecificity of the restoration 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 HOMO and LUMO orbitals of alkenes are the occupied and empty * orbitals. Consequently, the -orbital will participate in reactions with electrophiles (E +), and the *-orbital of the C=C bond will participate in reactions with nucleophiles (Nu -) (see Fig. 3). In most cases, simple alkenes react easily with electrophiles, but 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 the -HOMO of alkenes, while the HOMO of most nucleophiles lies significantly below the *-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 nucleophiles of average strength (ammonia, RO - , Nє C - , enolate anion, etc.).

As a result of the interaction of the electrophilic agent E + with an alkene, a carbocation is formed, which is highly reactive. The carbocation is further stabilized by the rapid addition of the nucleophilic agent Nu - :

Since the slow stage is the addition of an electrophile, the process of addition of any polar agent E + Nu - should be considered precisely as an electrophilic addition to the multiple bond of an 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 with 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 original reagents.

The rapid decolorization of a solution of bromine in CCl4 serves as one of the simplest tests for unsaturation, since alkenes, alkynes, and dienes react quickly 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, including several sequential stages. At the first stage, polarization of the halogen molecule occurs under the action of bonding electrons. The halogen atom, which acquires a certain fractional positive charge, forms an unstable intermediate with the electrons of the -bond, called an -complex or a charge transfer complex. It should be noted that in the -complex the halogen does not form a directed bond with any specific carbon atom; In this complex, the donor-acceptor interaction of an electron pair - bond as a donor and a halogen as an acceptor is simply realized.

Next, the -complex transforms into a cyclic bromonium ion. During the formation of this cyclic cation, heterolytic cleavage of the Br-Br bond occurs and an empty r-the sp 2 orbital of the hybridized carbon atom overlaps with r-orbital of the “lone pair” of electrons of the halogen atom, forming a cyclic bromonium ion.

In 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-near). This step can formally be considered as a nucleophilic substitution of SN 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, using the example of which one can consider the influence of the main factors that allow one to draw reasoned conclusions about the detailed mechanism of the process. To make informed conclusions about the mechanism of any reaction, you 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 influence of substituents in the original substrate on the reaction rate; 5) use of labeled substrates and (or) reagents; 6) the possibility of rearrangements during the reaction; 7) the effect of the solvent on the reaction rate.

Let us consider these factors using the example of the halogenation of alkenes. Kinetic data make it possible to establish the order of the reaction for each component and, on this basis, draw a conclusion about the overall molecularity of the reaction, i.e., the number of reacting molecules.

For the bromination of alkenes, the reaction rate is typically 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 bromine means that it is not the bromide ion Br - that reacts with the bromonium ion, but the tribromide ion formed by the interaction of bromine and bromide ion:

This balance is shifted to the right. Kinetic data do not allow us to draw any other conclusions about the structure of the transition state and the nature of the electrophilic species in the reaction of halogen addition to the double bond. The most valuable information about the mechanism of this reaction is provided by data on the stereochemistry of the 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, the predominant formation of one stereomer is observed) 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 this case, 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 manner relative to the average plane of the cyclohexane ring (without taking into account conformations):

When bromine combines with cyclohexene, it initially forms trance-1,2-dibromocyclohexane in a,a-conformation, which then immediately transforms into an energetically more favorable her-conformation. Anti-the addition of halogens to a double bond allows us to reject the mechanism of one-step synchronous addition of one halogen molecule to a 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 to the attack of the Br anion - to the formation of a mixture of products as anti- and so syn- accessions. Stereospecific anti-the addition of halogens was main reason creating the concept of bromonium or chloronium ions as discrete intermediate species. This concept perfectly satisfies the rule anti-addition, since nucleophilic attack of the halide ion is possible with anti-sides at either of the two carbon atoms of the halide ion via the S N 2 mechanism.

In the case of unsymmetrically substituted alkenes, this should result in two enantiomers trio-form upon addition of bromine to cis-isomer or enantiomer erythro-forms upon halogenation trance-isomer. This is actually observed when bromine is added to, for example, cis- And trance-isomers of pentene-2.

In the case of bromination of symmetrical alkenes, for example, cis- or trance-hexene-3 should be formed or a racemate ( D,L-form), or meso-form of the final dibromide, which is what is actually observed.

There is independent, direct evidence of the existence of halogenium ions in a non-nucleophilic, indifferent environment at low temperature. 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 of 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 environment, but is instantly destroyed by the action of any nucleophilic agents or upon heating.

Cyclic bromonium ions can sometimes be isolated in pure form if steric 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 bromine addition to the double bond of an alkene in alcohol, acetic acid and other electron-donating solvents. Such data should be considered only as independent confirmation of the fundamental possibility of the formation of halogenium ions in the process of electrophilic addition at the double bond.

The concept of the halide ion allows us to provide a rational explanation for the reversibility of the addition of iodine to the double bond. The halogenium 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 preferentially or even exclusively attack the carbon centers of the cation. For the bromonium cation, both directions of opening of the halogenium 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 starting reagents bromine and alkene:

The iodonium ion is revealed predominantly as a result of the attack of the iodide ion on the iodine atom, and therefore the equilibrium between the starting reagents and the iodonium ion is shifted to the left.

Besides, final product addition - the vicinal diiodide can undergo nucleophilic attack at 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. Vicinal dichlorides and dibromides do not dehalogenate under the conditions of the addition 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- accession takes place and syn-addition of a halogen, which can even become dominant in a polar environment.

In cases where the addition of a halogen to a double bond is carried out in a nucleophilic solvent environment, the solvent effectively competes with the halide ion in opening the three-membered ring of the halogenium ion:

The formation of addition products with the participation of a solvent or some other “external” nucleophilic agent is called a conjugate addition reaction. When bromine and styrene react in methanol, two products are formed: vicinal dibromide and bromine ester, the ratio of which depends on the concentration of bromine in methanol

In a highly dilute solution, the conjugate addition product dominates, while in a concentrated solution, on the contrary, vicinal dibromide predominates. In an aqueous solution, halohydrin (an alcohol containing a halogen at the -carbon atom) - the product of conjugate addition - always predominates.

her-conformer trance-2-chlorocyclohexanol is further stabilized by an O-H hydrogen bond . . . Cl. In the case of unsymmetrical alkenes, in conjugate 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 the least number of hydrogen atoms. An isomeric product with a different arrangement of joining groups is not formed. This means that the cyclic halogenonium ion formed as an intermediate must have an asymmetric structure with two bonds C 1 -Hal and C 2 -Hal that differ in energy and strength and a large positive charge on the internal carbon atom C 2, which can be graphically expressed in two ways:

Therefore, the C2 carbon atom of the halogenium ion is subject 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 bromohydrins is the hydroxybromination of alkenes using N-bromosuccinimide ( N.B.S.) in a binary mixture of dimethyl sulfoxide ( DMSO) and water.

This reaction can be carried out in water or without DMSO, however, the yields of bromohydrins in this case are somewhat lower.

The formation of conjugate addition products in the halogenation reaction of alkenes also allows us to reject the synchronous mechanism of addition of one halogen molecule. Conjugate addition to the double bond is in good agreement with a two-step mechanism involving the halogenium cation as an intermediate.

For the reaction of electrophilic addition to a double bond, one should expect an increase in the reaction rate in the presence of electron-donating alkyl substituents and a 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 when moving 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 1-butene. This enormous acceleration clearly indicates the high polarity of the transition state and the high degree of charge separation in the transition state and is consistent with the eletrophilic mechanism of addition.

In some cases, the addition of chlorine to alkenes containing electron-donating substituents is accompanied by the abstraction of a proton from the intermediate compound instead of the addition of a chloride ion. The abstraction of a proton results in the formation of a chlorine-substituted alkene, which can formally be considered as a direct substitution with double bond migration. However, experiments with isotope labeling indicate more complex character transformations taking place 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 at the double bond.

Formally, it seems as if there is a substitution, not an accession. The study of this reaction using isobutylene labeled at position 1 with the 14 C isotope showed that direct replacement of hydrogen with chlorine does not occur, since in the resulting metallyl chloride the label is located in the 14 CH 2 Cl group. This result can be explained by the following sequence of transformations:

In some cases, 1,2-migration of the alkyl group may also occur

In CCl 4 (non-polar solvent) this reaction gives almost 100% dichloride B- product of ordinary addition at a double bond (without rearrangement).

Skeletal rearrangements of this type are most typical for processes involving open carbocations as intermediate particles. It is possible that the addition of chlorine in these cases occurs not through the chloronium ion, but through a cationic particle close to the open carbocation. At the same time, it should be noted that skeletal rearrangements are a rather rare phenomenon in the processes of addition of halogens and mixed halogens at the double bond: they are more often observed during the addition of chlorine and much less frequently during the addition of bromine. The probability of such rearrangements increases when moving from non-polar solvents (CCl 4) to polar ones (nitromethane, acetonitrile).

Summarizing the presented data on stereochemistry, conjugate addition, the influence of substituents in the alkene, as well as rearrangements in the addition reactions of halogens at the double bond, it should be noted that they are in good agreement with the mechanism of electrophilic addition involving the cyclic halogenium ion. Data on the addition of mixed halogens to alkenes, for which the stages of addition are determined by the polarity of the bond of two halogen atoms, can be interpreted in the same way.

Let's find out what the alkene hydration reaction is. For this we will give brief description of this class hydrocarbons.

General formula

Alkenes are unsaturated organic compounds with the general formula SpH2n, the molecules of which have one double bond and also contain single (simple) bonds. The carbon atoms are in the sp2 hybrid state. Representatives of this class are called ethylene, since the ancestor of this series is ethylene.

Features of the nomenclature

In order to understand the mechanism of alkene hydration, it is necessary to highlight the features of their names. According to systematic nomenclature, when naming an alkene, a certain algorithm of actions is used.

First, you need to determine the longest carbon chain that includes a double bond. The numbers indicate the location of hydrocarbon radicals, starting with the smallest in the Russian alphabet.

If there are several identical radicals in the molecule, the qualifying prefixes di-, tri-, and tetra are added to the name.

Only after this the chain of carbon atoms itself is named, adding the suffix -ene at the end. To clarify the location of an unsaturated (double) bond in a molecule, it is indicated by a number. For example, 2methylpentene-2.

Hybridization in alkenes

To handle the following type of task: “Install molecular formula alkene, by hydration of which a secondary alcohol was obtained,” it is necessary to clarify the structural features of representatives of this class of hydrocarbons. The presence of a double bond explains the ability of CxHy to enter into addition reactions. The angle between double bonds is 120 degrees. No rotation is observed in the unsaturated bond, so representatives of this class are characterized by geometric isomerism. The main reaction site in alkene molecules is the double bond.

Physical properties

They are similar to saturated hydrocarbons. The lower representatives of this class of organic hydrocarbons are gaseous substances under normal conditions. Next, a gradual transition to liquids is observed, and alkenes, whose molecules contain more than seventeen carbon atoms, are characterized by a solid state. All compounds of this class have insignificant solubility in water, while they are perfectly soluble in polar organic solvents.

Features of isomerism

The presence of ethylene compounds in molecules explains their diversity structural formulas. In addition to the isomerization of the carbon skeleton, which is characteristic of representatives of all classes of organic compounds, they have interclass isomers. They are cycloparaffins. For example, for propene the interclass isomer is cyclopropane.

The presence of a double bond in molecules of this class explains the possibility of geometric cis- and trans-isomerism. Such structures are possible only for symmetrical unsaturated hydrocarbons containing a double bond.

Existence this option isomerism is determined by the impossibility of free rotation of carbon atoms along a double bond.

Specifics of chemical properties

The mechanism of alkene hydration has certain features. This reaction refers to electrophilic addition.

How does the hydration reaction of an alkene proceed? To answer this question, consider Markovnikov's rule. Its essence is that the hydration of alkenes with an asymmetric structure is carried out in a certain way. The hydrogen atom will attach to the carbon that is more hydrogenated. The hydroxyl group is attached to a carbon atom that has less H. Hydration of alkenes leads to the formation of secondary monohydric alcohols.

In order for the reaction to proceed fully, mineral acids are used as catalysts. They guarantee the introduction of the required amount of hydrogen cations into the reaction mixture.

It is impossible to obtain primary monohydric alcohols by hydration of alkenes, since Markovnikov’s rule will not be observed. This feature used in the organic synthesis of secondary alcohols. Any hydration of alkenes is carried out without the use of harsh conditions, so the process has found its practical use.

If ethylene is taken as the initial representative of the SpH2n class, Markovnikov’s rule does not work. What alcohols cannot be obtained by hydration of alkenes? It is impossible to obtain primary alcohols from unsymmetrical alkenes as a result of such a chemical process. How is the hydration of alkenes used? The production of secondary alcohols is carried out in exactly this way. If a representative of the acetylene series (alkynes) is chosen as a hydrocarbon, hydration leads to the production of ketones and aldehydes.

According to Markovnikov's rule, the hydration of alkenes is carried out. The reaction has an electrophilic addition mechanism, the essence of which is well studied.

Here are a few specific examples similar transformations. What does the hydration of alkenes lead to? Examples offered in a school chemistry course indicate that propanol-2 can be obtained from propene by reacting with water, and butanol-2 can be obtained from butene-1.

Alkene hydration is used commercially. Secondary alcohols are obtained in this way.

Halogenation

The interaction of unsaturated hydrocarbons with halogen molecules is considered a qualitative reaction to a double bond. We have already analyzed how the hydration of alkenes occurs. The mechanism of halogenation is similar.

Halogen molecules have a covalent nonpolar chemical bond. When temporary fluctuations occur, each molecule becomes electrophilic. As a result, the probability of addition increases, accompanied by the destruction of the double bond in the molecules of unsaturated hydrocarbons. After completion of the process, the reaction product is a dihalogen derivative of the alkane. Bromination is considered a qualitative reaction to unsaturated hydrocarbons, since the brown color of the halogen gradually disappears.

Hydrohalogenation

We have already looked at what the formula for the hydration of alkenes is. Reactions with hydrogen bromide have a similar option. This inorganic compound has a covalent polar chemical bond, so there is a shift in electron density to the more electronegative bromine atom. Hydrogen acquires a partial positive charge, giving an electron to the halogen and attacks the alkene molecule.

If an unsaturated hydrocarbon has an asymmetric structure, when it reacts with a hydrogen halide, two products are formed. Thus, from propene during hydrohalogenation, 1-bromoproane and 2-bromopropane are obtained.

For a preliminary assessment of interaction options, the electronegativity of the selected substituent is taken into account.

Oxidation

The double bond inherent in unsaturated hydrocarbon molecules is exposed to strong oxidizing agents. They are also electrophilic in nature and are used in the chemical industry. Of particular interest is the oxidation of alkenes with an aqueous (or weakly alkaline) solution of potassium permanganate. It is called the hydroxylation reaction because it results in dihydric alcohols.

For example, when ethylene molecules are oxidized with an aqueous solution of potassium permanganate, ethinediol-1,2 (ethylene glycol) is obtained. This interaction is considered a qualitative reaction to a double bond, since during the interaction, discoloration of the potassium permanganate solution is observed.

In an acidic environment (under harsh conditions), aldehyde can be noted among the reaction products.

When interacting with atmospheric oxygen, the corresponding alkene is oxidized to carbon dioxide and water vapor. The process is accompanied by the release of thermal energy, so in industry it is used to generate heat.

The presence of a double bond in an alkene molecule indicates the possibility of hydrogenation reactions occurring in this class. The interaction of SpH2n with hydrogen molecules occurs when platinum and nickel are used thermally as catalysts.

Many representatives of the class of alkenes are prone to ozonation. At low temperatures, representatives of this class react with ozone. The process is accompanied by the cleavage of the double bond, the formation of cyclic peroxide compounds called ozonides. Their molecules contain O-O bonds, so the substances are explosive. Ozonides are not synthesized in pure form, they are decomposed using a process of hydrolysis, then reduced with zinc. The products of this reaction are carbonyl compounds, which are isolated and identified by researchers.

Polymerization

This reaction involves the sequential combination of several alkene molecules (monomers) into a large macromolecule (polymer). From the initial ethene, polyethylene is produced, which has industrial applications. A polymer is a substance that has a high molecular weight.

Inside the macromolecule there is a certain number of repeating fragments called structural units. For the polymerization of ethylene, the group - CH2—CH2- is considered as a structural unit. The degree of polymerization indicates the number of units repeated in the polymer structure.

The degree of polymerization determines the properties of polymer compounds. For example, short chain polyethylene is a liquid that has lubricating properties. A macromolecule with long chains is characterized by a solid state. The flexibility and plasticity of the material is used in the manufacture of pipes, bottles, and films. Polyethylene, in which the degree of polymerization is five to six thousand, has increased strength, therefore it is used in the production of strong threads, rigid pipes, and cast products.

Among the products obtained by the polymerization of alkenes that are of practical importance, we highlight polyvinyl chloride. This compound is obtained by polymerization of vinyl chloride. The resulting product has valuable performance characteristics. It is characterized by increased resistance to aggressive influences chemicals, non-flammable, easy to paint. What can be made from polyvinyl chloride? Briefcases, raincoats, oilcloth, artificial leather, cables, electrical wire insulation.

Teflon is a product of the polymerization of tetrafluoroethylene. This organic inert compound is resistant to sudden temperature changes.

Polystyrene is an elastic transparent substance formed by polymerization of the original styrene. It is indispensable in the manufacture of dielectrics in radio and electrical engineering. In addition, polystyrene is used in large quantities for the production of acid-resistant pipes, toys, combs, and porous plastics.

Features of obtaining alkenes

Representatives of this class are in demand in the modern chemical industry, so various methods for their industrial and laboratory production have been developed. Ethylene and its homologues do not exist in nature.

Many laboratory options for obtaining representatives of this class of hydrocarbons involve reverse addition reactions called elimination. For example, the dehydrogenation of paraffins (saturated hydrocarbons) produces the corresponding alkenes.

By reacting halogen derivatives of alkanes with metallic magnesium, it is also possible to obtain compounds with the general formula SpH2n. Elimination is carried out according to Zaitsev's rule, the inverse of Markovnikov's rule.

In industrial quantities, unsaturated hydrocarbons of the ethylene series are produced by cracking oil. Gases from cracking and pyrolysis of oil and gas contain from ten to twenty percent unsaturated hydrocarbons. The mixture of reaction products contains both paraffins and alkenes, which are separated from each other by fractional distillation.

Some Applications

Alkenes are an important class of organic compounds. The possibility of their use is explained by their excellent reactivity, ease of preparation, and reasonable cost. Among the numerous industrial sectors that use alkenes, we highlight the polymer industry. Huge number ethylene, propylene, and their derivatives are used for the production of polymer compounds.

That is why questions regarding the search for new ways to produce alkene hydrocarbons are so relevant.

Polyvinyl chloride is considered one of the most important products obtained from alkenes. It is characterized by chemical and thermal stability and low flammability. Since this substance is insoluble in mineral solvents, but soluble in organic solvents, it can be used in various industrial sectors.

Its molecular weight is several hundred thousand. When the temperature rises, the substance is capable of decomposition, accompanied by the release of hydrogen chloride.

Of particular interest are its dielectric properties, used in modern electrical engineering. Among the industries in which polyvinyl chloride is used, we highlight the production artificial leather. The resulting material is in no way inferior to natural material in terms of performance characteristics, and at the same time has a much lower cost. Clothing made from such material is becoming increasingly popular among fashion designers who create bright and colorful collections of youth clothing made from polyvinyl chloride in different colors.

Polyvinyl chloride is used in large quantities as a sealant in refrigerators. Due to its elasticity and resilience, this chemical compound is in demand in the manufacture of films and modern suspended ceilings. Washable wallpaper is additionally covered with a thin PVC film. This allows you to add mechanical strength to them. Such finishing materials will become ideal option for cosmetic repairs in office premises.

In addition, hydration of alkenes leads to the formation of primary and secondary monohydric alcohols, which are excellent organic solvents.

Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the ring ( cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (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.

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 CnH2n. Its second name is 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, and 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 forming a double bond are in the same plane, and the orbitals forming a π bond are perpendicular to the plane of the molecule. A double bond (0.132 im) is shorter than a single bond, and its energy is greater, because it is stronger. However, the presence of a mobile, easily polarized π bond leads to the fact that alkenes are chemically more active than alkanes and are able to undergo addition reactions.

Structure of ethylene

Double bond formation in alkenes

Homologous series of ethene

Straight-chain 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.

Alkene isomerism

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 isomerism of the position of the double bond:

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 different from trans isomers the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the π-bond plane, and, consequently, properties.

Alkene nomenclature

1. Selection of 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 the 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 begins 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 is determined by the position of the substituents in the same way as for saturated hydrocarbons.

3. Formation of the name. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix -en, indicating 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.
Boiling and melting points naturally increase with increasing molecular weight of compounds.

Chemical properties of alkenes

Addition reactions. Let us remind you that distinctive feature representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism electrophilic addition.
1. Hydrogenation of alkenes. Alkenes are capable of adding hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction occurs at atmospheric and elevated pressure and does not require high temperature, because it is exothermic. When the temperature increases, the same catalysts can cause a reverse reaction - dehydrogenation.

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 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 obeys
When a hydrogen halide attaches to an alkene, the hydrogen attaches to the more hydrogenated carbon atom, i.e., the atom at which there are more hydrogen atoms, and the halogen to the less hydrogenated one.

4. Hydration (addition of water). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene is the basis of one of the industrial methods for producing ethyl alcohol.

Note that a primary alcohol (with a hydroxo group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, they form secondary alcohols.

This reaction also proceeds in accordance with Markovnikov's rule - a hydrogen cation attaches to a more hydrogenated carbon atom, and a hydroxo group to a less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:

This addition reaction occurs via 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 potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are added to those atoms between which a double bond existed before oxidation:

The first representative of the series of alkenes is ethene (ethylene); to construct the formula for the next representative of the series, you need to add the CH 2 group to the original formula; By repeating this procedure many times, a homologous series of alkenes can be constructed.

CH 2 +CH 2 +CH 2 +CH 2 +CH 2 +CH 2 +CH 2 +CH 2

C 2 H 4 ® C 3 H 6 ® C 4 H 8 ® C 5 H 10 ® C 6 H 12 ® C 7 H 14 ® C 8 H 16 ® C 9 H 18 ® C 10 H 20

To construct the name of an alkene, it is necessary to change the suffix in the name of the corresponding alkane (with the same number of carbon atoms as in the alkene) en on - en(or - ylene). For example, an alkane with four carbon atoms in the chain is called butane, and the corresponding alkene is butene (butylene). The exception is decane; the corresponding alkene will be called not decene, but decene (decilene). An alkene with five carbon atoms in the chain, in addition to the name pentene, is called amylene. The table below shows the formulas and names of the first ten representatives of the series of alkenes.

However, starting from the third, butene, a representative of a number of alkenes, in addition to the verbal name “butene”, after its writing there should be a number 1 or 2, which shows the location of the double bond in the carbon chain.

CH 2 = CH – CH 2 – CH 3 CH 3 – CH = CH – CH 3

butene 1 butene 2

In addition to the systematic nomenclature, rational names for alkenes are often used; alkenes are considered as derivatives of ethylene, in the molecule of which hydrogen atoms are replaced by radicals, and the name “ethylene” is taken as a basis.

For example, CH 3 – CH = CH – C 2 H 5 – symmetrical methylethylethylene.

(CH 3) – CH = CH – C 2 H 5 – symmetrical ethylisopropylethylene.

(CH 3)C – CH = CH – CH(CH 3) 2 – symmetrical isopropyl isobutylethylene.

Unsaturated hydrocarbon radicals are named according to systematic nomenclature by adding the suffix - enyl: ethenyl

CH 2 =CH -, propenyl-2 CH 2 = CH – CH 2 -. But much more often empirical names are used for these radicals - respectively vinyl And allyl.

Isomerism of alkenes.

It is typical for alkenes large number different types isomerism.

A) Isomerism of the carbon skeleton.

CH 2 = C – CH 2 – CH 2 – CH 3 CH 2 = CH – CH – CH 2 – CH 3

2-methyl pentene-1 3-methyl pentene-1

CH 2 = CH – CH 2 – CH – CH 3

4-methyl pentene-1

B) Isomerism of the position of a double bond.

CH 2 = CH – CH 2 – CH 3 CH 3 – CH = CH – CH 3

butene-1 butene-2

B) Spatial (stereoisomerism).

Isomers with identical substituents located on the same side of the double bond are called cis-isomers, but in different ways - trans-isomers:

H 3 C CH 3 H 3 C H

cis-butene trance-butene

Cis- And trance- isomers differ not only in their spatial structure, but also in many physical and chemical (and even physiological) properties. Trans - Isomers are more stable compared to cis isomers. This is explained by the greater distance in space of groups at atoms connected by a double bond, in the case trance– isomers.

G) Isomerism of substances of different classes of organic compounds.

Isomers of alkenes are cycloparaffins, which have a similar general formula - C n H 2 n.

CH 3 – CH = CH – CH 3

butene -2

cyclobutane

4. The location of alkenes in nature and methods of their preparation.

Just like alkanes, alkenes are found in nature in oil, associated petroleum and natural gases, brown and coal oil shale.

A) Preparation of alkenes by catalytic dehydrogenation of alkanes.

СH 3 – CH – CH 3 ® CH 2 = C – CH 3 + H 2

CH 3 cat. (K 2 O-Cr 2 O 3 -Al 2 O 3) CH 3

B) Dehydration of alcohols under the influence of sulfuric acid or with the participation of Al 2 O 3(paraphase dehydration).

ethanol H 2 SO 4 (conc.) ethene

C 2 H 5 OH ® CH 2 = CH 2 + H 2 O

ethanol Al2O3 ethene

Dehydration of alcohols proceeds according to the rule of A.M. Zaitsev, according to which hydrogen is split off from the least hydrogenated carbon atom, that is, secondary or tertiary.

H 3 C – CH – C ® H 3 C – CH = C – CH 3

3-methylbutanol-2 2-methylbutene

IN) Reaction of haloalkyls with alkalis(dehydrohalogenation).

H 3 C – C – CH 2 Cl + KOH ® H 3 C – C = CH 2 + H 2 O + KCl

1-chloro 2-methylpropane(alcohol solution) 2-methylpropene-1

D) The effect of magnesium or zinc on dihalogen derivatives of alkyls with halogen atoms at adjacent carbon atoms (dehalogenation).

alcohol. t

CH 3 -CHCl-CH 2 Cl + Zn ® CH 3 -CH = CH 2 + ZnCl 2

1.2-dichloropropane propene-1

D) Selective hydrogenation of alkynes on a catalyst.

СH º CH + H 2 ® CH 2 =CH 2

ethene ethene

5. Physical properties of alkenes.

The first three representatives of the homologous series of ethylene are gases.

Starting from C 5 H 10 to C 17 H 34 - liquids, starting from C 18 H 36 and then solids. As molecular weight increases, melting and boiling points increase. Alkenes with a normal carbon chain boil at more than high temperature than their isomers, which have an isostructure. Boiling point cis- isomers higher than trance– isomers, and the melting point is the opposite. Alkenes are slightly polar, but are easily polarized. Alkenes are poorly soluble in water (however, better than the corresponding alkanes). They dissolve well in organic solvents. Ethylene and propylene burn with a boiling flame.

The table below shows the main physical properties some representatives of a number of alkenes.

Alkene	Formula	t pl. ­ oC	t kip. ­ oC	d 4 20
Ethene (ethylene)	C2H4	-169,1	-103,7	0,5700
Propene (propylene)	C3H6	-187,6	-47,7	0.6100 (at t(kip))
Butene (butylene-1)	C4H8	-185,3	-6,3	0,5951
cis– Butene-2	C4H8	-138,9	3,7	0,6213
trance– Butene-2	C4H8	-105,5	0,9	0,6042
Isobutylene (2-methylpropene)	C4H8	-140,4	-7,0	0,6260
Penten-1 (amylene)	C5H10	-165,2	+30,1	0,6400
Hexene-1 (hexylene)	C6H12	-139,8	63,5	0,6730
Heptene-1 (heptylene)	C 7 H 14	-119	93,6	0,6970
Octene-1 (octylene)	C 8 H 16	-101,7	121,3	0,7140
Nonene-1 (nonylene)	C 9 H 18	-81,4	146,8	0,7290
Decene-1 (decylene)	C 10 H 20	-66,3	170,6	0,7410

6. Chemical properties of alkenes.

A) Hydrogen addition(hydrogenation).

CH 2 = CH 2 + H 2 ® CH 3 – CH 3

ethene ethane

B) Interaction with halogens(halogenation).

The addition of chlorine and bromine to alkenes is easier, but iodine is more difficult.

CH 3 – CH = CH 2 + Cl 2 ® CH 3 – CHCl – CH 2 Cl

propylene 1,2-dichloropropane

IN) Addition of hydrogen halides ( hydrohalogenation)

The addition of hydrogen halides to alkenes under normal conditions proceeds according to Markovnikov’s rule: during the ionic addition of hydrogen halides to unsymmetrical alkenes (under normal conditions), hydrogen is added at the site of the double bond to the most hydrogenated one (associated with the largest number hydrogen atoms) to the carbon atom, and halogen to the less hydrogenated one.

CH 2 =CH 2 + HBr ® CH 3 – CH 2 Br

ethene bromoethane

G) Addition of water to alkenes(hydration).

The addition of water to alkenes also occurs according to Markovnikov’s rule.

CH 3 – CH = CH 2 + H – OH ® CH 3 – CHOH – CH 3

propen-1 propanol-2

E) Alkylation of alkanes with alkenes.

Alkylation is a reaction by which various hydrocarbon radicals (alkyl) can be introduced into the molecules of organic compounds. Haloalkyls, unsaturated hydrocarbons, alcohols and other organic substances are used as alkylating agents. For example, in the presence of concentrated sulfuric acid, the alkylation reaction of isobutane with isobutylene actively occurs:

3CH 2 = CH 2 + 2KMnO 4 + 4H 2 O ® 3CH 2 OH – CH 2 OH + 2MnO 2 + 2KOH

ethene ethylene glycol

(ethanediol-1,2)

Cleavage of an alkene molecule at the double bond can lead to the formation of the corresponding carboxylic acid if a vigorous oxidizing agent is used (concentrated nitric acid or a chromic mixture).

HNO3(conc.)

CH 3 – CH = CH – CH 3 ® 2CH 3 COOH

butene-2 ​​ethanoic acid (acetic acid)

Oxidation of ethylene by atmospheric oxygen in the presence of metallic silver leads to the formation of ethylene oxide.

2CH 2 = CH 2 + O 2 ® 2CH 2 – CH 2

AND) Alkene polymerization reaction.

n CH 2 = CH 2 ® [–CH 2 – CH 2 –] n

ethylene cat. polyethylene

7.Application of alkenes.

A) Cutting and welding of metals.

B) Production of dyes, solvents, varnishes, new organic substances.

B) Production of plastics and other synthetic materials.

D) Synthesis of alcohols, polymers, rubbers

D) Synthesis of drugs.

IV. Diene hydrocarbons(alkadienes or diolefins) are unsaturated complex organic compounds with the general formula C n H 2 n -2, containing two double bonds between carbon atoms in the chain and capable of attaching molecules of hydrogen, halogens and other compounds due to the valence unsaturation of the carbon atom.

The first representative of the diene hydrocarbon series is propadiene (allen). The structure of diene hydrocarbons is similar to the structure of alkenes, the only difference is that the molecules of diene hydrocarbons have two double bonds, not one.

Alkenes- unsaturated hydrocarbons, which contain one double bond. Examples of alkenes:

Methods for obtaining alkenes.

1. Cracking of alkanes at 400-700°C. The reaction occurs via a free radical mechanism:

2. Dehydrogenation of alkanes:

3. Elimination reaction (elimination): 2 atoms or 2 groups of atoms are eliminated from neighboring carbon atoms, and a double bond is formed. Such reactions include:

A) Dehydration of alcohols (heating above 150°C, with the participation of sulfuric acid as a water-removing reagent):

B) Elimination of hydrogen halides when exposed to an alcoholic alkali solution:

The hydrogen atom is split off preferentially from the carbon atom that is bonded to fewer hydrogen atoms (the least hydrogenated atom) - Zaitsev's rule.

B) Dehalogenation:

Chemical properties of alkenes.

The properties of alkenes are determined by the presence of a multiple bond, therefore alkenes enter into electrophilic addition reactions, which occur in several stages (H-X - reagent):

1st stage:

2nd stage:

.

The hydrogen ion in this type of reaction belongs to the carbon atom that has a more negative charge. The density distribution is:

If the substituent is a donor, which manifests the +I- effect, then the electron density shifts towards the most hydrogenated carbon atom, creating a partially negative charge on it. Reactions go according to Markovnikov's rule: when joining polar molecules like NH (HCl, HCN, HOH etc.) to unsymmetrical alkenes, hydrogen attaches preferentially to the more hydrogenated carbon atom at the double bond.

A) Addition reactions:
1) Hydrohalogenation:

The reaction follows Markovnikov's rule. But if peroxide is present in the reaction, then the rule is not taken into account:

2) Hydration. The reaction follows Markovnikov's rule in the presence of phosphoric or sulfuric acid:

3) Halogenation. As a result, bromine water becomes discolored - this is a qualitative reaction to a multiple bond:

4) Hydrogenation. The reaction occurs in the presence of catalysts.