Alkenes hbr. Alkenes: methods of preparation, chemical properties and applications. Reaction of alkynes with bases
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 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 - accordingly vinyl And allyl.
Isomerism of alkenes.
Alkenes are characterized by a large number of different types of 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 in which identical substituents are 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 petroleum, associated petroleum and natural gases, brown coal 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 H2SO4 (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 a higher temperature than their isomers, which have an isostructure. Boiling temperature 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 basic physical properties of 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 |
| Decen-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 double bond to the most hydrogenated (bonded to the largest number of hydrogen atoms) carbon atom, and the halogen is added 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.
General formula of alkenes: CnH2n(n 2)
The first representatives of the homologous series of alkenes:
The formulas of alkenes can be compiled from the corresponding formulas of alkanes (saturated hydrocarbons). The names of alkenes are formed by replacing the suffix -ane of the corresponding alkane with -ene or –ylene: butane - butylene, pentane - pentene, etc. The number of the carbon atom with a double bond is indicated by an Arabic numeral after the name.
The carbon atoms involved in the formation of the double bond are in a state of sp-hybridization. Three -bonds formed by hybrid orbitals and are located in the same plane at an angle of 120° to each other. An additional -bond is formed by lateral overlap of non-hybrid p-orbitals:
The length of the C=C double bond (0.133 nm) is shorter than the length of the single bond (0.154 nm). The energy of a double bond is less than twice the energy of a single bond because the energy of the -bond is less than the energy of the -bond.
Alkene isomers
All alkenes except ethylene have isomers. Alkenes are characterized by isomerism of the carbon skeleton, isomerism of the position of the double bond, interclass and spatial isomerism.
The interclass isomer of propene (C 3 H 6) is cyclopropane. Starting with butene (C 4 H 8), isomerism appears by the position of the double bond (butene-1 and butene-2), isomerism of the carbon skeleton (methylpropene or isobutylene), as well as spatial isomerism (cis-butene-2 and trans-butene-2 ). In cis isomers, the substituents are located on one side, and in trans isomers, they are located on opposite sides of the double bond.
The chemical properties and chemical activity of alkenes are determined by the presence of a double bond in their molecules. The most common reactions for alkenes are electrophilic addition: hydrohalogenation, hydration, halogenation, hydrogenation, polymerization.
Qualitative reaction to a double bond – discoloration of bromine water:
Examples of solving problems on the topic “formula of alkenes”
EXAMPLE 1
| Exercise | How many isomers capable of decolorizing bromine water does a substance with the composition C 3 H 5 Cl have? Write the structural formulas of these isomers |
| Solution | C 3 H 5 Cl is a monochlor derivative of the hydrocarbon C 3 H 6 . This formula corresponds to either propene, a hydrocarbon with one double bond, or cyclopropane (a cyclic hydrocarbon). This substance discolors bromine water, which means it contains a double bond. Three carbon atoms can only form this structure: since isomerism of the carbon skeleton and the position of the double bond is impossible with such a number of carbon atoms. Structural isomerism in a given molecule is possible only due to a change in the position of the chlorine atom relative to the double bond: For 1-chloropropene, cis-trans isomerism is possible: |
| Answer | The problem conditions are satisfied by 4 isomers |
EXAMPLE 2
| Exercise | A mixture of isomeric hydrocarbons (gases with a hydrogen density of 21) with a volume of 11.2 liters (n.s.) reacted with bromine water. The result was 40.4 g of the corresponding dibromo derivative. What structure do these hydrocarbons have? Determine their volumetric content in the mixture (in%). |
| Solution | The general formula of hydrocarbons is C x H y. Let's calculate the molar mass of hydrocarbons: Therefore, the formula of hydrocarbons is C 3 H 6. Only two substances have this formula - propene and cyclopropane. Only propene reacts with bromine water: Let's calculate the amount of dibromo derivative substance: According to the reaction equation: n(propene) mol The total amount of hydrocarbons in the mixture is equal to: |
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). The -C bond in ethylene is stronger than the -bond, 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 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, 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 to different sides of a multiple bond (double or triple), 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.
A major 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 cleavage of single carbon-carbon bonds (hydrogenolysis). Homogeneous hydrogenation does not have these disadvantages. In 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, allowing 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 preferential 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 middle 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 occur 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-addition of halogens was the main reason for 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.
In addition, the final addition product, vicinal diiodide, can be subject to 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 isotopic tracers 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 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.
Knowledge Hypermarket >>Chemistry >>Chemistry 10th grade >> Chemistry: Alkenes
Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unsaturated 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) also have an unsaturated character. 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
Alkenes are acyclic, containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula C n H 2n.
Alkenes received their second name - “olefins” by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from the English oil - oil).
Carbon atoms that have a double bond between them, as you know, are in a state of sp 2 hybridization. This means that one s and two p orbitals are involved in hybridization, and one p orbital remains unhybridized. The overlap of the hybrid orbitals leads to the formation of an a-bond, and due to the unhybridized -orbitals of the neighboring carbon atoms of the ethylene molecule, a second one is formed, P-connection. Thus, a double bond consists of one Þ-bond and one p-bond.
The hybrid orbitals of the atoms forming a double bond are in the same plane, and the orbitals forming an n-bond are located perpendicular to the plane of the molecule (see Fig. 5).
The double bond (0.132 nm) is shorter than the single bond, and its energy is higher, i.e. it is stronger. Nevertheless, the presence of a mobile, easily polarizable 7g-bond leads to the fact that alkenes are chemically more active than alkanes and are capable of undergoing addition reactions.
Homologous series of ethene
Straight-chain alkenes form the homologous series of ethene (ethylene).
C2H4 - ethene, C3H6 - propene, C4H8 - butene, C5H10 - pentene, C6H12 - hexene, etc.
Isomerism and nomenclature
Alkenes, like alkanes, are characterized by structural isomerism. Structural isomers, as you remember, differ from each other in the structure of the carbon skeleton. The simplest alkene, characterized by structural isomers, is butene.
CH3-CH2-CH=CH2 CH3-C=CH2
l
CH3
butene-1 methylpropene
A special type of structural isomerism is isomerism of the position of the double bond:
CH3-CH2-CH=CH2 CH3-CH=CH-CH3
butene-1 butene-2
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-trans isomerism.
Cis isomers differ from thorax isomers in the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the plane P-connections, and therefore properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
CH2 = CH-CH2-CH2-CH2-CH3
hexene-1 cyclohexane
Nomenclature alkenes, developed by IUPAC, is similar to the nomenclature of alkanes.
1. Main circuit selection
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
dn3-dn-dn2-dn=dn-dn3 dn3
5-methylhexene-2, not 2-methylhexene-4, as one might expect.
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.
CH3- CH2-CH=CH-CH-CH3
l
CH3
2-methylhexene-3
3. Formation of the name
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and the suffix indicating that the compound belongs to the class of alkenes, -ene.
Receipt
1. Cracking of petroleum products. In the process of thermal cracking of saturated hydrocarbons, along with the formation of alkanes, the formation of alkenes occurs.
2. Dehydrogenation of saturated hydrocarbons. When alkanes are passed over a catalyst at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed: 
3. Dehydration of alcohols (elimination of water). The effect of water-removing agents (H2804, Al203) on monohydric alcohols at high temperatures leads to the elimination of a water molecule and the formation of a double bond:
This reaction is called intramolecular dehydration (in contrast to intermolecular dehydration, which leads to the formation of ethers and will be studied in § 16 “Alcohols”).
4. Dehydrohalogenation (elimination of hydrogen halide).
When a haloalkane reacts with an alkali in an alcohol solution, a double bond is formed as a result of the elimination of a hydrogen halide molecule.
Note that this reaction produces predominantly butene-2 rather than butene-1, which corresponds to Zaitsev's rule:
When a hydrogen halide is eliminated from secondary and tertiary haloalkanes, a hydrogen atom is eliminated from the least hydrogenated carbon atom.
5. Dehalogenation. When zinc acts on a dibromo derivative of an alkane, halogen atoms located at neighboring carbon atoms are eliminated and a double bond is formed: 
Physical properties
The first three representatives of the homologous series of alkenes are gases, substances of the composition C5H10-C16H32 are liquids, and higher alkenes are solids.
Boiling and melting points naturally increase with increasing molecular weight of compounds.
Chemical properties
Addition reactions
Let us recall that a distinctive feature of representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.
1. Hydrogenation of alkenes. Alkenes are capable of adding hydrogen in the presence of hydrogenation catalysts - metals - platinum, palladium, nickel:
CH3-CH2-CH=CH2 + H2 -> CH3-CH2-CH2-CH3
This reaction occurs at both atmospheric and elevated pressure and does not require high temperature, since 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 (CCl4) 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.
Markovnikov Vladimir Vasilievich
(1837-1904)
Russian organic chemist. Formulated (1869) rules on the direction of substitution, elimination, addition at a double bond and isomerization reactions depending on the chemical structure. He studied (since 1880) the composition of oil and laid the foundations of petrochemistry as an independent science. Discovered (1883) a new class of organic substances - cyclo-paraffins (naphthenes).
3. Hydrohalogenation (addition of hydrogen halide).
The hydrogen halide addition reaction will be discussed in more detail below. This reaction obeys Markovnikov's rule:
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 underlies one of the industrial methods for producing ethyl alcohol:
CH2=CH2 + H2O -> CH3-CH2OH
ethene ethanol
Note that a primary alcohol (with a hydroxy group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed. 
This reaction also proceeds in accordance with Markovnikov's rule - a hydrogen cation attaches to a more hydrogenated carbon atom, and a hydroxy group attaches 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
Like any organic compounds, alkenes burn in oxygen to form CO2 and H20.
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by the action of aqueous solutions of potassium permanganate. In neutral or slightly alkaline solutions, oxidation of alkenes to diols (dihydric alcohols) occurs, and hydroxyl groups are added to those atoms between which a double bond existed before oxidation.
As you already know, unsaturated hydrocarbons - alkenes are capable of entering into addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.
Electrophilic connection
Electrophilic reactions are reactions that occur under the influence of electrophiles - particles that have a lack of electron density, for example, an unfilled orbital. The simplest electrophilic particle is the hydrogen cation. It is known that the hydrogen atom has one electron in the 3rd orbital. A hydrogen cation is formed when an atom loses this electron, thus the hydrogen cation has no electrons at all:
Н· - 1е - -> Н +
In this case, the cation has a fairly high electron affinity. The combination of these factors makes the hydrogen cation a fairly strong electrophilic particle.
The formation of a hydrogen cation is possible during the electrolytic dissociation of acids:
НВr -> Н + + Вr -
It is for this reason that many electrophilic reactions occur in the presence and participation of acids.
Electrophilic particles, as mentioned earlier, act on systems containing areas of increased electron density. An example of such a system is a multiple (double or triple) carbon-carbon bond.
You already know that carbon atoms between which a double bond is formed are in a state of sp 2 hybridization. Unhybridized p-orbitals of neighboring carbon atoms located in the same plane overlap, forming P-bond, which is less strong than the Þ-bond, and, most importantly, is easily polarized under the influence of an external electric field. This means that when a positively charged particle approaches, the electrons of the CS bond shift towards it and the so-called P- complex.
It turns out P-complex and upon addition of a hydrogen cation to P- connections. The hydrogen cation seems to bump into the electron density protruding from the plane of the molecule P-connection and joins it. 
At the next stage, a complete displacement of the electron pair occurs P-bond to one of the carbon atoms, which leads to the appearance of a lone pair of electrons on it. The orbital of the carbon atom on which this pair is located and the unoccupied orbital of the hydrogen cation overlap, which leads to the formation of a covalent bond through the donor-acceptor mechanism. The second carbon atom still has an unfilled orbital, i.e., a positive charge.
The resulting particle is called a carbocation because it contains a positive charge on the carbon atom. This particle can combine with any anion, a particle that has a lone electron pair, i.e., a nucleophile.
Let us consider the mechanism of the electrophilic addition reaction using the example of hydrobromination (addition of hydrogen bromide) of ethene:
СН2= СН2 + НВг --> СНВr-СН3
The reaction begins with the formation of an electrophilic particle - a hydrogen cation, which occurs as a result of the dissociation of a hydrogen bromide molecule.
Hydrogen cation attacks P- connection, forming P- a complex that is quickly converted into a carbocation: 
Now let's look at a more complex case.
The reaction of the addition of hydrogen bromide to ethene proceeds unambiguously, and the interaction of hydrogen bromide with propene can theoretically give two products: 1-bromopropane and 2-bromopropane. Experimental data show that 2-bromopropane is mainly produced.
In order to explain this, we will have to consider the intermediate particle - the carbocation.
The addition of a hydrogen cation to propene can lead to the formation of two carbocations: if a hydrogen cation joins the first carbon atom, the atom located at the end of the chain, then the second one will have a positive charge, i.e., in the center of the molecule (1); if it joins the second, then the first atom will have a positive charge (2). 
The preferential direction of the reaction will depend on which carbocation is more abundant in the reaction medium, which, in turn, is determined by the stability of the carbocation. The experiment shows the predominant formation of 2-bromopropane. This means that the formation of carbocation (1) with a positive charge on the central atom occurs to a greater extent.
The greater stability of this carbocation is explained by the fact that the positive charge on the central carbon atom is compensated by the positive inductive effect of two methyl groups, the total effect of which is higher than the +/- effect of one ethyl group:
The laws of the reactions of hydrohalogenation of alkenes were studied by the famous Russian chemist V.V. Markovnikov, a student of A.M. Butlerov, who, as mentioned above, formulated the rule that bears his name.
This rule was established empirically, that is, experimentally. At present, we can give a completely convincing explanation for it.
Interestingly, other electrophilic addition reactions also obey Markovnikov’s rule, so it would be correct to formulate it in a more general form.
In electrophilic addition reactions, an electrophile (a particle with an unfilled orbital) adds to a more hydrogenated carbon atom, and a nucleophile (a particle with a lone pair of electrons) adds to a less hydrogenated one.
Polymerization
A special case of addition reaction is the polymerization reaction of alkenes and their derivatives. This reaction proceeds by the free radical addition mechanism:
Polymerization is carried out in the presence of initiators - peroxide compounds, which are a source of free radicals. Peroxide compounds are substances whose molecules include the -O-O- group. The simplest peroxide compound is hydrogen peroxide HOOH.
At a temperature of 100 °C and a pressure of 100 MPa, homolysis of the unstable oxygen-oxygen bond and the formation of radicals - polymerization initiators - occur. Under the influence of KO- radicals, polymerization is initiated, which develops as a free radical addition reaction. Chain growth stops when recombination of radicals occurs in the reaction mixture - the polymer chain and radicals or COCH2CH2-.
Using the reaction of free radical polymerization of substances containing a double bond, a large number of high molecular weight compounds are obtained: 
The use of alkenes with various substituents makes it possible to synthesize a wide range of polymeric materials with a wide range of properties. 
All these polymer compounds are widely used in a variety of areas of human activity - industry, medicine, used for the manufacture of equipment for biochemical laboratories, some are intermediates for the synthesis of other high-molecular compounds.
Oxidation
You already know that in neutral or slightly alkaline solutions, oxidation of alkenes to diols (dihydric alcohols) occurs. In an acidic environment (a solution acidified with sulfuric acid), the double bond is completely destroyed and the carbon atoms between which the double bond existed are converted into carbon atoms of the carboxyl group: 
Destructive oxidation of alkenes can be used to determine their structure. So, for example, if acetic and propionic acids are obtained during the oxidation of a certain alkene, this means that pentene-2 has undergone oxidation, and if butyric acid and carbon dioxide are obtained, then the original hydrocarbon is pentene-1.
Application
Alkenes are widely used in the chemical industry as raw materials for the production of a variety of organic substances and materials.
For example, ethene is the starting material for the production of ethanol, ethylene glycol, epoxides, and dichloroethane.
A large amount of ethene is processed into polyethylene, which is used to make packaging film, tableware, pipes, and electrical insulating materials.
Glycerin, acetone, isopropanol, and solvents are obtained from propene. By polymerizing propene, polypropylene is obtained, which is superior to polyethylene in many respects: it has a higher melting point and chemical resistance.
Currently, fibers with unique properties are produced from polymers - analogues of polyethylene. For example, polypropylene fiber is stronger than all known synthetic fibers.
Materials made from these fibers are promising and are increasingly used in various areas of human activity.
1. What types of isomerism are characteristic of alkenes? Write the formulas for possible isomers of pentene-1.
2. From what compounds can be obtained: a) isobutene (2-methylpropene); b) butene-2; c) butene-1? Write the equations for the corresponding reactions.
3. Decipher the following chain of transformations. Name compounds A, B, C. 4. Suggest a method for obtaining 2-chloropropane from 1-chloropropane. Write the equations for the corresponding reactions.
5. Suggest a method for purifying ethane from ethylene impurities. Write the equations for the corresponding reactions.
6. Give examples of reactions that can be used to distinguish between saturated and unsaturated hydrocarbons.
7. For complete hydrogenation of 2.8 g of alkene, 0.896 liters of hydrogen (n.e.) were consumed. What is the molecular weight and structural formula of this compound, which has a normal chain of carbon atoms?
8. What gas is in the cylinder (ethene or propene), if it is known that the complete combustion of 20 cm3 of this gas required 90 cm3 (n.s.) of oxygen?
9*. When an alkene reacts with chlorine in the dark, 25.4 g of dichloride is formed, and when this alkene of the same mass reacts with bromine in carbon tetrachloride, 43.2 g of dibromide is formed. Determine all possible structural formulas of the starting alkene.
History of discovery
From the above material, we have already understood that ethylene is the ancestor of the homologous series of unsaturated hydrocarbons, which has one double bond. Their formula is C n H 2n and they are called alkenes.
In 1669, the German physician and chemist Becher was the first to obtain ethylene by reacting sulfuric acid with ethyl alcohol. Becher found that ethylene is more chemically active than methane. But, unfortunately, at that time the scientist could not identify the resulting gas, and therefore did not assign any name to it.
A little later, Dutch chemists used the same method for producing ethylene. And since, when interacting with chlorine, it tended to form an oily liquid, it accordingly received the name “oil gas.” Later it became known that this liquid was dichloroethane.
In French, the term "oil" is oléfiant. And after other hydrocarbons of this type were discovered, Antoine Fourcroix, a French chemist and scientist, introduced a new term that became common to the entire class of olefins or alkenes.
But already at the beginning of the nineteenth century, the French chemist J. Gay-Lussac discovered that ethanol consists not only of “oil” gas, but also of water. In addition, the same gas was discovered in ethyl chloride.
And although chemists determined that ethylene consists of hydrogen and carbon, and already knew the composition of the substances, they could not find its real formula for a long time. And only in 1862 E. Erlenmeyer managed to prove the presence of a double bond in the ethylene molecule. This was also recognized by the Russian scientist A.M. Butlerov and confirmed the correctness of this point of view experimentally.
Occurrence in nature and physiological role of alkenes
Many people are interested in the question of where alkenes can be found in nature. So, it turns out that they practically do not occur in nature, since its simplest representative, ethylene, is a hormone for plants and is synthesized in them only in small quantities.
It is true that in nature there is such an alkene as muskalur. This one of the natural alkenes is a sexual attractant of the female house fly.
It is worth paying attention to the fact that, having a high concentration, lower alkenes have a narcotic effect that can cause convulsions and irritation of the mucous membranes.
Applications of alkenes
It is difficult to imagine the life of modern society today without the use of polymer materials. Since, unlike natural materials, polymers have different properties, they are easy to process, and if you look at the price, they are relatively cheap. Another important aspect in favor of polymers is that many of them can be recycled.
Alkenes have found their use in the production of plastics, rubbers, films, Teflon, ethyl alcohol, acetaldehyde and other organic compounds.
In agriculture, it is used as a means that accelerates the ripening process of fruits. Propylene and butylenes are used to produce various polymers and alcohols. But in the production of synthetic rubber, isobutylene is used. Therefore, we can conclude that it is impossible to do without alkenes, since they are the most important chemical raw materials.
Industrial uses of ethylene
On an industrial scale, propylene is usually used for the synthesis of polypropylene and for the production of isopropanol, glycerol, butyraldehydes, etc. Every year the demand for propylene increases.
Alkenes are unsaturated aliphatic hydrocarbons with one or more carbon-carbon double bonds. A double bond transforms two carbon atoms into a planar structure with bond angles between adjacent bonds of 120°C:
The homologous series of alkenes has a general formula; its first two members are ethene (ethylene) and propene (propylene):
Members of a number of alkenes with four or more carbon atoms exhibit bond position isomerism. For example, an alkene with the formula has three isomers, two of which are bond position isomers:
Note that the alkene chain is numbered from the end closest to the double bond. The position of the double bond is indicated by the lower of the two numbers, which correspond to the two carbon atoms connected by the double bond. The third isomer has a branched structure:
The number of isomers of any alkene increases with the number of carbon atoms. For example, hexene has three bond position isomers:
The diene is buta-1,3-diene, or simply butadiene:
Compounds containing three double bonds are called trienes. Compounds with multiple double bonds are collectively called polyenes.
Physical properties
Alkenes have slightly lower melting and boiling points than their corresponding alkanes. For example, pentane has a boiling point. Ethylene, propene and three isomers of butene are gaseous at room temperature and normal pressure. Alkenes with the number of carbon atoms from 5 to 15 are in a liquid state under normal conditions. Their volatility, like that of alkanes, increases in the presence of branching in the carbon chain. Alkenes with more than 15 carbon atoms are solids under normal conditions.
Obtained in laboratory conditions
The two main methods for producing alkenes in the laboratory are the dehydration of alcohols and the dehydrohalogenation of haloalkanes. For example, ethylene can be obtained by dehydration of ethanol under the action of an excess of concentrated sulfuric acid at a temperature of 170 ° C (see section 19.2):
Ethylene can also be produced from ethanol by passing ethanol vapor over the surface of heated alumina. For this purpose, you can use the installation schematically shown in Fig. 18.3.
The second common method for the preparation of alkenes is based on the dehydrohalogenation of halogenated alkanes under basic catalysis conditions
The mechanism of this type of elimination reaction is described in Section. 17.3.
Alkene reactions
Alkenes are much more reactive than alkanes. This is due to the ability of the -electrons of the double bond to attract electrophiles (see Section 17.3). Therefore, the characteristic reactions of alkenes are mainly electrophilic addition reactions at the double bond:
Many of these reactions have ionic mechanisms (see Section 17.3).
Hydrogenation
If any alkene, for example ethylene, is mixed with hydrogen and passed this mixture over the surface of a platinum catalyst at room temperature or a nickel catalyst at a temperature of about 150 ° C, then addition will occur
hydrogen at the double bond of the alkene. This produces the corresponding alkane:
This type of reaction is an example of heterogeneous catalysis. Its mechanism is described in Section. 9.2 and is shown schematically in Fig. 9.20.
Addition of halogens
Chlorine or bromine easily adds to the double bond of the alkene; this reaction occurs in non-polar solvents, such as tetrachloromethane or hexane. The reaction proceeds by an ionic mechanism, which involves the formation of a carbocation. The double bond polarizes the halogen molecule, turning it into a dipole:
Therefore, a solution of bromine in hexane or tetrachloromethane becomes colorless when shaken with an alkene. The same thing happens if you shake an alkene with bromine water. Bromine water is a solution of bromine in water. This solution contains hypobromous acid. A hypobromous acid molecule attaches to the double bond of the alkene, resulting in the formation of a bromo-substituted alcohol. For example
Addition of hydrogen halides
The mechanism of this type of reaction is described in Section. 18.3. As an example, consider the addition of hydrogen chloride to propene:
Note that the product of this reaction is 2-chloropropane, not 1-chloropropane:
In such addition reactions, the most electronegative atom or the most electronegative group always adds to the carbon atom bonded to
the smallest number of hydrogen atoms. This pattern is called Markovnikov's rule.
The preferential attachment of an electronegative atom or group to the carbon atom associated with the smallest number of hydrogen atoms is due to an increase in the stability of the carbocation as the number of alkyl substituents on the carbon atom increases. This increase in stability is in turn explained by the inductive effect that occurs in alkyl groups, since they are electron donors:
In the presence of any organic peroxide, propene reacts with hydrogen bromide, i.e., not according to Markovnikov’s rule. Such a product is called anti-Markovnikov. It is formed as a result of a reaction occurring by a radical rather than an ionic mechanism.
Hydration
Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogen sulfates. For example
This reaction is an addition because it involves the addition of an acid at a double bond. It is the reverse reaction to the dehydration of ethanol to form ethylene. The mechanism of this reaction is similar to the mechanism of addition of hydrogen halides at the double bond. It involves the formation of a carbocation intermediate. If the product of this reaction is diluted with water and heated gently, it hydrolyzes to form ethanol:
The reaction of addition of sulfuric acid to alkenes obeys Markovnikov’s rule:
Reaction with acidified solution of potassium permanganate
The violet color of an acidified solution of potassium permanganate disappears if this solution is shaken in a mixture with any alkene. Hydroxylation of the alkene occurs (the introduction of a hydroxy group formed as a result of oxidation), which as a result is converted into a diol. For example, when an excess amount of ethylene is shaken with an acidified solution, ethane-1,2-diol (ethylene glycol) is formed.
If an alkene is shaken with an excess amount of -ion solution, oxidative cleavage of the alkene occurs, leading to the formation of aldehydes and ketones:
The aldehydes formed in this case undergo further oxidation to form carboxylic acids.
Hydroxylation of alkenes to form diols can also be carried out using an alkaline solution of potassium permanganate.
Reaction with perbenzoic acid
Alkenes react with peroxyacids (peracids), such as perbenzoic acid, to form cyclic ethers (epoxy compounds). For example
When epoxyethane is gently heated with a dilute solution of an acid, ethane-1,2-diol is formed:
Reactions with oxygen
Like all other hydrocarbons, alkenes burn and, with plenty of air, form carbon dioxide and water:
With limited air access, combustion of alkenes leads to the formation of carbon monoxide and water:
Because alkenes have a higher relative carbon content than the corresponding alkanes, they burn with a smokier flame. This is due to the formation of carbon particles:
If you mix any alkene with oxygen and pass this mixture over the surface of a silver catalyst, epoxyethane is formed at a temperature of about 200 ° C:
Ozonolysis
When ozone gas is passed through a solution of an alkene in trichloromethane or tetrachloromethane at temperatures below 20 °C, the ozonide of the corresponding alkene (oxirane) is formed.
Ozonides are unstable compounds and can be explosive. They undergo hydrolysis to form aldehydes or ketones. For example
In this case, part of the methanal (formaldehyde) reacts with hydrogen peroxide, forming methane (formic) acid:
Polymerization
The simplest alkenes can polymerize to form high molecular weight compounds that have the same empirical formula as the parent alkene:
This reaction occurs at high pressure, a temperature of 120°C and in the presence of oxygen, which acts as a catalyst. However, ethylene polymerization can be carried out at lower pressure if a Ziegler catalyst is used. One of the most common Ziegler catalysts is a mixture of triethylaluminum and titanium tetrachloride.
The polymerization of alkenes is discussed in more detail in Section. 18.3.
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