Lesson topic: Alkenes. Preparation, chemical properties and applications of alkenes.
Goals and objectives of the lesson:
- review the specific chemical properties of ethylene and the general properties of alkenes;
- deepen and concretize the concepts of?-bonds and the mechanisms of chemical reactions;
- give initial ideas about polymerization reactions and the structure of polymers;
- analyze laboratory and general industrial methods for producing alkenes;
- continue to develop the ability to work with the textbook.
Equipment: device for producing gases, KMnO 4 solution, ethyl alcohol, concentrated sulfuric acid, matches, alcohol lamp, sand, tables “Structure of the ethylene molecule”, “Basic chemical properties of alkenes”, demonstration samples “Polymers”.
DURING THE CLASSES
I. Organizational moment
We continue to study the homologous series of alkenes. Today we have to look at the methods of preparation, chemical properties and applications of alkenes. We must characterize the chemical properties caused by the double bond, gain an initial understanding of polymerization reactions, and consider laboratory and industrial methods for producing alkenes.
II. Activating students' knowledge
- What hydrocarbons are called alkenes?
- What are the features of their structure?
- In what hybrid state are the carbon atoms that form a double bond in an alkene molecule?
Bottom line: alkenes differ from alkanes in the presence of one double bond in their molecules, which determines the peculiarities of the chemical properties of alkenes, methods of their preparation and use.
III. Learning new material
1. Methods for producing alkenes
Draw up reaction equations confirming methods for producing alkenes
– cracking of alkanes C 8 H 18 ––> C 4 H 8 + C 4 H 10 ; (thermal cracking at 400-700 o C)
octane butene butane
– dehydrogenation of alkanes C 4 H 10 ––> C 4 H 8 + H 2; (t, Ni)
butane butene hydrogen
– dehydrohalogenation of haloalkanes C 4 H 9 Cl + KOH ––> C 4 H 8 + KCl + H 2 O;
chlorobutane hydroxide butene chloride water
potassium potassium
– dehydrohalogenation of dihaloalkanes 
– dehydration of alcohols C 2 H 5 OH ––> C 2 H 4 + H 2 O (when heated in the presence of concentrated sulfuric acid)
Remember!
In the reactions of dehydrogenation, dehydration, dehydrohalogenation and dehalogenation, it must be remembered that hydrogen is preferentially abstracted from less hydrogenated carbon atoms (Zaitsev’s rule, 1875)
2. Chemical properties of alkenes
The nature of the carbon-carbon bond determines the type of chemical reactions in which organic substances enter. The presence of a double carbon-carbon bond in the molecules of ethylene hydrocarbons determines the following features of these compounds:
– the presence of a double bond allows alkenes to be classified as unsaturated compounds. Their transformation into saturated ones is possible only as a result of addition reactions, which is the main feature of the chemical behavior of olefins;
– the double bond represents a significant concentration of electron density, so addition reactions are electrophilic in nature;
– a double bond consists of one - and one - bond, which is quite easily polarized.
Reaction equations characterizing the chemical properties of alkenes
a) Addition reactions
Remember! Substitution reactions are characteristic of alkanes and higher cycloalkanes, which have only single bonds; addition reactions are characteristic of alkenes, dienes and alkynes, which have double and triple bonds.
Remember! The following mechanisms for breaking the -bond are possible:
a) if alkenes and the reagent are non-polar compounds, then the -bond is broken to form a free radical:
H 2 C = CH 2 + H: H ––> + +
b) if the alkene and the reagent are polar compounds, then the cleavage of the -bond leads to the formation of ions:
c) when reagents containing hydrogen atoms in the molecule join at the site of a broken -bond, hydrogen always attaches to a more hydrogenated carbon atom (Morkovnikov’s rule, 1869).
– polymerization reaction nCH 2 = CH 2 ––> n – CH 2 – CH 2 –– > (– CH 2 – CH 2 –)n
ethene polyethylene
b) oxidation reaction
Laboratory experience. Obtain ethylene and study its properties (instructions on student desks)
Instructions for obtaining ethylene and experiments with it
1. Place 2 ml of concentrated sulfuric acid, 1 ml of alcohol and a small amount of sand into a test tube.
2. Close the test tube with a stopper with a gas outlet tube and heat it in the flame of an alcohol lamp.
3. Pass the released gas through a solution with potassium permanganate. Note the change in color of the solution.
4. Light the gas at the end of the gas outlet tube. Pay attention to the color of the flame.
– alkenes burn with a luminous flame. (Why?)
C 2 H 4 + 3O 2 ––> 2CO 2 + 2H 2 O (with complete oxidation, the reaction products are carbon dioxide and water)
Qualitative reaction: “mild oxidation (in aqueous solution)”
– alkenes decolorize a solution of potassium permanganate (Wagner reaction)
Under more severe conditions in an acidic environment, the reaction products can be carboxylic acids, for example (in the presence of acids):
CH 3 – CH = CH 2 + 4 [O] ––> CH 3 COOH + HCOOH
– catalytic oxidation
Remember the main thing!
1. Unsaturated hydrocarbons actively participate in addition reactions.
2. The reactivity of alkenes is due to the fact that the bond is easily broken under the influence of reagents.
3. As a result of addition, the transition of carbon atoms from sp 2 to sp 3 - a hybrid state occurs. The reaction product has a limiting character.
4. When ethylene, propylene and other alkenes are heated under pressure or in the presence of a catalyst, their individual molecules are combined into long chains - polymers. Polymers (polyethylene, polypropylene) are of great practical importance.
3. Application of alkenes(student message according to the following plan).
1 – production of fuel with a high octane number;
2 – plastics;
3 – explosives;
4 – antifreeze;
5 – solvents;
6 – to accelerate fruit ripening;
7 – production of acetaldehyde;
8 – synthetic rubber.
III. Reinforcing the material learned
Homework:§§ 15, 16, ex. 1, 2, 3 p. 90, ex. 4, 5 p. 95.
The simplest alkene is ethene C 2 H 4. According to the IUPAC nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix “-ane” with “-ene”; The position of the double bond is indicated by an Arabic numeral.
Spatial structure of ethylene
By the name of the first representative of this series - ethylene - such hydrocarbons are called ethylene.
Nomenclature and isomerism
Nomenclature
Alkenes of simple structure are often named by replacing the suffix -ane in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.
According to systematic nomenclature, the names of ethylene hydrocarbons are made by replacing the suffix -ane in the corresponding alkanes with the suffix -ene (alkane - alkene, ethane - ethene, propane - propene, etc.). The choice of the main chain and the naming order are the same as for alkanes. However, the chain must necessarily include a double bond. The numbering of the chain begins from the end to which this connection is located closest. For example:
Sometimes rational names are also used. In this case, all alkene hydrocarbons are considered as substituted ethylene:
Unsaturated (alkene) radicals are called by trivial names or by systematic nomenclature:
H 2 C = CH - - vinyl (ethenyl)
H 2 C = CH - CH 2 - -allyl (propenyl-2)
Isomerism
Alkenes are characterized by two types of structural isomerism. In addition to the isomerism associated with the structure of the carbon skeleton (as in alkanes), there is an isomerism that depends on the position of the double bond in the chain. This leads to an increase in the number of isomers in the series of alkenes.
The first two members of the homologous series of alkenes - (ethylene and propylene) - do not have isomers and their structure can be expressed as follows:
H 2 C = CH 2 ethylene (ethene)
H 2 C = CH - CH 3 propylene (propene)
Multiple bond position isomerism
H 2 C = CH - CH 2 - CH 3 butene-1
H 3 C - CH = CH - CH 3 butene-2
Geometric isomerism - cis-, trans-isomerism.
This isomerism is typical for compounds with a double bond.
If a simple σ-bond allows free rotation of individual links of the carbon chain around its axis, then such rotation does not occur around a double bond. This is the reason for the appearance of geometric ( cis-, trans-) isomers.
Geometric isomerism is one of the types of spatial isomerism.
Isomers in which the same substituents (at different carbon atoms) are located on one side of the double bond are called cis-isomers, and on the opposite side - trans-isomers:
Cis- And trance- isomers differ not only in their spatial structure, but also in many physical and chemical properties. Trance- isomers are more stable than cis- isomers.
Preparation of alkenes
Alkenes are rare in nature. Typically, gaseous alkenes (ethylene, propylene, butylenes) are isolated from oil refining gases (during cracking) or associated gases, as well as from coal coking gases.
In industry, alkenes are obtained by dehydrogenation of alkanes in the presence of a catalyst (Cr 2 O 3).
Dehydrogenation of alkanes
H 3 C - CH 2 - CH 2 - CH 3 → H 2 C = CH - CH 2 - CH 3 + H 2 (butene-1)
H 3 C - CH 2 - CH 2 - CH 3 → H 3 C - CH = CH - CH 3 + H 2 (butene-2)
Among the laboratory methods of production, the following can be noted:
1. Elimination of hydrogen halide from alkyl halides under the action of an alcoholic alkali solution on them:
2. Hydrogenation of acetylene in the presence of a catalyst (Pd):
H-C ≡ C-H + H 2 → H 2 C = CH 2
3. Dehydration of alcohols (elimination of water).
Acids (sulfuric or phosphoric) or Al 2 O 3 are used as a catalyst:
In such reactions, hydrogen is split off from the least hydrogenated (with the smallest number of hydrogen atoms) carbon atom (A.M. Zaitsev’s rule):
Physical properties
The physical properties of some alkenes are shown in the table below. The first three representatives of the homologous series of alkenes (ethylene, propylene and butylene) are gases, starting with C 5 H 10 (amylene, or pentene-1) are liquids, and with C 18 H 36 are solids. As molecular weight increases, melting and boiling points increase. Normal alkenes boil at a higher temperature than their isomers. Boiling points cis-isomers higher than trance-isomers, and the melting points are the opposite.
Alkenes are poorly soluble in water (however, better than the corresponding alkanes), but well - in organic solvents. Ethylene and propylene burn with a smoky flame.
Physical properties of some alkenes
Name |
t pl,°С |
t kip, °C |
||
Ethylene (ethene) |
||||
Propylene (propene) |
||||
Butylene (butene-1) |
||||
Cis-butene-2 |
||||
Trans-butene-2 |
||||
Isobutylene (2-methylpropene) |
||||
Amylene (pentene-1) |
||||
Hexylene (hexene-1) |
||||
Heptylene (heptene-1) |
||||
Octylene (octene-1) |
||||
Nonylene (nonene-1) |
||||
Decylene (decene-1) |
Alkenes are slightly polar, but are easily polarized.
Chemical properties
Alkenes are highly reactive. Their chemical properties are determined mainly by the carbon-carbon double bond.
The π-bond, as the least strong and more accessible, breaks under the action of the reagent, and the released valences of carbon atoms are spent on attaching the atoms that make up the reagent molecule. This can be represented as a diagram:
Thus, in addition reactions, the double bond is broken, as it were, by half (with the preservation of the σ-bond).
For alkenes, in addition to addition, oxidation and polymerization reactions are also characteristic.
Addition reactions
More often, addition reactions proceed according to the heterolytic type, being electrophilic addition reactions.
1. Hydrogenation (addition of hydrogen). Alkenes, adding hydrogen in the presence of catalysts (Pt, Pd, Ni), pass into saturated hydrocarbons - alkanes:
H 2 C = CH 2 + H 2 → H 3 C - CH 3 (ethane)
2. Halogenation (addition of halogens). Halogens easily add at the site of double bond rupture to form dihalogen derivatives:
H 2 C = CH 2 + Cl 2 → ClH 2 C - CH 2 Cl (1,2-dichloroethane)
The addition of chlorine and bromine is easier, and iodine is more difficult. Fluorine reacts with alkenes, as well as with alkanes, explosively.
Compare: in alkenes, the halogenation reaction is a process of addition, not substitution (as in alkanes).
The halogenation reaction is usually carried out in a solvent at ordinary temperature.
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 π-bond electrons. The halogen atom, which acquires a certain fractional positive charge, forms an unstable intermediate with the electrons of the π bond, called a π complex or a charge transfer complex. It should be noted that in the π-complex the halogen does not form a directional bond with any specific carbon atom; in this complex the donor-acceptor interaction of the electron pair of the π bond as the donor and the halogen as the acceptor is simply realized.
The π-complex then 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 stage can formally be considered as a nucleophilic substitution of SN 2 at the carbon atom, where the leaving group is Br +.
The result of this reaction is not difficult to predict: the bromine anion attacks the carbocation to form dibromoethane.
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 to alkenes (bromination reaction) is a qualitative reaction to saturated hydrocarbons. When unsaturated hydrocarbons are passed through bromine water (a solution of bromine in water), the yellow color disappears (in the case of saturated hydrocarbons, it remains).
3. Hydrohalogenation (addition of hydrogen halides). Alkenes easily add hydrogen halides:
H 2 C = CH 2 + HBr → H 3 C - CH 2 Br
The addition of hydrogen halides to ethylene homologues follows the rule of V.V. Markovnikov (1837 - 1904): under normal conditions, the hydrogen of the hydrogen halide is added at the site of the double bond to the most hydrogenated carbon atom, and the halogen to the less hydrogenated one:
Markovnikov's rule can be explained by the fact that in unsymmetrical alkenes (for example, in propylene), the electron density is unevenly distributed. Under the influence of the methyl group bonded directly to the double bond, the electron density shifts towards this bond (to the outermost carbon atom).
As a result of this displacement, the p-bond is polarized and partial charges arise on the carbon atoms. It is easy to imagine that a positively charged hydrogen ion (proton) will attach to a carbon atom (electrophilic addition) that has a partial negative charge, and a bromine anion will attach to a carbon that has a partial positive charge.
This addition is a consequence of the mutual influence of atoms in an organic molecule. As you know, the electronegativity of the carbon atom is slightly higher than that of hydrogen.
Therefore, in the methyl group there is some polarization of C-H σ bonds associated with a shift in electron density from hydrogen atoms to carbon. In turn, this causes an increase in the electron density in the region of the double bond and especially on its outermost atom. Thus, the methyl group, like other alkyl groups, acts as an electron donor. However, in the presence of peroxide compounds or O 2 (when the reaction is radical), this reaction can also go against Markovnikov’s rule.
For the same reasons, Markovnikov’s rule is observed when adding not only hydrogen halides, but also other electrophilic reagents (H 2 O, H 2 SO 4, HOCl, ICl, etc.) to unsymmetrical alkenes.
4. Hydration (addition of water). In the presence of catalysts, water adds to alkenes to form alcohols. For example:
H 3 C - CH = CH 2 + H - OH → H 3 C - CHOH - CH 3 (isopropyl alcohol)
Oxidation reactions
Alkenes are oxidized more easily than alkanes. The products formed during the oxidation of alkenes and their structure depend on the structure of the alkenes and on the conditions of this reaction.
1. Combustion
H 2 C = CH 2 + 3O 2 → 2СO 2 + 2H 2 O
2. Incomplete catalytic oxidation
3. Oxidation at ordinary temperature. When an aqueous solution of KMnO 4 acts on ethylene (under normal conditions, in a neutral or alkaline medium - the Wagner reaction), a dihydric alcohol - ethylene glycol is formed:
3H 2 C = CH 2 + 2KMnO 4 + 4H 2 O → 3HOCH 2 - CH 2 OH (ethylene glycol) + 2MnO 2 + KOH
This reaction is qualitative: the purple color of the potassium permanganate solution changes when an unsaturated compound is added to it.
Under more severe conditions (oxidation of KMnO4 in the presence of sulfuric acid or a chromium mixture), the double bond in the alkene breaks to form oxygen-containing products:
H 3 C - CH = CH - CH 3 + 2O 2 → 2H 3 C - COOH (acetic acid)
Isomerization reaction
When heated or in the presence of catalysts, alkenes are capable of isomerization - the movement of the double bond occurs or the establishment of isostructure.
Polymerization reactions
By breaking π bonds, alkene molecules can connect with each other, forming long chain molecules.
Occurrence in nature and physiological role of alkenes
Acyclic alkenes are practically never found in nature. The simplest representative of this class of organic compounds - ethylene C 2 H 4 - is a hormone for plants and is synthesized in them in small quantities.
One of the few natural alkenes is muskalur ( cis- tricosen-9) is a sexual attractant of the female house fly (Musca domestica).
Lower alkenes in high concentrations have a narcotic effect. Higher members of the series also cause convulsions and irritation of the mucous membranes of the respiratory tract
Individual representatives
Ethylene (ethene) is an organic chemical compound described by the formula C 2 H 4. It is the simplest alkene. Contains a double bond and therefore belongs to unsaturated or unsaturated hydrocarbons. It plays an extremely important role in industry, and is also a phytohormone (low molecular weight organic substances produced by plants and having regulatory functions).
Ethylene - causes anesthesia, has an irritating and mutagenic effect.
Ethylene is the most produced organic compound in the world; Total global ethylene production in 2008 was 113 million tons and continues to grow at 2-3% per year.
Ethylene is the leading product of basic organic synthesis and is used to produce polyethylene (1st place, up to 60% of the total volume).
Polyethylene is a thermoplastic polymer of ethylene. The most common plastic in the world.
It is a waxy mass of white color (thin sheets are transparent and colorless). Chemical and frost-resistant, insulator, not sensitive to shock (shock absorber), softens when heated (80-120°C), hardens when cooled, adhesion (adhesion of surfaces of dissimilar solids and/or liquid bodies) is extremely low. Sometimes in the popular consciousness it is identified with cellophane - a similar material of plant origin.
Propylene - causes anesthesia (more powerful than ethylene), has a general toxic and mutagenic effect.
Resistant to water, does not react with alkalis of any concentration, with solutions of neutral, acidic and basic salts, organic and inorganic acids, even concentrated sulfuric acid, but decomposes under the action of 50% nitric acid at room temperature and under the influence of liquid and gaseous chlorine and fluorine. Over time, thermal aging occurs.
Plastic film (especially packaging film, such as bubble wrap or tape).
Containers (bottles, jars, boxes, canisters, garden watering cans, seedling pots.
Polymer pipes for sewerage, drainage, water and gas supply.
Electrical insulating material.
Polyethylene powder is used as hot melt adhesive.
Butene-2 - causes anesthesia and has an irritating effect.
The physical properties of alkenes are similar to those of alkanes, although they all have slightly lower melting and boiling points than the corresponding alkanes. For example, pentane has a boiling point of 36 °C, and pentene-1 - 30 °C. Under normal conditions, alkenes C 2 - C 4 are gases. C 5 – C 15 are liquids, starting from C 16 are solids. Alkenes are insoluble in water but highly soluble in organic solvents.
Alkenes are rare in nature. Since alkenes are valuable raw materials for industrial organic synthesis, many methods for their preparation have been developed.
1. The main industrial source of alkenes is the cracking of alkanes that are part of oil:
3. In laboratory conditions, alkenes are obtained by elimination reactions, in which two atoms or two groups of atoms are eliminated from neighboring carbon atoms, and an additional p-bond is formed. Such reactions include the following.
1) Dehydration of alcohols occurs when they are heated with water-removing agents, for example with sulfuric acid at temperatures above 150 ° C:
When H 2 O is eliminated from alcohols, HBr and HCl from alkyl halides, the hydrogen atom is preferentially eliminated from that of the neighboring carbon atoms that is bonded to the smallest number of hydrogen atoms (from the least hydrogenated carbon atom). This pattern is called Zaitsev's rule.
3) Dehalogenation occurs when dihalides that have halogen atoms at adjacent carbon atoms are heated with active metals:
CH 2 Br -CHBr -CH 3 + Mg → CH 2 =CH-CH 3 + Mg Br 2.
The chemical properties of alkenes are determined by the presence of a double bond in their molecules. The electron density of the p-bond is quite mobile and easily reacts with electrophilic particles. Therefore, many reactions of alkenes proceed according to the mechanism electrophilic addition, designated by the symbol A E (from English, addition electrophilic). Electrophilic addition reactions are ionic processes that occur in several stages.
In the first stage, an electrophilic particle (most often this is an H + proton) interacts with the p-electrons of the double bond and forms a p-complex, which is then converted into a carbocation by forming a covalent s-bond between the electrophilic particle and one of the carbon atoms:
alkene p-complex carbocation
In the second stage, the carbocation reacts with the X - anion, forming a second s-bond due to the electron pair of the anion:
In electrophilic addition reactions, a hydrogen ion attaches to the carbon atom at the double bond that has a greater negative charge. The charge distribution is determined by the shift in p-electron density under the influence of substituents: 
.
Electron-donating substituents exhibiting the +I effect shift the p-electron density to a more hydrogenated carbon atom and create a partial negative charge on it. This explains Markovnikov's rule: when adding polar molecules like HX (X = Hal, OH, CN, etc.) to unsymmetrical alkenes, hydrogen preferentially attaches to the more hydrogenated carbon atom at the double bond.
Let's look at specific examples of addition reactions.
1) Hydrohalogenation. When alkenes interact with hydrogen halides (HCl, HBr), alkyl halides are formed:
CH 3 -CH = CH 2 + HBr ® CH 3 -CHBr-CH 3 .
The reaction products are determined by Markovnikov's rule.
It should, however, be emphasized that in the presence of any organic peroxide, polar HX molecules do not react with alkenes according to Markovnikov’s rule:
| R-O-O-R | ||
| CH 3 -CH = CH 2 + HBr | CH 3 -CH 2 -CH 2 Br |
This is due to the fact that the presence of peroxide determines the radical rather than ionic mechanism of the reaction.
2) Hydration. When alkenes react with water in the presence of mineral acids (sulfuric, phosphoric), alcohols are formed. Mineral acids act as catalysts and are sources of protons. The addition of water also follows Markovnikov’s rule:
CH 3 -CH = CH 2 + HON ® CH 3 -CH (OH) -CH 3 .
3) Halogenation. Alkenes discolor bromine water:
CH 2 = CH 2 + Br 2 ® B-CH 2 -CH 2 Br.
This reaction is qualitative for a double bond.
4) Hydrogenation. The addition of hydrogen occurs under the action of metal catalysts:
where R = H, CH 3, Cl, C 6 H 5, etc. The CH 2 =CHR molecule is called a monomer, the resulting compound is called a polymer, the number n is the degree of polymerization.
Polymerization of various alkene derivatives produces valuable industrial products: polyethylene, polypropylene, polyvinyl chloride and others.
In addition to addition, alkenes also undergo oxidation reactions. During the mild oxidation of alkenes with an aqueous solution of potassium permanganate (Wagner reaction), dihydric alcohols are formed:
ZSN 2 =CH 2 + 2KMn O 4 + 4H 2 O ® ZNOSN 2 -CH 2 OH + 2MnO 2 ↓ + 2KOH.
As a result of this reaction, the purple solution of potassium permanganate quickly becomes discolored and a brown precipitate of manganese (IV) oxide precipitates. This reaction, like the decolorization reaction of bromine water, is qualitative for a double bond. During the severe oxidation of alkenes with a boiling solution of potassium permanganate in an acidic environment, the double bond is completely broken with the formation of ketones, carboxylic acids or CO 2, for example:
| [ABOUT] | ||
| CH 3 -CH=CH-CH 3 | 2CH 3 -COOH |
Based on the oxidation products, the position of the double bond in the original alkene can be determined.
Like all other hydrocarbons, alkenes burn and, with plenty of air, form carbon dioxide and water:
C n H 2 n + Zn /2O 2 ® n CO 2 + n H 2 O.
When air is limited, combustion of alkenes can lead to the formation of carbon monoxide and water:
C n H 2n + nO 2 ® nCO + nH 2 O .
If you mix an alkene with oxygen and pass this mixture over a silver catalyst heated to 200°C, an alkene oxide (epoxyalkane) is formed, for example:
At any temperature, alkenes are oxidized by ozone (ozone is a stronger oxidizing agent than oxygen). If ozone gas is passed through a solution of an alkene in methane tetrachloride at temperatures below room temperature, an addition reaction occurs and the corresponding ozonides (cyclic peroxides) are formed. Ozonides are very unstable and can explode easily. Therefore, they are usually not isolated, but immediately after production they are decomposed with water - this produces carbonyl compounds (aldehydes or ketones), the structure of which indicates the structure of the alkene that was subjected to ozonation.
Lower alkenes are important starting materials for industrial organic synthesis. Ethyl alcohol, polyethylene, and polystyrene are produced from ethylene. Propene is used for the synthesis of polypropylene, phenol, acetone, and glycerin.
Alkenes (olefins, ethylene hydrocarbons C n H 2n
Homologous series.
|
ethene (ethylene) | |
The simplest alkene is ethylene (C 2 H 4). According to the IUPAC nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix “-ane” with “-ene”; The position of the double bond is indicated by an Arabic numeral.
Hydrocarbon radicals formed from alkenes have the suffix "-enil". Trivial names: CH 2 =CH- "vinyl", CH 2 =CH-CH 2 - "allyl".
The carbon atoms at the double bond are in a state of sp² hybridization and have a bond angle of 120°.
Alkenes are characterized by isomerism of the carbon skeleton, double bond positions, interclass and spatial.
Physical properties
The melting and boiling points of alkenes (simplified) increase with molecular weight and length of the carbon backbone.
Under normal conditions, alkenes from C 2 H 4 to C 4 H 8 are gases; from pentene C 5 H 10 to hexadecene C 17 H 34 inclusive - liquids, and starting from octadecene C 18 H 36 - solids. Alkenes are insoluble in water, but are highly soluble in organic solvents.
Dehydrogenation of alkanes
This is one of the industrial methods for producing alkenes
Hydrogenation of alkynes
Partial hydrogenation of alkynes requires special conditions and the presence of a catalyst
A double bond is a combination of sigma and pi bonds. A sigma bond occurs when sp2 orbitals overlap axially, and a pi bond occurs when there is lateral overlap.
Zaitsev's rule:
The abstraction of a hydrogen atom in elimination reactions occurs predominantly from the least hydrogenated carbon atom.
13. Alkenes. Structure. sp 2 hybridization, multiple coupling parameters. Reactions of electrophilic addition of halogens, hydrogen halides, hypochlorous acid. Hydration of alkenes. Morkovnikov's rule. Mechanisms of reactions.
Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n
One s- and 2 p-orbitals mix and form 2 equivalent sp2-hybrid orbitals located in the same plane at an angle of 120.
If a bond is formed by more than one pair of electrons, then it is called multiple.
A multiple bond is formed when there are too few electrons and bonding atoms for each bond-forming valence orbital of the central atom to overlap with any orbital of the surrounding atom.
Electrophilic addition reactions
In these reactions, the attacking particle is an electrophile.
Halogenation:
Hydrohalogenation
Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov’s rule
Markovnikov rule
Addition of hypochlorous acid to form chlorohydrins:
Hydration
The addition of water to alkenes occurs in the presence of sulfuric acid:
Carbocation- a particle in which a positive charge is concentrated on the carbon atom; the carbon atom has a vacant p-orbital.
14. Ethylene hydrocarbons. Chemical properties: reactions with oxidizing agents. Catalytic oxidation, reaction with peracids, oxidation reaction to glycols, with cleavage of the carbon-carbon bond, ozonation. Wacker process. Substitution reactions.
Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n
Oxidation
Oxidation of alkenes can occur, depending on the conditions and types of oxidizing reagents, both with the cleavage of the double bond and with the preservation of the carbon skeleton.
When burned in air, olefins produce carbon dioxide and water.
H 2 C=CH 2 + 3O 2 => 2CO 2 + 2H 2 O
C n H 2n+ 3n/O 2 => nCO 2 + nH 2 O – general formula
Catalytic oxidation
In the presence of palladium salts, ethylene is oxidized to acetaldehyde. Acetone is formed from propene in the same way.
When alkenes are exposed to strong oxidizing agents (KMnO 4 or K 2 Cr 2 O 7 in H 2 SO 4), the double bond breaks when heated:
When alkenes are oxidized with a dilute solution of potassium permanganate, diatomic alcohols are formed - glycols (E.E. Wagner reaction). The reaction takes place in the cold.
Acyclic and cyclic alkenes, when reacting with peracids RCOOOH in a non-polar environment, form epoxides (oxiranes), therefore the reaction itself is called the epoxidation reaction.
Ozonation of alkenes.
When alkenes react with ozone, peroxide compounds are formed, which are called ozonides. The reaction of alkenes with ozone is the most important method for the oxidative cleavage of alkenes at the double bond.
Alkenes do not undergo substitution reactions.
Wacker process-the process of producing acetaldehyde by direct oxidation of ethylene.
The Wacker process is based on the oxidation of ethylene with palladium dichloride:
CH 2 = CH 2 + PdCl 2 + H 2 O = CH 3 CHO + Pd + 2HCl
15. Alkenes: chemical properties. Hydrogenation. Lebedev's rule. Isomerization and oligomerization of alkenes. Radical and ionic polymerization. The concept of polymer, oligomer, monomer, elementary unit, degree of polymerization. Telomerization and copolymerization.
Hydrogenation
Hydrogenation of alkenes directly with hydrogen occurs only in the presence of a catalyst. Hydrogenation catalysts include platinum, palladium, and nickel.
Hydrogenation can also be carried out in the liquid phase with homogeneous catalysts
Isomerization reactions
When heated, isomerization of alkene molecules is possible, which
can lead to both double bond movement and skeletal changes
hydrocarbon.
CH2=CH-CH2-CH3 CH3-CH=CH-CH3
Polymerization reactions
This is a type of addition reaction. Polymerization is the reaction of sequential combination of identical molecules into larger molecules, without isolating any low-molecular-weight product. During polymerization, a hydrogen atom is added to the most hydrogenated carbon atom located at the double bond, and the rest of the molecule is added to the other carbon atom.
CH2=CH2 + CH2=CH2 + ... -CH2-CH2-CH2-CH2- ...
or n CH2=CH2 (-CH2-CH2-)n (polyethylene)
A substance whose molecules undergo a polymerization reaction is called monomer. A monomer molecule must have at least one double bond. The resulting polymers consist of a large number of repeating chains having the same structure ( elementary units). The number showing how many times a structural (elementary) unit is repeated in a polymer is called degree of polymerization(n).
Depending on the type of intermediate particles formed during polymerization, there are 3 polymerization mechanisms: a) radical; b) cationic; c) anionic.
The first method produces high-density polyethylene:
The reaction catalyst is peroxides.
The second and third methods involve the use of acids (cationic polymerization) and organometallic compounds as catalysts.
In chemistry oligomer) - a molecule in the form of a chain of small number of identical constituent links.
Telomerization
Telomerization is the oligomerization of alkenes in the presence of chain transfer agents (telogens). As a result of the reaction, a mixture of oligomers (telomeres) is formed, the end groups of which are parts of telogen. For example, in the reaction of CCl 4 with ethylene, the telogen is CCl 4 .
CCl 4 + nCH 2 =CH 2 => Cl(CH 2 CH 2) n CCl 3
The initiation of these reactions can be carried out by radical initiators or g-radiation.
16. Alkenes. Reactions of radical addition of halogens and hydrogen halides (mechanism). Addition of carbenes to olefins. Ethylene, propylene, butylenes. Industrial sources and main uses.
Alkenes readily add halogens, especially chlorine and bromine (halogenation).
A typical reaction of this type is the discoloration of bromine water
CH2=CH2 + Br2 → CH2Br-CH2Br (1,2-dibromoethane)
Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov’s rule:
Markovnikov rule: When adding protic acids or water to unsymmetrical alkenes or alkynes, hydrogen is added to the most hydrogenated carbon atom
A hydrogenated carbon atom is one that has hydrogen attached to it. Most hydrogenated - where there is most H
Carbene addition reactions
CR 2 carbenes: - highly reactive short-lived species that can easily add to the double bond of alkenes. As a result of the carbene addition reaction, cyclopropane derivatives are formed
Ethylene is an organic chemical described by the formula C 2 H 4. Is the simplest alkene ( olefin)compound. Under normal conditions, it is a colorless flammable gas with a faint odor. Partially soluble in water. Contains a double bond and therefore belongs to unsaturated or unsaturated hydrocarbons. Plays an extremely important role in industry. Ethylene is the most produced organic compound in the world: Ethylene oxide; polyethylene, acetic acid, ethyl alcohol.
Basic chemical properties(don’t teach me, just let them be there just in case, in case they can write it off)
Ethylene is a chemically active substance. Since there is a double bond between the carbon atoms in the molecule, one of them, which is less strong, is easily broken, and at the site of the bond break the attachment, oxidation, and polymerization of molecules occurs.
Halogenation:
CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br
Bromine water becomes discolored. This is a qualitative reaction to unsaturated compounds.
Hydrogenation:
CH 2 =CH 2 + H - H → CH 3 - CH 3 (under the influence of Ni)
Hydrohalogenation:
CH 2 =CH 2 + HBr → CH 3 - CH 2 Br
Hydration:
CH 2 =CH 2 + HOH → CH 3 CH 2 OH (under the influence of a catalyst)
This reaction was discovered by A.M. Butlerov, and it is used for the industrial production of ethyl alcohol.
Oxidation:
Ethylene oxidizes easily. If ethylene is passed through a solution of potassium permanganate, it will become discolored. This reaction is used to distinguish between saturated and unsaturated compounds. Ethylene oxide is a fragile substance; the oxygen bridge breaks and water joins, resulting in the formation of ethylene glycol. Reaction equation:
3CH 2 =CH 2 + 2KMnO 4 + 4H 2 O → 3HOH 2 C - CH 2 OH + 2MnO 2 + 2KOH
C 2 H 4 + 3O 2 → 2CO 2 + 2H 2 O
Polymerization (production of polyethylene):
nCH 2 =CH 2 → (-CH 2 -CH 2 -) n
Propylene(propene) CH 2 = CH-CH 3 - unsaturated (unsaturated) hydrocarbon of the ethylene series, flammable gas. Propylene is a gaseous substance with a low boiling point t boil = −47.6 °C
Typically, propylene is isolated from oil refining gases (during cracking of crude oil, pyrolysis of gasoline fractions) or associated gases, as well as from coal coking gases.
