EntranceSpecific properties of alkenes. Characteristic chemical properties of hydrocarbons
Containing a pi bond are unsaturated hydrocarbons. They are derivatives of alkanes, in the molecules of which two hydrogen atoms have been eliminated. The resulting free valencies form a new type of bond, which is located perpendicular to the plane of the molecule. This is how a new group of compounds arises - alkenes. We will consider the physical properties, production and use of substances of this class in everyday life and industry in this article.
Homologous series of ethylene
The general formula of all compounds called alkenes, reflecting their qualitative and quantitative composition, is C n H 2 n. The names of hydrocarbons according to the systematic nomenclature have the following form: in the term of the corresponding alkane the suffix changes from -ane to -ene, for example: ethane - ethene, propane - propene, etc. In some sources you can find another name for compounds of this class - olefins. Next, we will study the process of double bond formation and the physical properties of alkenes, and also determine their dependence on the structure of the molecule.
How is a double bond formed?
Using the example of ethylene, the electronic nature of the pi bond can be represented as follows: the carbon atoms in its molecule are in the form of sp 2 hybridization. In this case, a sigma bond is formed. Two more hybrid orbitals, one each from carbon atoms, form simple sigma bonds with hydrogen atoms. The two remaining free hybrid clouds of carbon atoms overlap above and below the plane of the molecule - a pi bond is formed. It is this that determines the physical and chemical properties of alkenes, which will be discussed further.
Spatial isomerism
Compounds that have the same quantitative and qualitative composition of molecules, but different spatial structures, are called isomers. Isomerism occurs in a group of substances called organics. The characteristics of olefins are greatly influenced by the phenomenon of optical isomerism. It is expressed in the fact that homologues of ethylene, containing different radicals or substituents at each of the two carbon atoms at the double bond, can occur in the form of two optical isomers. They differ from each other in the position of the substituents in space relative to the plane of the double bond. The physical properties of alkenes in this case will also be different. For example, this concerns the boiling and melting points of substances. Thus, olefins with a straight carbon skeleton have higher boiling points than isomer compounds. Also, the boiling points of cis isomers of alkenes are higher than trans isomers. With regard to melting temperatures, the picture is the opposite.
Comparative characteristics of the physical properties of ethylene and its homologues
The first three representatives of olefins are gaseous compounds, then, starting with pentene C 5 H 10 and up to the alkene with the formula C 17 H 34, they are liquids, and then there are solids. The following trend can be observed among ethene homologues: the boiling points of the compounds decrease. For example, for ethylene this indicator is -169.1°C, and for propylene -187.6°C. But boiling temperatures increase with increasing molecular weight. So, for ethylene it is -103.7°C, and for propene -47.7°C. To summarize what has been said, we can draw a short conclusion: the physical properties of alkenes depend on their molecular weight. With its increase, the state of aggregation of compounds changes in the direction: gas - liquid - solid, and the melting point decreases, and the boiling point increases.
Characteristics of ethene
The first representative of the homologous series of alkenes is ethylene. It is a gas, slightly soluble in water, but highly soluble in organic solvents and has no color. Molecular weight - 28, ethene is slightly lighter than air, has a subtle sweetish odor. It reacts easily with halogens, hydrogen and hydrogen halides. The physical properties of alkenes and paraffins are nevertheless quite similar. For example, the state of aggregation, the ability of methane and ethylene to undergo severe oxidation, etc. How can alkenes be distinguished? How to identify the unsaturated nature of an olefin? For this purpose, there are qualitative reactions, which we will dwell on in more detail. Let us recall what peculiarity alkenes have in the structure of the molecule. The physical and chemical properties of these substances are determined by the presence of a double bond in their composition. To prove its presence, pass the hydrocarbon gas through a violet solution of potassium permanganate or bromine water. If they become discolored, it means that the compound contains pi bonds in its molecules. Ethylene enters into an oxidation reaction and discolors solutions of KMnO 4 and Br 2.
Mechanism of addition reactions
The cleavage of the double bond ends with the addition of atoms of other chemical elements to the free valences of carbon. For example, when ethylene reacts with hydrogen, called hydrogenation, it produces ethane. A catalyst such as powdered nickel, palladium or platinum is required. The reaction with HCl ends with the formation of chloroethane. Alkenes containing more than two carbon atoms in their molecules undergo the addition of hydrogen halides taking into account V. Markovnikov’s rule.
How ethene homologues interact with hydrogen halides
If we are faced with the task “Characterize the physical properties of alkenes and their preparation,” we need to consider V. Markovnikov’s rule in more detail. It has been established in practice that homologues of ethylene react with hydrogen chloride and other compounds at the site of double bond cleavage, obeying a certain pattern. It consists in the fact that a hydrogen atom is attached to the most hydrogenated carbon atom, and a chlorine, bromine or iodine ion is attached to the carbon atom containing the least number of hydrogen atoms. This feature of the occurrence of addition reactions is called V. Markovnikov’s rule.
Hydration and polymerization
Let us continue to consider the physical properties and applications of alkenes using the example of the first representative of the homologous series - ethene. Its reaction with water is used in the organic synthesis industry and is of great practical importance. The process was first carried out in the 19th century by A.M. Butlerov. The reaction requires the fulfillment of a number of conditions. This is, first of all, the use of concentrated sulfuric acid or oleum as a catalyst and ethene solvent, a pressure of about 10 atm and a temperature within 70°. The hydration process occurs in two phases. First, at the site where the pi bond is broken, sulfate acid molecules join ethene, resulting in the formation of ethyl sulfuric acid. Then the resulting substance reacts with water to produce ethyl alcohol. Ethanol is an important product used in the food industry to produce plastics, synthetic rubbers, varnishes and other organic chemical products.
Olefin based polymers
Continuing to study the issue of using substances belonging to the class of alkenes, we will study the process of their polymerization, in which compounds containing unsaturated chemical bonds in the composition of their molecules can participate. There are several types of polymerization reactions that produce high molecular weight products - polymers, for example polyethylene, polypropylene, polystyrene, etc. The free radical mechanism leads to the production of high-density polyethylene. It is one of the most widely used compounds in industry. The cation-ion type ensures the production of a polymer with a stereoregular structure, for example polystyrene. It is considered one of the safest and most convenient polymers to use. Polystyrene products are resistant to aggressive substances: acids and alkalis, non-flammable, and easy to paint. Another type of polymerization mechanism is dimerization, which leads to the production of isobutene, which is used as an anti-knock additive for gasoline.
Methods of obtaining
Alkenes, the physical properties of which we study, are obtained in the laboratory and in industry by various methods. In experiments in the school course of organic chemistry, the process of dehydration of ethyl alcohol with the help of water-removing agents, for example, such as phosphorus pentoxide or sulfate acid, is used. The reaction is carried out by heating and is the reverse of the process for producing ethanol. Another common method for producing alkenes has found its application in industry, namely: heating halogen derivatives of saturated hydrocarbons, for example, chlorpropane with concentrated alcohol solutions of alkalis - sodium or potassium hydroxide. In the reaction, a hydrogen chloride molecule is eliminated, and a double bond is formed at the site where the free valences of the carbon atoms appear. The end product of the chemical process will be an olefin - propene. Continuing to consider the physical properties of alkenes, let us dwell on the main process of producing olefins - pyrolysis.
Industrial production of unsaturated hydrocarbons of the ethylene series
Cheap raw materials - gases formed during the cracking of oil, serve as a source for the production of olefins in the chemical industry. For this purpose, a technological scheme of pyrolysis is used - the splitting of a gas mixture, which occurs with the rupture of carbon bonds and the formation of ethylene, propene and other alkenes. Pyrolysis is carried out in special ovens consisting of individual pyrolysis coils. They create a temperature of about 750-1150°C and contain water vapor as a diluent. Reactions occur via a chain mechanism with the formation of intermediate radicals. The final product is ethylene or propene, they are produced in large volumes.
We have studied in detail the physical properties, as well as the applications and methods of producing alkenes.
Alkenes are characterized primarily by reactions accession through a double bond. Basically, these reactions proceed by an ionic mechanism. The pi bond is broken and two new sigma bonds are formed. Let me remind you that substitution reactions were typical for alkanes and they followed a radical mechanism. Hydrogen molecules can attach to alkenes; these reactions are called hydrogenation, water molecules, hydration, halogens halogenation, hydrogen halides hydrohalogenation. But first things first.
Double bond addition reactions
So, first chemical property ability to add hydrogen halides, hydrohalogenation.
Propene and other alkenes react with hydrogen halides according to Markovnikov's rule.
A hydrogen atom attaches to the most hydrogenated, or more correctly hydrogenated, carbon atom.
Second number on our list of properties would be hydration, the addition of water.
The reaction takes place when heated in the presence of an acid, usually sulfuric or phosphoric. The addition of water also occurs according to Markovnikov’s rule, that is, primary alcohol can only be obtained by hydration of ethylene, the remaining unbranched alkenes give secondary alcohols.
There are exceptions to Markovnikov's rule for both hydrohalogenation and hydration. Firstly, contrary to this rule, the addition occurs in the presence of peroxides.
Secondly, for derivatives of alkenes in which electron-withdrawing groups are present. For example, for 3,3,3-trifluoropropene-1.
Fluorine atoms, due to their high electronegativity, attract electron density to themselves along a chain of sigma bonds. This phenomenon is called a negative inductive effect.
Because of this, the mobile pi electrons of the double bond are displaced and the outermost carbon atom ends up with a partial positive charge, which is usually designated as delta plus. It is to this that the negatively charged bromine ion will go, and the hydrogen cation will attach to the least hydrogenated carbon atom.
In addition to the trifluoromethyl group, for example, the trichloromethyl group, nitro group, carboxyl group and some others have a negative inductive effect.
This second case of violation of the Markovnikov rule in the Unified State Exam is very rare, but it is still advisable to keep it in mind if you plan to pass the exam with the maximum score.
Third chemical property attachment of halogen molecules.
This primarily concerns bromine, since this reaction is qualitative for a multiple bond. When, for example, ethylene is passed through bromine water, that is, a solution of bromine in water that is brown in color, it becomes discolored. If you pass a mixture of gases, for example, ethane and ethene, through bromine water, you can get pure ethane without ethene impurities, since it will remain in the reaction flask in the form of dibromoethane, which is a liquid.
Of particular note is the reaction of alkenes in the gas phase with strong heating, for example, with chlorine.
Under such conditions, it is not an addition reaction that occurs, but a substitution reaction. Moreover, exclusively at the alpha carbon atom, that is, the atom adjacent to the double bond. In this case, 3-chloropropene-1 is obtained. These reactions are infrequent in the exam, so most students do not remember them and, as a rule, make mistakes.
Fourth number is the hydrogenation reaction, and with it the dehydrogenation reaction. That is, the addition or removal of hydrogen.
Hydrogenation occurs at a not very high temperature on a nickel catalyst. At higher temperatures, dehydrogenation is possible to produce alkynes.
Fifth A property of alkenes is the ability to polymerize, when hundreds and thousands of alkene molecules form very long and strong chains due to the breaking of the pi bond and the formation of sigma bonds with each other.
In this case, the result was polyethylene. Please note that the resulting molecule contains no multiple bonds. Such substances are called polymers, the original molecules are called monomers, the repeating fragment is the elementary unit of the polymer, and the number n is the degree of polymerization.
Reactions to produce other important polymeric materials, such as polypropylene, are also possible.
Another important polymer is polyvinyl chloride.
The starting material for the production of this polymer is chloroethene, whose common name is vinyl chloride. Because this unsaturated substituent is called vinyl. The frequently encountered abbreviation on plastic products, PVC, stands for polyvinyl chloride.
We discussed five properties that represented double bond addition reactions. Now let's look at the reactions oxidation.
Alkene oxidation reactions
Sixth chemical property in our general list is mild oxidation or Wagner reaction. It occurs when an alkene is exposed to an aqueous solution of potassium permanganate in the cold, which is why the temperature of zero degrees is often indicated in exam tasks.
The result is a dihydric alcohol. In this case, ethylene glycol, and in general such alcohols are collectively called glycols. During the reaction, the purple-pink permanganate solution becomes discolored, so this reaction is also qualitative for a double bond. Manganese in a neutral environment is reduced from oxidation state +7 to oxidation state +4. Let's look at a few more examples. THE EQUATION
Here we get propanediol-1,2. However, cyclic alkenes will react in the same way. THE EQUATION
Another option is when the double bond is located, for example, in the side chain of aromatic hydrocarbons. The Wagner reaction involving styrene, its other name is vinylbenzene, is regularly encountered in exam assignments.
I hope that I have provided enough examples for you to understand that the mild oxidation of a double bond always follows a fairly simple rule: the pi bond is broken and a hydroxy group is added to each carbon atom.
Now, regarding hard oxidation. It will be ours seventh property. This oxidation occurs when an alkene reacts with an acidic solution of potassium permanganate when heated.
The destruction of the molecule occurs, that is, its destruction at the double bond. In the case of butene-2, two molecules of acetic acid were obtained. In general, the position of the multiple bond in the carbon chain can be judged from the oxidation products.
The oxidation of butene-1 produces a molecule of propionic (propanoic) acid and carbon dioxide.
In the case of ethylene, you get two molecules of carbon dioxide. In all cases, in an acidic environment, manganese is reduced from oxidation state +7 to +2.
And finally eighth property complete oxidation or combustion.
Alkenes burn, like other hydrocarbons, to carbon dioxide and water. Let us write the equation for the combustion of alkenes in general form.
There will be as many carbon dioxide molecules as there are carbon atoms in the alkene molecule, since the CO 2 molecule contains one carbon atom. That is, n CO 2 molecules. There will be two times fewer water molecules than hydrogen atoms, that is, 2n/2, which means just n.
There are the same number of oxygen atoms on the left and right. On the right there are 2n of carbon dioxide plus n of water, for a total of 3n. On the left there are the same number of oxygen atoms, which means there are two times fewer molecules, because the molecule contains two atoms. That is, 3n/2 oxygen molecules. You can write 1.5n.
We have reviewed eight chemical properties of alkenes.
Alkenes- unsaturated hydrocarbons, which contain one double bond. Examples of alkenes:
Methods for obtaining alkenes.
1. Cracking of alkanes at 400-700°C. The reaction occurs via a free radical mechanism:
2. Dehydrogenation of alkanes:
3. Elimination reaction (elimination): 2 atoms or 2 groups of atoms are eliminated from neighboring carbon atoms, and a double bond is formed. Such reactions include:
A) Dehydration of alcohols (heating above 150°C, with the participation of sulfuric acid as a water-removing reagent):
B) Elimination of hydrogen halides when exposed to an alcoholic alkali solution:
The hydrogen atom is split off preferentially from the carbon atom that is bonded to fewer hydrogen atoms (the least hydrogenated atom) - Zaitsev's rule.
B) Dehalogenation:
Chemical properties of alkenes.
The properties of alkenes are determined by the presence of a multiple bond, therefore alkenes enter into electrophilic addition reactions, which occur in several stages (H-X - reagent):
1st stage:
2nd stage:
.
The hydrogen ion in this type of reaction belongs to the carbon atom that has a more negative charge. The density distribution is:
If the substituent is a donor, which manifests the +I- effect, then the electron density shifts towards the most hydrogenated carbon atom, creating a partially negative charge on it. Reactions go according to Markovnikov's rule: when joining polar molecules like NH (HCl, HCN, HOH etc.) to unsymmetrical alkenes, hydrogen attaches preferentially to the more hydrogenated carbon atom at the double bond.
A) Addition reactions:
1) Hydrohalogenation:
The reaction follows Markovnikov's rule. But if peroxide is present in the reaction, then the rule is not taken into account:
2) Hydration. The reaction follows Markovnikov's rule in the presence of phosphoric or sulfuric acid:
3) Halogenation. As a result, bromine water becomes discolored - this is a qualitative reaction to a multiple bond:
4) Hydrogenation. The reaction occurs in the presence of catalysts.
DEFINITION
Alkenes- unsaturated hydrocarbons, the molecules of which contain one double bond; The names of alkenes contain the suffix –ene or –ylene.
The general formula of the homologous series of alkenes (Table 2) is C n H 2n
Table 2. Homologous series of alkenes.
Hydrocarbon radicals formed from alkenes: -CH = CH 2 - vinyl and -CH 2 -CH = CH 2 - allyl.
Alkenes, starting with butene, are characterized by isomerism of the carbon skeleton:
CH 2 -C(CH 3)-CH 3 (2-methylpropen-1)
and positions of the double bond:
CH 2 = CH-CH 2 -CH 3 (butene-1)
CH 3 -C = CH-CH 3 (butene-2)
Alkenes, starting with butene-2, are characterized by geometric (cis-trans) isomerism (Fig. 1).
Rice. 1. Geometric isomers of butene-2.
Alkenes, starting with propene, are characterized by interclass isomerism with cycloalkanes. Thus, the composition of C 4 H 8 corresponds to substances of the class of alkenes and cycloalkanes - butene-1(2) and cyclobutane.
The carbon atoms in alkene molecules are in sp 2 hybridization: 3σ bonds are located in the same plane at an angle of 120 to each other, and the π bond is formed by p-electrons of neighboring carbon atoms. A double bond is a combination of σ and π bonds.
Chemical properties of alkenes
Most chemical reactions of alkenes proceed by the mechanism of electrophilic addition:
- hydrohalogenation - the interaction of alkenes with hydrogen halides (HCl, HBr), proceeding according to Markovnikov’s rule (when polar molecules like HX are added to unsymmetrical alkenes, hydrogen attaches to the more hydrogenated carbon atom at the double bond)
CH 3 -CH = CH 2 + HCl = CH 3 -CHCl-CH 3
- hydration - the interaction of alkenes with water in the presence of mineral acids (sulfuric, phosphoric) with the formation of alcohols, proceeding according to Markovnikov’s rule
CH 3 -C(CH 3) = CH 2 + H 2 O = CH 3 -C(CH 3)OH-CH 3
- halogenation - the interaction of alkenes with halogens, for example, with bromine, in which bromine water becomes discolored
CH 2 = CH 2 + Br 2 = BrCH 2 -CH 2 Br
When a mixture of an alkene and a halogen is heated to 500C, it is possible to replace the hydrogen atom of the alkene by a radical mechanism:
CH 3 -CH = CH 2 + Cl 2 = Cl-CH 2 -CH = CH 2 + HCl
The hydrogenation reaction of alkenes proceeds according to the radical mechanism. The condition for the reaction to occur is the presence of catalysts (Ni, Pd, Pt), as well as heating of the reaction mixture:
CH 2 = CH 2 + H 2 = CH 3 -CH 3
Alkenes can be oxidized to form various products, the composition of which depends on the conditions of the oxidation reaction. Thus, during oxidation under mild conditions (the oxidizing agent is potassium permanganate), the π bond is broken and dihydric alcohols are formed:
3CH 2 = CH 2 + 2KMnO 4 +4H 2 O = 3CH 2 (OH)-CH 2 (OH) +2MnO 2 + 2KOH
During the severe oxidation of alkenes with a boiling solution of potassium permanganate in an acidic environment, complete cleavage of the bond (σ-bond) occurs with the formation of ketones, carboxylic acids or carbon dioxide:
Oxidation of ethylene with oxygen at 200C in the presence of CuCl 2 and PdCl 2 leads to the formation of acetaldehyde:
CH 2 = CH 2 +1/2O 2 = CH 3 -CH = O
Alkenes undergo polymerization reactions. Polymerization is the process of forming a high-molecular compound - a polymer - by combining with each other using the main valences of the molecules of the original low-molecular substance - the monomer. Polymerization can be caused by heat, ultra-high pressure, irradiation, free radicals or catalysts. Thus, the polymerization of ethylene occurs under the action of acids (cationic mechanism) or radicals (radical mechanism):
n CH 2 = CH 2 = -(-CH 2 -CH 2 -) n —
Physical properties of alkenes
Under normal conditions, C 2 -C 4 are gases, C 5 -C 17 are liquids, and starting from C 18 are solids. Alkenes are insoluble in water but highly soluble in organic solvents.
Preparation of alkenes
The main methods for obtaining alkenes:
— dehydrohalogenation of halogenated alkanes under the influence of alcoholic solutions of alkalis
CH 3 -CH 2 -CHBr-CH 3 + KOH = CH 3 -CH = CH-CH 3 + KBr + H 2 O
— dehalogenation of dihalogen derivatives of alkanes under the influence of active metals
CH 3 -CHCl-CHCl-CH 3 + Zn = ZnCl 2 + CH 3 -CH = CH-CH 3
— dehydration of alcohols when heated with sulfuric acid (t >150 C) or passing alcohol vapor over a catalyst
CH 3 -CH(OH)- CH 3 = CH 3 -CH = CH 2 + H 2 O
— dehydrogenation of alkanes by heating (500C) in the presence of a catalyst (Ni, Pt, Pd)
CH 3 -CH 2 - CH 3 = CH 3 -CH = CH 2 + H 2
Alkenes are used as starting products in the production of polymeric materials (plastics, rubbers, films) and other organic substances.
Examples of problem solving
EXAMPLE 1
| Exercise | Establish the molecular formula of an alkene if it is known that the same amount of it, interacting with halogens, forms, respectively, either 56.5 g of a dichloro derivative or 101 g of a dibromo derivative. |
| Solution | The chemical properties of alkenes are determined by their ability to add substances through the mechanism of electrophilic addition, in which the double bond is converted into a single bond: CnH 2 n + Cl 2 → CnH 2 nCl 2 CnH 2 n + Br 2 → CnH 2 nBr 2 The mass of the alkene that entered into the reaction is the same, which means that the same number of moles of alkene participates in the reaction. Let us express the number of moles of hydrocarbon if the molar mass of the dichloro derivative is 12n+2n+71, the molar mass of the dibromo derivative (12n+2n+160): m(CnH 2 nCl 2) \ (12n+2n+71) = m(СnH 2 nBr 2) \ (12n+2n+160) 56.5 \ (12n+2n+71) = 101 \ (12n+2n+160) Therefore, the alkene has the formula C 3 H 6 is propene. |
| Answer | Alkene formula C 3 H 6 is propene |
EXAMPLE 2
| Exercise | Carry out a series of transformations ethane → ethene → ethanol → ethene → chloroethane → butane |
| Solution | To obtain ethene from ethane, it is necessary to use the ethane dehydrogenation reaction, which occurs in the presence of a catalyst (Ni, Pd, Pt) and upon heating: C 2 H 6 →C 2 H 4 + H 2 Ethanol is produced from ethene by a hydration reaction with water in the presence of mineral acids (sulfuric, phosphoric): C 2 H 4 + H 2 O = C 2 H 5 OH To obtain ethene from ethanol, a dehydration reaction is used: C 2 H 5 OH → (t, H 2 SO 4) → C 2 H 4 + H 2 O The production of chloroethane from ethene is carried out by the hydrohalogenation reaction: C 2 H 4 + HCl → C 2 H 5 Cl To obtain butane from chloroethane, the Wurtz reaction is used: 2C 2 H 5 Cl + 2Na → C 4 H 10 + 2NaCl |
In organic chemistry, you can find hydrocarbon substances with different amounts of carbon in the chain and C=C bond. They are homologues and are called alkenes. Due to their structure, they are chemically more reactive than alkanes. But what kind of reactions are typical for them? Let's consider their distribution in nature, different methods of production and use.
What are they?
Alkenes, which are also called olefins (oily), get their name from ethene chloride, a derivative of the first member of this group. All alkenes have at least one C=C double bond. C n H 2n is the formula of all olefins, and the name is formed from an alkane with the same number of carbons in the molecule, only the suffix -ane changes to -ene. The Arabic numeral at the end of the name, separated by a hyphen, indicates the number of carbon from which the double bond begins. Let's look at the main alkenes, the table will help you remember them:
If the molecules have a simple, unbranched structure, then the suffix -ylene is added, this is also reflected in the table.
Where can you find them?
Since the reactivity of alkenes is very high, their representatives are extremely rare in nature. The principle of life of an olefin molecule is “let’s be friends.” There are no other substances around - no problem, we will be friends with each other, forming polymers.
But they exist, and a small number of representatives are included in the accompanying petroleum gas, and higher ones are in the oil produced in Canada.
The very first representative of alkenes, ethene, is a hormone that stimulates fruit ripening, so it is synthesized in small quantities by representatives of the flora. There is an alkene, cis-9-tricosene, which plays the role of a sexual attractant in female house flies. It is also called muscalur. (An attractant is a substance of natural or synthetic origin that causes attraction to the source of odor in another organism). From a chemical point of view, this alkene looks like this:
Since all alkenes are very valuable raw materials, the methods for producing them artificially are very diverse. Let's look at the most common ones.
What if you need a lot?
In industry, the class of alkenes is mainly obtained by cracking, i.e. cleavage of the molecule under the influence of high temperatures, higher alkanes. The reaction requires heating in the range of 400 to 700 °C. The alkane splits the way it wants, forming alkenes, the methods of obtaining which we are considering, with a large number of molecular structure options:
C 7 H 16 -> CH 3 -CH=CH 2 + C 4 H 10.
Another common method is called dehydrogenation, in which a hydrogen molecule is separated from a representative of an alkane series in the presence of a catalyst.
In laboratory conditions, alkenes and methods of preparation differ; they are based on elimination reactions (elimination of a group of atoms without their substitution). The most commonly eliminated water atoms from alcohols are halogens, hydrogen or hydrogen halides. The most common way to obtain alkenes is from alcohols in the presence of an acid as a catalyst. It is possible to use other catalysts
All elimination reactions are subject to Zaitsev’s rule, which states:
A hydrogen atom is split off from the carbon adjacent to the carbon bearing the -OH group, which has fewer hydrogens.
Having applied the rule, answer which reaction product will predominate? Later you will find out if you answered correctly.
Chemical properties
Alkenes react actively with substances, breaking their pi bond (another name for the C=C bond). After all, it is not as strong as a single bond (sigma bond). A hydrocarbon is converted from unsaturated to saturated without forming other substances after the reaction (addition).
- addition of hydrogen (hydrogenation). The presence of a catalyst and heating is necessary for its passage;
- addition of halogen molecules (halogenation). It is one of the qualitative reactions to the pi bond. After all, when alkenes react with bromine water, it turns from brown to transparent;
- reaction with hydrogen halides (hydrohalogenation);
- addition of water (hydration). The conditions for the reaction to occur are heating and the presence of a catalyst (acid);
Reactions of unsymmetrical olefins with hydrogen halides and water obey Markovnikov's rule. This means that hydrogen will attach itself to the carbon from the carbon-carbon double bond that already has more hydrogen atoms.
- combustion;
- incomplete oxidation catalytic. The product is cyclic oxides;
- Wagner reaction (oxidation with permanganate in a neutral environment). This alkene reaction is another qualitative C=C bond. As it flows, the pink solution of potassium permanganate becomes discolored. If the same reaction is carried out in a combined acidic environment, the products will be different (carboxylic acids, ketones, carbon dioxide);
- isomerization. All types are characteristic: cis- and trans-, double bond movement, cyclization, skeletal isomerization;
- Polymerization is the main property of olefins for industry.
Application in medicine
The reaction products of alkenes are of great practical importance. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if it is used instead of water, the solutions will be more concentrated. For medical purposes, alkaloids, thymol, iodine, bromine, etc. are dissolved in it. Glycerin is also used in the preparation of ointments, pastes and creams. It prevents them from drying out. Glycerin itself is an antiseptic.
When reacted with hydrogen chloride, derivatives are obtained that are used as local anesthesia when applied to the skin, as well as for short-term anesthesia during minor surgical interventions, using inhalation.
Alkadienes are alkenes with two double bonds in one molecule. Their main use is the production of synthetic rubber, from which various heating pads and syringes, probes and catheters, gloves, pacifiers and much more are then made, which are simply irreplaceable when caring for the sick.
Industrial Applications
| Type of industry | What is used | How can they use |
| Agriculture | ethene | accelerates the ripening of vegetables and fruits, defoliation of plants, films for greenhouses |
| Varnish and colorful | ethene, butene, propene, etc. | for the production of solvents, ethers, solvents |
| Mechanical engineering | 2-methylpropene, ethene | production of synthetic rubber, lubricating oils, antifreeze |
| Food industry | ethene | production of teflon, ethyl alcohol, acetic acid |
| Chemical industry | ethene, polypropylene | alcohols, polymers (polyvinyl chloride, polyethylene, polyvinyl acetate, polyisobtylene, acetaldehyde) are obtained |
| Mining | ethene etc. | explosives |
Alkenes and their derivatives have found wider use in industry. (Where and how are alkenes used, table above).
This is only a small part of the use of alkenes and their derivatives. Every year the demand for olefins only increases, which means that the need for their production also increases.