Hydrocarbons of different classes (alkanes, alkenes, alkynes, alkadienes, arenes) can be obtained in various ways.
Preparation of alkanes
Cracking of alkanes from initially b O longer chain length
The process used in industry takes place in the temperature range 450-500 o C in the presence of a catalyst and at a temperature of 500-700 o C in the absence of a catalyst:
The importance of the industrial cracking process lies in the fact that it allows increasing the yield of gasoline from heavy fractions of oil, which are not of significant value in themselves.
Hydrogenation of unsaturated hydrocarbons
- alkenes:
- alkynes and alkadienes:
Coal gasification
in the presence of a nickel catalyst at elevated temperatures and pressures can be used to produce methane:
Fischer-Tropsch process
Using this method, saturated hydrocarbons of normal structure can be obtained, i.e. alkanes. The synthesis of alkanes is carried out using synthesis gas (a mixture of carbon monoxide CO and hydrogen H2), which is passed through catalysts at high temperature and pressure:
Wurtz reaction
Using this reaction, hydrocarbons with b O higher number of carbon atoms in the chain than in the parent hydrocarbons. The reaction occurs when metallic sodium acts on haloalkanes:
Decarboxylation of carboxylic acid salts
Fusion of solid salts of carboxylic acids with alkalis leads to a decarboxylation reaction, which produces a hydrocarbon with a smaller number of carbon atoms and a metal carbonate (Dumas reaction):
Hydrolysis of aluminum carbide
The interaction of aluminum carbide with water, as well as non-oxidizing acids, leads to the formation of methane:
Al 4 C 3 + 12H 2 O = 4Al(OH) 3 + 3CH 4
Al 4 C 3 + 12HCl = 4AlCl 3 + 3CH 4
Preparation of alkenes
Cracking of alkanes
The reaction in general form has already been discussed above (production of alkanes). Example of a cracking reaction:
Dehydrohalogenation of haloalkanes
Dehydrohalogenation of haloalkanes occurs when they are exposed to an alcoholic alkali solution:
Dehydration of alcohols
This process takes place in the presence of concentrated sulfuric acid and heating to a temperature of more than 140 o C:
Please note that in both the case of dehydration and dehydrohalogenation, the elimination of a low molecular weight product (water or hydrogen halide) occurs according to Zaitsev's rule: hydrogen is eliminated from a less hydrogenated carbon atom.
Dehalogenation of vicinal dihaloalkanes
Vicinal dihaloalkanes are those derivatives of hydrocarbons in which chlorine atoms are attached to adjacent atoms of the carbon chain.
Dehydrohalogenation of vicinal haloalkanes can be accomplished using zinc or magnesium:
Dehydrogenation of alkanes
Passing alkanes over a catalyst (Ni, Pt, Pd, Al 2 O 3 or Cr 2 O 3) at high temperature (400-600 o C) leads to the formation of the corresponding alkenes:
Preparation of alkadienes
Dehydrogenation of butane and butene-1
Currently, the main method for the production of butadiene-1,3 (divinyl) is the catalytic dehydrogenation of butane, as well as butene-1 contained in gases from secondary oil refining. The process is carried out in the presence of a catalyst based on chromium (III) oxide at 500-650°C:
The action of high temperatures in the presence of catalysts on isopentane (2-methylbutane) produces an industrially important product - isoprene (the starting material for the production of so-called “natural” rubber):
Lebedev method
Previously (in the Soviet Union) butadiene-1,3 was obtained using the Lebedev method from ethanol:
Dehydrohalogenation of dihalogenated alkanes
It is carried out by the action of an alcoholic alkali solution on halogen derivatives:
Preparation of alkynes
Acetylene production
Methane pyrolysis
When heated to a temperature of 1200-1500 o C, methane undergoes a dehydrogenation reaction with simultaneous doubling of the carbon chain - acetylene and hydrogen are formed:
Hydrolysis of alkali and alkaline earth metal carbides
Acetylene is produced in the laboratory by reacting carbides of alkali and alkaline earth metals with water or non-oxidizing acids. The cheapest and, as a result, the most accessible for use is calcium carbide:
Dehydrohalogenation of dihaloalkanes
Preparation of acetylene homologues
Dehydrohalogenation of dihaloalkanes:
Dehydrogenation of alkanes and alkenes:
Preparation of aromatic hydrocarbons (arenes)
Decarboxylation of salts of aromatic carboxylic acids
By fusing salts of aromatic carboxylic acids with alkalis, it is possible to obtain aromatic hydrocarbons with fewer carbon atoms in the molecule compared to the original salt:
Trimerization of acetylene
When passing acetylene at a temperature of 400°C over activated carbon, benzene is formed in good yield:
In a similar way, symmetrical trialkyl-substituted benzenes can be prepared from acetylene homologues. For example:
Dehydrogenation of cyclohexane homologues
When cycloalkanes with 6 carbon atoms are exposed to a high temperature cycle in the presence of platinum, dehydrogenation occurs with the formation of the corresponding aromatic hydrocarbon:
Dehydrocyclization
It is also possible to obtain aromatic hydrocarbons from non-cyclic hydrocarbons in the presence of a carbon chain with a length of 6 or more carbon atoms (dehydrocyclization). The process is carried out at high temperatures in the presence of platinum or any other hydrogenation-dehydrogenation catalyst (Pd, Ni):
Alkylation
Preparation of benzene homologues by alkylation of aromatic hydrocarbons with chlorinated alkanes, alkenes or alcohols.
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.
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 application.
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.
DEFINITION
Alkenes are called unsaturated hydrocarbons whose molecules contain one double bond. The structure of the alkene molecule using ethylene as an example is shown in Fig. 1.
Rice. 1. The structure of the ethylene molecule.
In terms of physical properties, alkenes differ little from alkanes with the same number of carbon atoms in the molecule. Lower homologs C 2 - C 4 under normal conditions are gases; C 5 - C 17 - liquids; higher homologues are solids. Alkenes are insoluble in water. Highly soluble in organic solvents.
Preparation of alkenes
In industry, alkenes are obtained during oil refining: cracking and dehydrogenation of alkanes. We divided laboratory methods for obtaining alkenes into two groups:
- Elimination reactions
– dehydration of alcohols
CH 3 -CH 2 -OH → CH 2 =CH 2 + H 2 O (H 2 SO 4 (conc), t 0 = 170).
— dehydrohalogenation of monohaloalkanes
CH 3 -CH(Br)-CH 2 -CH 3 + NaOH alcohol → CH 3 -CH=CH-CH 3 + NaBr + H 2 O (t 0).
— dehalogenation of dihaloalkanes
CH 3 -CH(Cl)-CH(Cl)-CH 2 -CH 3 + Zn(Mg) → CH 3 -CH=CH-CH 2 -CH 3 + ZnCl 2 (MgCl 2).
- Incomplete hydrogenation of alkynes
CH≡CH + H 2 →CH 2 =CH 2 (Pd, t 0).
Chemical properties of alkenes
Alkenes are highly reactive organic compounds. This is explained by their structure. The chemistry of alkenes is the chemistry of double bonds. Typical reactions for alkenes are electrophilic addition reactions.
Chemical transformations of alkenes proceed with splitting:
1) π-C-C bonds (addition, polymerization and oxidation)
- hydrogenation
CH 3 -CH=CH 2 + H 2 → CH 3 -CH 2 -CH 2 (kat = Pt).
- halogenation
CH 3 -CH 2 -CH=CH 2 + Br 2 → CH 3 -CH 2 -CH(Br)-CH 2 Br.
— hydrohalogenation (proceeds according to Markovnikov’s rule: a hydrogen atom attaches preferentially to a more hydrogenated carbon atom)
CH 3 -CH=CH 2 + H-Cl → CH 3 -CH(Cl)-CH 3 .
- hydration
CH 2 =CH 2 + H-OH → CH 3 -CH 2 -OH (H + , t 0).
- polymerization
nCH 2 =CH 2 → -[-CH 2 -CH 2 -]- n (kat, t 0).
- oxidation
CH 2 =CH 2 + 2KMnO 4 + 2KOH → HO-CH 2 -CH 2 -OH + 2K 2 MnO 4;
2CH 2 =CH 2 + O 2 → 2C 2 OH 4 (epoxide) (kat = Ag,t 0);
2CH 2 =CH 2 + O 2 → 2CH 3 -C(O)H (kat = PdCl 2, CuCl).
2) σ- and π-C-C bonds
CH 3 -CH=CH-CH 2 -CH 3 + 4[O] → CH 3 COOH + CH 3 CH 2 COOH (KMnO 4, H +, t 0).
3) bonds C sp 3 -H (in the allylic position)
CH 2 =CH 2 + Cl 2 → CH 2 =CH-Cl + HCl (t 0 =400).
4) Breaking all ties
C 2 H 4 + 2O 2 → 2CO 2 + 2H 2 O;
C n H 2n + 3n/2 O 2 → nCO 2 + nH 2 O.
Applications of alkenes
Alkenes have found application in various sectors of the national economy. Let's look at the example of individual representatives.
Ethylene is widely used in industrial organic synthesis to produce a variety of organic compounds, such as halogen derivatives, alcohols (ethanol, ethylene glycol), acetaldehyde, acetic acid, etc. Ethylene is consumed in large quantities for the production of polymers.
Propylene is used as a raw material for the production of some alcohols (for example, 2-propanol, glycerin), acetone, etc. Polypropylene is produced by polymerization of propylene.
Examples of problem solving
EXAMPLE 1
| Exercise | When hydrolyzed with an aqueous solution of sodium hydroxide NaOH dichloride, obtained by adding 6.72 liters of chlorine to ethylene hydrocarbon, 22.8 g of dihydric alcohol was formed. What is the formula of the alkene if it is known that the reactions proceed in quantitative yields (without losses)? |
| Solution | Let us write the equation for the chlorination of an alkene in general form, as well as the reaction for producing a dihydric alcohol: C n H 2 n + Cl 2 = C n H 2 n Cl 2 (1); C n H 2 n Cl 2 + 2NaOH = C n H 2 n (OH) 2 + 2HCl (2). Let's calculate the amount of chlorine: n(Cl 2) = V(Cl 2) / V m; n(Cl 2) = 6.72 / 22.4 = 0.3 mol, therefore, ethylene dichloride will also be 0.3 mol (equation 1), dihydric alcohol should also be 0.3 mol, and according to the conditions of the problem this is 22.8 g. This means its molar mass will be equal to: M(C n H 2 n (OH) 2) = m(C n H 2 n (OH) 2) / n(C n H 2 n (OH) 2); M(C n H 2 n (OH) 2) = 22.8 / 0.3 = 76 g/mol. Let's find the molar mass of the alkene: M(C n H 2 n) = 76 - (2×17) = 42 g/mol, which corresponds to the formula C 3 H 6 . |
| Answer | Alkene formulaC 3 H 6 |
EXAMPLE 2
| Exercise | How many grams will be required to brominate 16.8 g of an alkene, if it is known that during the catalytic hydrogenation of the same amount of alkene, 6.72 liters of hydrogen were added? What is the composition and possible structure of the original hydrocarbon? |
| Solution | Let us write in general form the equations for the bromination and hydrogenation of an alkene: C n H 2 n + Br 2 = C n H 2 n Br 2 (1); C n H 2 n + H 2 = C n H 2 n +2 (2). Let's calculate the amount of hydrogen substance: n(H 2) = V(H 2) / V m; n(H 2) = 6.72 / 22.4 = 0.3 mol, therefore, the alkene will also be 0.3 mol (equation 2), and according to the conditions of the problem it is 16.8 g. This means its molar mass will be equal to: M(C n H 2n) = m(C n H 2n) / n(C n H 2n); M(C n H 2 n) = 16.8 / 0.3 = 56 g/mol, which corresponds to the formula C 4 H 8 . According to equation (1) n(C n H 2 n) : n(Br 2) = 1:1, i.e. n(Br 2) = n(C n H 2 n) = 0.3 mol. Let's find the mass of bromine: m(Br 2) = n(Br 2) × M(Br 2); M(Br 2) = 2×Ar(Br) = 2×80 = 160 g/mol; m(MnO 2) = 0.3 × 160 = 48 g. Let's create the structural formulas of the isomers: butene-1 (1), butene-2 (2), 2-methylpropene (3), cyclobutane (4). CH 2 =CH-CH 2 -CH 3 (1); CH 3 -CH=CH-CH 3 (2); CH 2 =C(CH 3)-CH 3 (3); |
| Answer | The mass of bromine is 48 g |
Alkenes or olefins (C n H 2n) are a class of organic substances that actively react with other compounds. Therefore, in nature, alkenes are rarely found in their pure form. Industrial chemistry deals with the production of alkenes. There are several methods for isolating olefins from natural raw materials.
Receipt
In modern chemistry, alkenes are obtained by industrial and laboratory methods. The raw materials for isolating olefins are oil, gas, alkanes and their derivatives. The main methods for obtaining alkenes are given in the table.
|
Type of receipt |
Way |
Example |
|
Industrial |
Cracking and pyrolysis of petroleum products, coal coking - high-temperature (400-700°C) processing of minerals. Using cracking and pyrolysis of petroleum products, the first four alkenes in the homologous series are obtained - ethylene, propylene, butylene, pentene. Coking coal releases ethylene and propylene |
C n H 2n+2 (alkanes) → C n H 2n (alkenes) + C n H 2n+2: C 8 H 18 → CH 2 =CH 2 -CH 2 -CH 2 + C 4 H 10; C 7 H 16 → CH 3 -CH=CH 2 + C 4 H 10 |
|
Dehydrogenation of alkanes is the elimination of hydrogen atoms due to the cleavage of the C-H bond. Occurs at high temperature under the influence of a catalyst |
C n H 2n+2 → C n H 2n + H 2: CH 3 -CH 3 → CH 2 =CH 2 + H 2; CH 3 -CH 2 -CH 2 -CH 3 → CH 3 -CH=CH-CH 3 + H 2 |
|
|
Hydrogenation of alkynes is the addition of hydrogen in the presence of a low-activity catalyst (Pb(CH 3 COO) 2). Reaction time converts alkynes to alkanes |
C n H 2n-2 + H 2 → C n H 2n: 2HC ≡CH + 2H 2 → CH 3 -C(CH 3)=CH 2 (isobutylene) |
|
|
Laboratory |
Dehydration of alcohols is the elimination of a water molecule under the influence of temperatures above 150°C and in the presence of reagents that can remove water. For example, in the presence of concentrated sulfuric acid |
R-CH 2 -CH 2 -OH → R-CH=CH 2 + H 2 O: CH 3 -CH-H-CH 2 -OH → CH 3 -CH=CH 2 + H 2 O |
|
Dehydrogenation of monohaloalkanes - elimination of halogen and hydrogen atoms under the influence of an alcoholic alkali solution |
CH 3 -CH 2 -CH 2 -Br + NaOH (alcohol solution) → CH 3 -CH=CH 2 + NaBr + H 2 O |
|
|
Dehalogenation of dihaloalkanes - elimination of halogen atoms under the influence of metals |
CH 2 -Br-CH-Br-CH 3 + Mg → CH 2 =CH-CH 3 + MgBr 2 |
Rice. 1. Cracking.
There are also other methods for the synthesis of alkenes from carbonyl compounds, aldehydes, ketones, alcohols, ammonium bases and other compounds.
The reactions of dehydration and dehydrogenation in the production of alkenes proceed according to Alexander Zaitsev’s rule. In 1875, the chemist Zaitsev determined experimentally that hydrogen is split off from the less hydrogenated carbon atom.
Rice. 2. Alexander Zaitsev.
Application
Alkenes are used as industrial raw materials. From them they produce:
- Teflon;
- plastics;
- rubber;
- polyethylene;
- ethanol;
- acetic acid;
- oils;
- solvents.
Rice. 3. Materials that are made from alkenes.
Ethylene is widely used, so more than 100 million tons of ethylene are produced per year in the world.
What have we learned?
Alkenes are synthesized for chemical needs using industrial and laboratory methods. In industry, petroleum products and coal are used to produce alkenes. When alkanes are heated, dehydrogenated, or hydrogenated, alkenes are released. In laboratories, alkenes are obtained by dehydration of alcohols, dehydrogenation of monohaloalkanes, and dehalogenation of dihaloalkanes. There are other methods for synthesizing olefins. Alkenes are used to make durable materials, solvents, and oils.
Test on the topic
Evaluation of the report
Average rating: 4.6. Total ratings received: 222.