The isomerism of halogen derivatives is associated with the structural features of the carbon skeleton (linear or branched structure), the position of the halogen atoms in the carbon chain:
1. CH 3 -CH 2 -CH 2 -CH 2 -Br 2. CH 3 -CH-CH 2 -CH 3
primary bromide
(linear structure secondary bromide
carbon skeleton, butyl
halogen atom y (linear structure
terminal atom of the carbon skeleton,
carbon) halogen atom at the middle
carbon atom)
3. CH 3 -CH-CH 2 -Br CH 3
CH 3 4. CH 3 -C-CH 3
primary bromide
isobutyl Cl
(branched structure tertiary chloride
carbon skeleton, isobutyl atom
halogen at the terminal atom (branched structure
carbon) carbon skeleton,
halogen atom at the middle
carbon atom)
and different arrangements of atoms and groups in space (cis-, trans-isomerism; optical isomerism):
CH 3 H C = C
Cl CH 3 Cl H
trans form cis form
When naming halogenated hydrocarbons, the following nomenclature is used: trivial, rational and systematic (IUPAC).
Trivial nomenclature in halogen derivatives is used in some cases: chloroform CHCl 3, iodoform CHI 3.
According to rational nomenclature, the name of halogen derivatives is formed from the name of the hydrocarbon radical and halogen, the position of the latter, if necessary, is indicated:
C 2 H 5 Cl CH 3 -CH-CH 2 -CH 3 CH 2 = CH-Br C 6 H 5 CH 2 Br
ethyl chloride bromide (ethyl chloride) Br vinyl benzyl
sec-butyl bromide (vinyl bromide) (benzyl bromide)
(sec-butyl bromide)
If a molecule of a halogen derivative contains two halogen atoms, then the hydrocarbon radical is named depending on the position of these atoms in the carbon chain. Thus, when halogen atoms are located at neighboring carbon atoms, the suffix - ene is added to the name of the radical (in this case, a divalent radical is formed by subtracting two hydrogen atoms from two neighboring carbon atoms):
CH 2 Cl-CH 2 Cl CH 3 -CHCl-CH 2 Cl
ethylene chloride propylene chloride
(ethylene chloride) (propylene chloride)
If both halogen atoms are located at the same terminal carbon atom, then the suffix - idene is added to the name of the radical (in this case, a divalent radical is obtained by subtracting two hydrogen atoms from one extreme carbon atom):
CH 3 -CHCl 2 CH 3 -CH 2 -CHI 2
ethylidene chloride propylidene iodide
(ethylidene chloride) (propylidene iodide)
Hydrocarbon radicals of dihalogen derivatives, in which two halogen atoms are located at the terminal carbon atoms, contain a number of methylene (-CH 2 -) groups, depending on the number of which their names are formed:
CH 2 Cl-CH 2 -CH 2 Cl CH 2 Br-CH 2 -CH 2 -CH 2 Br
trimethylene chloride tetramethylene bromide
(trimethylene chloride) (tetramethylene bromide)
Halogen derivatives, in which all the hydrogen atoms present in the molecule are replaced by halogen, are called perhalogen derivatives:
CF 3 -CF 3 CF 2 =CF 2
perfluoroethane perfluoroethylene
According to systematic nomenclature (IUPAC), when naming halogen derivatives, the longest chain of carbon atoms is selected, including, if present, a short bond (main chain). The carbon atoms of this chain are numbered. Numbering starts from the end to which the halogen atom is located closest. The name of halogen-containing compounds is derived from the corresponding alkane, preceded by the name of the halogen and a number indicating at which carbon atom from the beginning of the chain the halogen is located (other substituents in the molecule are indicated similarly):
CH 3 Cl 1 2 3 1 2 CH 2 -CH 3
chloromethane CH 3 -CHCl-CH 3 Cl H 2 C-C
2-chloropropane CH 3
1-chloro-2-methylbutane
If a halogen-containing hydrocarbon contains a halogen atom and a multiple bond, then the beginning of the numbering is determined by the multiple bond:
1 2 3 4 1 2 3 4 5
CH 2 =CH-CH 2 -CH 2 Br CH 3 -C=C-CH 2 -CH 2 Br
4-bromo-1-butene
5-bromo-2-methyl-3-chloro-2-pentene
Di- and polyhalogen derivatives are named according to the same rules as monohalogen derivatives:
CH 2 Cl-CH 2 Cl CH 3 -CHCl 2
1,2-dichloroethane 1,1-dichloroethane
- Using the diagram below, identify substances A–E, write down the reaction equations
- Amalgam is an alloy, one of the components of which is mercury. A zinc-aluminum amalgam weighing 10.00 g was treated with an excess of dilute sulfuric acid solution. In this case, 0.896 liters of hydrogen (n.s.) were released. The mass of the resulting insoluble residue was found to be 8.810 g.
Calculate the mass fractions (in %) of each amalgam component.SOLUTION POINTS Mercury does not dissolve in dilute sulfuric acid, therefore
the mass of mercury in the amalgam is 8.810 g.1 point The release of hydrogen occurs due to the interaction
zinc and aluminum with sulfuric acid solution:
Zn + H 2 SO 4 = ZnSO 4 + H 2 (1)1 point 2Al + 3H 2 SO 4 = Al 2 (SO 4) 3 + 3H 2 (2) 1 point m(Al + Zn) = 10.00 – 8.810 = 1.190 g 0.5 points n(H 2) = 0.896 / 22.4 = 0.04 mol 1 point Let n(Zn) = x mol; n(Al) = y mole, then 65x +27y = 1.19 2 points According to the reaction equation:
n(H 2) = n(Zn) + 1.5n(Al) = (x + 1.5y) mol, then2 points 65x +27y = 1.19
x +1.5y = 0.04
x = 0.01 mol; y = 0.02 mol2.5 points m(Zn) = 65 0.01 = 0.65 g; m(Al) = 27 0.02 = 0.54 g 1 point ω(Zn) = 0.65/10 = 0.065 (6.5%); ω(Al) = 0.54/10 = 0.054 (5.4%) 1 point TOTAL FOR TASK 13 POINTS - The reaction involved 3.700 g of calcium hydroxide and 1.467 l of carbon dioxide, measured at 760 mm Hg. Art. and 25°C. The resulting precipitate was filtered and calcined at 1000°C.
Calculate the mass of the dry residue.SOLUTION POINTS Let us bring the volume of carbon dioxide to normal conditions, taking into account
that 760 mm Hg. Art. - normal pressure corresponding to 101.3 kPa,
and T’ = 273 + 25 = 298 K:1 point According to Gay-Lussac's law, the volume of carbon dioxide at normal temperature
(0°C or 273 K) at constant pressure is equal to:
V/T = V’/T’
V/273 = 1.467/298
V = 1.344 l2 points When CO 2 is passed through a solution of calcium hydroxide, the following reactions occur:
Ca(OH) 2 + CO 2 = CaCO 3 ↓ + H 2 O (1)1 point CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2 (2) 1 point n(Ca(OH) 2) = 3.7/74 = 0.05 mol; n(CO2) = 1.344/22.4 = 0.06 mol. 2 points According to reaction equation (1) n(Ca(OH) 2) = n(CO 2) = n(CaCO 3) = 0.05 mol 1 point Reaction (1) consumes 0.05 mol CO 2, therefore, 0.01 mol CO 2
remains in excess and enters into reaction (2), interacting with 0.01 mol of CaCO 3 .
0.04 mol of CaCO 3 remains in the precipitate.1 point When the precipitate is calcined, the decomposition reaction of CaCO 3 occurs:
CaCO 3 = CaO + CO 2 (3)1 point According to the reaction equation, 0.04 mol CaO 3 is formed from 0.04 mol CaCO 3,
which represents the dry residue after calcination.1 point m(CaO) = 0.04 56 = 2.24 g. 1 point TOTAL FOR TASK 12 POINTS - When a colorless gas interacts A and iron(III) chloride, a yellow precipitate forms B. When it reacts with concentrated nitric acid, a brown gas is released IN, which reacts with ozone to form a white crystalline substance G, which forms only nitric acid when interacting with water.
Identify the substances A, B, IN, G. Write down the equations of the chemical reactions occurring. - Calculate the mass of glucose that was subjected to alcoholic fermentation if the same amount of carbon dioxide was released as it is formed during the combustion of 120 g of acetic acid, taking into account that the yield of the fermentation reaction is 92% of the theoretical one.
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Synonyms |
, Methyl bromide (Bromomethyl), methyl bromide, monobromomethyl, monobromoethane, methyl bromide, bromomethyl, bromomethane, metabromine, panobrome, therabol, broson |
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In English |
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Empirical Formula |
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Group on the site |
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Chemical class |
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Penetration method |
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Effect on organisms |
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Application methods |
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Click on photo to enlarge
Methyl bromide- a broad-spectrum insecticide and acaricide, used in the practice of quarantine fumigation to control pests of stocks, pests of industrial wood in wooden containers and plant pests when planting material is infected.
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Physicochemical characteristics
In the gaseous state, chemically pure methyl bromide is a colorless, odorless, and tasteless gas. Chloropicrin is added as an odorant.
Under the influence of high temperatures (500°C) it decomposes to form HBr. It is well hydrolyzed by alcoholic alkali solution.
Sometimes technical methyl bromide has an unpleasant odor of mercaptan (rotting protein substances), which can persist in the air of rooms exposed to carbonation () for several days, even after its vapors have been completely removed, but this odor is not transmitted to carbonated products.
At high humidity and ambient temperatures below the boiling point, liquid methyl bromide can form a hydrate (a dense white crystalline mass), which at temperatures below 10 °C slowly releases gas (decomposes into water and gas). To prevent these phenomena and damage to liquid products, methyl bromide should be introduced into the container only through a gas evaporator, where it turns into a gaseous state.
Methyl bromide vapors are heavier than air, they penetrate deeply into sorbent materials, are weakly absorbed by them and are easily removed when ventilated, remaining only on the surface in the form of bound inorganic bromides, the amount of which depends on the concentration of the drug used and the duration of exposure.
Increased humidity of products does not prevent the penetration of vapors. In the concentrations used, the mixture of vapors and air is non-explosive.
In terms of chemical properties, methyl bromide is a characteristic representative of monohaloalkanes. It easily undergoes substitution reactions, its reactivity is much higher than methyl chloride.
physical characteristics
Effect on harmful organisms
The substance is toxic to all stages of development of insects and mites in any form of contamination of products, vehicles and containers.
. Methyl bromide has a nerve-paralytic effect. For harmful insects and mites, it is associated with a high methylating ability when interacting with enzymes containing sulfhydryl groups, as a result of which redox processes and carbohydrate metabolism are disrupted. Apparently, this is the reason for the effect of the fumigant on ticks and insects.The effect of methyl bromide appears slowly, so effectiveness should be determined no earlier than 24 hours after decontamination.
. There is no information on acquired resistance to the drug.However, during the treatment process, at a sublethal concentration of the fumigant in the air, many insects fall into a protective torpor and do not die at the subsequent lethal concentration.
Some species of thrips and scale insects are naturally resistant to drugs based on methyl bromide, but they also die quickly with increasing doses of the fumigant and increasing exposure.
Application
A registered preparation based on methyl bromide can be used for fumigation:
Previously, methyl bromide was also used for:
Methyl bromide was also used for disinfestation and deratization of warehouses, refrigerators, elevators, mills, ship holds and dwellings.
In industry it was used as an alkylating agent, as well as for refilling fire extinguishers, in medical practice for the sterilization of polymers, medical equipment, instruments, optical instruments, military clothing and footwear.
Methyl bromide is similar in action to hydrogen cyanide, but is safer for plants and seeds.
Mixes. At the end of the 90s of the last century, the disinfection department of VNIIKR conducted research to obtain experimental data on the possibility of reducing the concentration of methyl bromide when carrying out. It was supposed to be used in mixtures with others, in particular, with preparations based on hydrogen phosphorous (). As a result of the research, data on effective concentrations were obtained; dissertations were defended on the basis of these data, however, due to the sharp reduction in the use of methyl bromide, these studies did not find practical application. (editor's note)
Reduced seed germination. According to the results of studies using a carbon-labeled drug, at normal pressure and temperature, methyl bromide behaves as a methylating agent, reacting with substances that are part of the grain. Thus, it disrupts the course of normal life processes and reduces germination.
Effect on grain quality. Methyl bromide is sorbed physically from grains and then enters into chemical interaction with protein substances. In this case, methylation of the imidazole rings of histidine residues of lysine and methionine occurs. However, the substance does not have a significant effect on the quality of grain, although it leads to a slight loss of the nutritional value of bread.
Toxicological data |
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| (mg/kg human body weight) | 1,0 |
| in soil (mg/kg) | () |
| in soil (mg/kg) | () |
| in water of reservoirs (mg/dm 3) | 0,2 |
| in the air of the working area (mg/m 3) | 1,0 |
| in atmospheric air (mg/m 3) | 0,1 |
| in imported products (mg/kg): | |
in cereal grains |
5,0 |
in grain products, including ground |
1,0 |
in cocoa beans |
5,0 |
in dried fruits |
2,0 |
Toxicological properties and characteristics
Methyl bromide is highly toxic to humans and warm-blooded animals, and is a strong neutropic poison. When the active substance enters the animal’s body, it changes the blood picture and disrupts the functions of the nervous system. As a strong methylating agent, the drug has a negative effect on the processes of synthesis and breakdown of hydrocarbons.
The toxic effect is usually associated with the formation in the body of methanol and its products (formaldehyde and formic acid), as well as bromides.
The glycogen content in the liver drops especially sharply. In addition, poisoning can be accompanied by damage to the optic nerve and blindness.
In the body of a mammal, the toxicant quickly decomposes to form methyl alcohol and then formaldehyde, which further enhances the toxic effect.
Irritates mucous membranes. Contact with the skin should be avoided, and if contact occurs, rinse immediately with plenty of water (Melnikov, Novozhilov, 80). Belongs to a group of compounds that primarily damage the nervous system, kidneys and lungs.
LC 50 at 30-minute exposure for:
- mice - 6.6;
- rats and rabbits - 28.9 g/m3.
with a six-hour exposure, LC 50 for rats and guinea pigs is 0.63-0.56 g/m 3 .
Table Toxicological data compiled in accordance with GN 1.2.3111-13.
Symptoms
Clinical picture
a person is characterized, as a rule, by the presence of a latent period. There is general weakness, dizziness, headache, nausea, sometimes vomiting, an uncertain shaky gait, trembling of the limbs, blurred vision, increased tendon reflexes, hyperemia of the skin of the face, rapid or slow pulse, hypotension. These symptoms may disappear after you stop working. The second period, which can begin after 2-12 hours or even 1-2 days, is characterized by the rapid development of muscle twitching, epileptiform seizures, trembling of the tongue and limbs, scanned speech, double vision, dilated pupils and their lack of reaction to light, coordination disorder movements.Chronic intoxication
occurs several weeks or months after starting work and is accompanied by headaches, dizziness, drowsiness, weakness in the limbs, numbness in the fingers, increased salivation and sweating, nausea, pain in the heart, blurred vision and auditory hallucinations.Skin resorptive effect
. Poisoning of a person is possible if the active substance comes into contact with the skin, and contact with open areas of the body does not cause burns, since the substance instantly evaporates. Poisoning can occur through the skin and when methyl bromide gas gets under clothing. If clothing is well ventilated, the substance will easily evaporate from it. In places where clothing fits tightly to the body, it lingers, and bubbles may appear here.Children and the elderly are more sensitive to the effects of the drug.
Story
Methyl bromide was first synthesized by Perkinson in 1884. In 1932 in France and later in the USA it was proposed as a barn pest control agent (). Since that time, it began to be widely used for quarantine disinfection, since most plants, fruits and vegetables turned out to be resistant to concentrations and effective against insects.
In the territory of the former USSR, methyl bromide was first used in 1958 in the Kherson port, where it was used to disinfect cargo in the holds of a ship.
By 1984, global consumption of this had reached 45,500 tons. In 1992, it was already used in the amount of 71,500 tons. Such a large amount had a serious impact on the environment, causing the United Nations Environment Program to designate it as an ozone-depleting substance.
Since January 1, 1998, methyl bromide can only be used for decontamination of ships and quarantine purposes. Canada agreed to this condition; in Germany, since January 1, 1996, the use of the substance has been reduced by approximately 70% and since January 1, 1998, its use has been prohibited. In the Scandinavian countries, methyl bromide has been banned since January 1, 1998, including quarantine and ships. The Netherlands has completely banned the use of methyl bromide, including on soil; in Italy its use has been prohibited since January 1, 1999.
However, in the USA, among farmers who could not do without this drug in their crop production practices, a petition was created to limit or prohibit the use of methyl bromide, especially in the state of California.
The UN Montreal Protocol calls for a complete phase-out of methyl bromide in industrialized countries by 2010, with a phase-out reduction of 25% by 2001 and 50% by 2005. Consequently, there is a need to find the use of alternative substances or methods.
In Russia, methyl bromide was removed from the official list of pesticides approved for use in the country in 2005. In 2011, under the name "Metabrom-RFO", it was again included in the list and approved for use for the disinfection of various products.
Alternatives to Methyl Bromide
There is no doubt among experts that methyl bromide is superior, and that is why it is difficult to replace. A lot of users continue to insist on its use. On the other hand, its replacement is necessary, since the ozone-depleting potential of methyl bromide has been scientifically proven. A decrease in stratospheric ozone invariably leads to an increase in dangerous ultraviolet radiation from the sun. The negative impact of this radiation on humans, animals and plants is reliably known.
Hydrogen cyanide
(HCN). Colorless liquid, has the smell of bitter almonds. The substance is lighter than air and has a boiling point of 26°C.Hydrogen cyanide is non-flammable, but when used for fumigation purposes, its concentrations approach explosive levels. The substance is very toxic and acts extremely quickly on many living beings. It is easily soluble in water, which is very important to consider when fumigating, as the hydrogen cyanide can become hydrated and difficult to remove.
Receipt
Methyl bromide is obtained in good yield by reacting methanol with hydrobromic acid salts or with bromine in the presence of hydrogen sulphide or sulfur dioxide. The industrial production method is based on the reaction of methanol with bromine and sulfur:
6CH 3 OH+ 3Br 2 + S → 6CH 3 Br + H 2 SO 4 + 2 H 2 O Hygienic standards for the content of pesticides in environmental objects (list). Hygienic standards GN 1.2.3111-13
4.State catalog of pesticides and agrochemicals approved for use on the territory of the Russian Federation, 2013. Ministry of Agriculture of the Russian Federation (Ministry of Agriculture of Russia)
5.Gruzdev G.S. Chemical plant protection. Edited by G.S. Gruzdeva - 3rd ed., revised. and additional - M.: Agropromizdat, 1987. - 415 p.: ill.
6.Maslov M.I., Magomedov U.Sh., Mordkovich Ya.B. Fundamentals of quarantine disinfection: monograph. – Voronezh: Scientific book, 2007. – 196 p.
7.Medved L.I. Handbook of pesticides (use hygiene and toxicology) / Team of authors, ed. Academician of the USSR Academy of Medical Sciences, Professor L.I. Medved -K.: Harvest, 1974. 448 p.
8.Melnikov N.N. Pesticides. Chemistry, technology and application. - M.: Chemistry, 1987. 712 p.
Alkenes – these are hydrocarbons whose molecules have ONE double C=C bond.
Alkene nomenclature: a suffix appears in the name -EN.
The first member of the homologous series is C2H4 (ethene).
For the simplest alkenes, historical names are also used:
ethylene (ethene),
· propylene (propene),
The following monovalent alkene radicals are often used in nomenclature:
CH2-CH=CH2 |
Types of isomerism of alkenes:
1. Carbon skeleton isomerism:(starting from C4H8 – butene and 2-methylpropene)
2. Isomerism of multiple bond position:(starting from C4H8): butene-1 and butene-2.
3. Interclass isomerism: With cycloalkanes(starting with propene):
C4H8 - butene and cyclobutane.
4. Spatial isomerism of alkenes:
Due to the fact that free rotation around the double bond is impossible, it becomes possible cis-trans- isomerism.
Alkenes with each of two carbon atoms at a double bond various substituents, can exist in the form of two isomers, differing in the arrangement of substituents relative to the π-bond plane:
Chemical properties of alkenes.
Alkenes are characterized by:
· addition reactions to a double bond,
· oxidation reactions,
· substitution reactions in the “side chain”.
1. Double bond addition reactions: the weaker π bond is broken and a saturated compound is formed. These are electrophilic addition reactions - AE. | 1) Hydrogenation: CH3-CH=CH2 + H2 à CH3-CH2-CH3 2) Halogenation: CH3-CH=CH2 + Br2 (solution)à CH3-CHBr-CH2Br Discoloration of bromine water is a qualitative reaction to a double bond. 3) Hydrohalogenation: CH3-CH=CH2 + HBr à CH3-CHBr-CH3 (MARKOVNIKOV'S RULE: hydrogen attaches to the most hydrogenated carbon atom). 4) Hydration - water connection: CH3-CH=CH2 + HOH à CH3-CH-CH3 (annexation also occurs according to Markovnikov’s rule) |
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2. Addition of hydrogen bromide to presence of peroxides (Harash effect) - this is a radical addition - AR | CH3-CH=CH2 + HBr -(H2O2)à CH3-CH2-CH2Br (the reaction with hydrogen bromide in the presence of peroxide proceeds against Markovnikov's rule ) |
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3. Combustion– complete oxidation of alkenes with oxygen to carbon dioxide and water. | С2Н4 + 3О2 = 2СО2 + 2Н2О |
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4. Mild oxidation of alkenes – Wagner reaction : reaction with a cold aqueous solution of potassium permanganate. | 3CH3- CH=CH2+ 2KMnO4 + 4H2O à 2MnO2 + 2KOH + 3 CH3 - CH - CH2 Oh Oh ( diol is formed) Discoloration of an aqueous solution of potassium permanganate by alkenes is a qualitative reaction to alkenes. |
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5. Severe oxidation of alkenes– hot neutral or acidic solution of potassium permanganate. Comes with the cleavage of the C=C double bond. | 1. When potassium permanganate acts in an acidic environment, depending on the structure of the alkene skeleton, the following is formed:
CH3-C-1 N=S-2Н2 +2 KMn+7O4 + 3H2SO4 а CH3-C+3 OOH+ C+4 O2 + 2Mn+2SO4 + K2SO4 + 4H2O 2. If the reaction occurs in a neutral environment when heated, then the following results are obtained: potassium salt:
3CH3C-1N=WITH-2Н2 +10 K MnO4 - tà 3 CH3 C+3OO K + + 3K 2C+4O3 + 10MnO2 +4H2O+ K Oh
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6. Oxidation oxygen of ethylene in the presence of palladium salts. | CH2=CH2 + O2 –(kat)à CH3CHO (acetic aldehyde) |
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7. Chlorination and bromination to the side chain: if the reaction with chlorine is carried out in the light or at high temperature, hydrogen is replaced in the side chain. | CH3-CH=CH2 + Cl2 –(light)à CH2-CH=CH2 +HCl |
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8. Polymerization: | n CH3-CH=CH2 à(-CH–CH2-)n propylene ô polypropylene |
OBTAINING ALKENES
I . Cracking alkanes: | С7Н16 –(t)а CH3- CH=CH2 + C4H10 Alkene alkane |
II. Dehydrohalogenation of haloalkanes under the action of an alcohol solution of alkali - reaction ELIMINATION. |
Zaitsev's rule: The abstraction of a hydrogen atom in elimination reactions occurs predominantly from the least hydrogenated carbon atom.
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III. Dehydration of alcohols at elevated temperatures (above 140°C) in the presence of oxidation-removing reagents - aluminum oxide or concentrated sulfuric acid - an elimination reaction. | CH3- CH-CH2-CH3 – (H2SO4,t>140o)à à H2O+CH3- CH=CH-CH3 (also obeys Zaitsev's rule) |
IV. Dehalogenation of dihaloalkanes having halogen atoms at neighboring carbon atoms, under the action of active metals. | CH2 Br-CH Br-CH3+ MgàCH2=CH-CH3+ MgBr2 Zinc can also be used. |
V. Dehydrogenation of alkanes at 500°C: |
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VI. Incomplete hydrogenation of dienes and alkynes | C2H2 + H2 (deficiency) –(kat)à C2H4 |
ALCADIENES.
These are hydrocarbons containing two double bonds. The first member of the series is C3H4 (propadiene or allene). The suffix appears in the name - DIEN .
Types of double bonds in dienes:
1.Insulateddouble bonds separated in a chain by two or more σ-bonds: CH2=CH–CH2–CH=CH2. Dienes of this type exhibit properties characteristic of alkenes. |
2. Cumulateddouble bonds located at one carbon atom: CH2=C=CH2(allen) Such dienes (allenes) belong to a rather rare and unstable type of compounds. |
3. Conjugatedouble bonds separated by one σ bond: CH2=CH–CH=CH2 Conjugated dienes have characteristic properties due to the electronic structure of the molecules, namely, a continuous sequence of four sp2 carbon atoms. |
Isomerism of dienes
1. Isomerism positions of double bonds: |
2. Isomerism carbon skeleton: |
3. Interclass isomerism with alkynes And cycloalkenes . For example, the following compounds correspond to the formula C4H6:
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4. Spatial isomerism Dienes, which have different substituents on the carbon atoms of their double bonds, like alkenes, exhibit cis-trans isomerism.
(1)Cis isomer (2)Trans isomer |
Electronic structure of conjugated dienes.
Butadiene-1,3 molecule CH2=CH-CH=CH2 contains four carbon atoms sp2
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hybridized state and has a flat structure.
π-Electrons of double bonds form a single π-electron cloud (conjugate system ) and are delocalized between all carbon atoms.
The multiplicity of bonds (the number of shared electron pairs) between carbon atoms has an intermediate value: there are no purely single and purely double bonds. The structure of butadiene is more accurately reflected by the formula with delocalized “one and a half” bonds.
CHEMICAL PROPERTIES OF CONJUGATED ALKADIENES.
ADDITION REACTIONS TO CONJUGATED DIENEES. The addition of halogens, hydrogen halides, water and other polar reagents occurs by an electrophilic mechanism (as in alkenes). In addition to addition at one of the two double bonds (1,2-addition), conjugated dienes are characterized by the so-called 1,4-addition, when the entire delocalized system of two double bonds participates in the reaction: The ratio of 1,2- and 1,4-addition products depends on the reaction conditions (with increasing temperature, the probability of 1,4-addition usually increases). |
1. Hydrogenation. CH3-CH2-CH=CH2 (1,2-product) CH2=CH-CH=CH2 + H2 CH3-CH=CH-CH3 (1,4-product) In the presence of a Ni catalyst, the product of complete hydrogenation is obtained: CH2=CH-CH=CH2 + 2 H2 –(Ni, t)à CH3-CH2-CH2-CH3 |
2. Halogenation, hydrohalogenation and hydration 1,4-attachment. 1,2 connection. When there is an excess of bromine, another molecule of it joins at the site of the remaining double bond to form 1,2,3,4-tetrabromobutane. |
3. Polymerization reaction. The reaction proceeds predominantly via the 1,4-mechanism, resulting in the formation of a polymer with multiple bonds, called rubber : nCH2=CH-CH=CH2 à (-CH2-CH=CH-CH2-)n polymerization of isoprene: nCH2=C–CH=CH2 à(–CH2 –C =CH –CH2 –)n CH3 CH3 (polyisoprene) |
OXIDATION REACTIONS – soft, hard, and combustion. They proceed in the same way as in the case of alkenes - soft oxidation leads to a polyhydric alcohol, and hard oxidation leads to a mixture of various products, depending on the structure of the diene: CH2=CH –CH=CH2 + KMnO4 + H2O à CH2 – CH – CH – CH2 + MnO2 + KOH |
Alkadienes burn– to carbon dioxide and water. С4Н6 + 5.5О2 à 4СО2 + 3Н2О |
OBTAINING ALKADIENES.
1. Catalytic dehydrogenation alkanes (through the stage of formation of alkenes). In this way, divinyl is produced industrially from butane contained in oil refining gases and associated gases: Isoprene is obtained by catalytic dehydrogenation of isopentane (2-methylbutane): |
2. Lebedev synthesis: (catalyst – mixture of oxides Al2O3, MgO, ZnO 2 C2H5OH –(Al2O3,MgO, ZnO, 450˚C)à CH2=CH-CH=CH2 + 2H2O + H2 |
3. Dehydration of dihydric alcohols: |
4. Effect of an alcohol solution of alkali on dihaloalkanes (dehydrohalogenation): |
