Characteristic chemical properties of hydrocarbons. Reaction mechanisms
alkanes, alkenes, alkynes, arenes - characteristics, use, reactions
1) Alkanes- these are saturated hydrocarbons, in the molecules of which all atoms are connected by single bonds. Their composition is reflected by one general formula: C n H 2n + 2.
Physical properties alkanes depend on the composition of their molecules, i.e. on relative molecular weight. With an increase in the relative molecular weight of alkanes, the boiling point and density increase, and the state of aggregation also changes: the first four alkanes are gaseous substances, the next eleven are liquids, and starting from hexadecane, solids.
Main chemical property saturated hydrocarbons, which determines the use of alkanes as a fuel, is combustion reaction.
For alkanes, as for saturated hydrocarbons, the most characteristic substitution reactions. So the hydrogen atoms in the methane molecule can be successively replaced by halogen atoms.
Nitration
Alkanes react with nitric acid or N 2 O 4 in the gas phase to form nitro derivatives. All available data point to a free radical mechanism. As a result of the reaction, mixtures of products are formed.
Cracking
When heated above 500°C, alkanes undergo pyrolytic decomposition with the formation of a complex mixture of products, the composition and ratio of which depend on the reaction temperature and time.
Receipt
The main source of alkanes is oil and natural gas, which usually occur together.
Application
Gaseous alkanes are used as a valuable fuel. Liquids, in turn, make up a significant proportion in motor and rocket fuels.
2) Alkenes- these are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, one double carbon-carbon bond. Their composition is displayed by the formula: C n H 2n.
Physical properties
The melting and boiling points of alkenes increase with molecular weight and the length of the main carbon chain. Alkenes are insoluble in water, but readily soluble in organic solvents.
Chemical properties
Alkenes are chemically active. Their chemical properties are largely determined by the presence of a double bond. For alkenes, the most typical addition reactions are:
1) Hydrogen, 2) Water, 3) Halogens, 4) Hydrogen halides.
Alkenes easily enter into oxidation reactions, the oxidation of alkenes can occur, depending on the conditions and types of oxidizing reagents, both with the breaking of the double bond and with the preservation of the carbon skeleton. Polymerization of alkenes can proceed both by free radical and cation-anion mechanism.
Methods for obtaining alkenes
The main industrial method for obtaining alkenes is the catalytic and high-temperature cracking of hydrocarbons in oil and natural gas. For the production of lower alkenes, the dehydration reaction of the corresponding alcohols is also used.
In laboratory practice, the method of dehydration of alcohols in the presence of strong mineral acids is usually used. In nature, acyclic alkenes are practically not found. 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.
Application
Alkenes are the most important chemical raw materials. Alkenes are used as raw materials in the production of polymeric materials (plastics, films) and other organic substances. Higher alkenes are used to obtain higher alcohols.
3) Alkynes- these are unsaturated hydrocarbons, the molecules of which contain, in addition to single bonds, one triple carbon-carbon bond. The composition displays the formula: C n H 2n-2.
Physical properties
Alkynes are similar in physical properties to the corresponding alkenes. Lower (up to C 4) - gases without color and odor, having higher boiling points than their counterparts in alkenes. Alkynes are poorly soluble in water, but better in organic solvents. The presence of a triple bond in the chain leads to an increase in the boiling point, density and solubility in water.
Chemical properties
Like all unsaturated compounds, alkynes actively enter into addition reactions: 1) halogens, 2) hydrogen, 3) hydrogen halides, 4) water. They enter into oxidation reactions. Due to the presence of a triple bond, they are prone to polymerization reactions that can proceed in several directions:
a) Under the influence of complex copper salts, dimerization occurs and linear
trimerization of acetylene.
b) When acetylene is heated in the presence of activated carbon (Zelinsky reaction), cyclic trimerization occurs with the formation of benzene.
Acquisition Methods
The main industrial method for producing acetylene is the electro- or thermal cracking of methane, the pyrolysis of natural gas, and the carbide method. Alkynes can be obtained from dihalogenated paraffins by splitting off hydrogen halide under the action of an alcoholic solution of alkali.
Application
Serious industrial value is only acetylene, which is the most important chemical raw material. When burning acetylene in oxygen, the flame temperature reaches 3150 ° C, so acetylene is used for cutting and welding metals.
4) Arenas- aromatic hydrocarbons containing one or more benzene rings.
Physical properties
As a rule, aromatic compounds are solid or liquid substances. They have high refractive and absorption indices. They are insoluble in water, but highly soluble in many organic liquids. Flammable, benzene is toxic.
Chemical properties
Aromatic compounds are characterized by substitution reactions of hydrogen atoms associated with the cycle. Addition and oxidation reactions are possible, but they are difficult, since they violate aromaticity.
Acquisition Methods
The main natural sources of aromatic hydrocarbons are
coal and oil. Trimerization of acetylene and its homologues over activated carbon at 600 °C. Catalytic dehydrogenation of cyclohexane and its derivatives.
Application- Aromatic hydrocarbons, primarily benzene, are widely used in industry: as an additive to gasoline, in the production of solvents, explosives, aniline dye, medicines.
10. Struktura
halové deriváty, nitrosloučeniny, aminosloučeniny, alkoholy a fenoly, aldehydy a ketony, karboxylové sloučeniny – charakteristika, použití, reakce
10. Structure, properties and significance of derivatives of hydrocarbons
haloalkanes, nitro compounds, amino compounds, alcohols and phenols, aldehydes and ketones, carboxylic acids - characteristics, use, reactions
1) Halogenalkanes- organic compounds that contain a carbon-halogen bond in their composition. Due to the fact that the halogen atoms are more electronegative than the carbon atom, the C-X bond is polarized in such a way that the halogen atom acquires a partial negative charge, and the carbon atom acquires a partial positive charge. Most halogenoalkanes in their pure form are colorless compounds. The more atoms carbon, the higher the melting and boiling points. If one carbon atom contains 2 or 3 halogen atoms, then the melting and boiling points of such a compound, on the contrary, decrease. Typical reactions are the Wurtz reaction, nucleophilic substitution, elimination, interaction with alkali and alkaline earth metals. Halogenalkanes are obtained by chlorination of alkanes in the light, by hydrochlorination of unsaturated carbons, or obtained from alcohols. Halogenalkanes are used: as solvents for fats and oils; teflon; as coolants.
2) Nitro compounds- organic compounds containing one or more nitro groups - NO 2 . Nitro compounds are usually understood to mean C-nitro compounds in which the nitro group is bonded to a carbon atom. Nitro compounds are colorless, sparingly soluble in water and highly soluble in organic solvents, liquids with a characteristic almond odor. All nitro compounds are quite strong poisons for the central nervous system. Due to the high polarity, nitro compounds can dissolve substances that do not dissolve in ordinary solvents. Polynitro compounds are usually weakly colored, explosive on impact and detonation.
According to the chemical behavior of nitro compounds, they show a certain similarity with nitric acid. This similarity is manifested in redox reactions: Reduction of nitro compounds (Zinin Reaction), condensation reactions, Tautomerism (the phenomenon of reverse isomerism) of nitro compounds.
Nitro compounds are widely used in organic synthesis to obtain various substances used in the production of dyes and drugs. Some of the nitro compounds are used as antifungal and antimicrobial agents. Polynitro derivatives - TNT, picric acid and its salts - are used as explosives.
4) Amino compounds- these are organic compounds that are derivatives of ammonia, in the molecule of which one, two or three hydrogen atoms are replaced by a hydrocarbon radical. Amines are classified according to two structural features: 1) By the number of radicals associated with the nitrogen atom, primary, secondary and tertiary amines are distinguished. 2) According to the nature of the hydrocarbon radical, amines are divided into aliphatic, aromatic and mixed.
Methylamine, dimethylamine and trimethylamine are gases, the middle members of the aliphatic series are liquids, the higher ones are solids. Like ammonia, lower amines dissolve perfectly in water, forming alkaline solutions. With an increase in molecular weight, the solubility of amines in water deteriorates. The smell of amines resembles the smell of ammonia, higher amines are practically odorless. The boiling points of primary amines are much lower than those of the corresponding alcohols.
The fatty amines, like ammonia, are capable of combining with acids, even those as weak as carbonic acid, and in doing so give the corresponding salts of substituted ammonium bases. The action of nitrous acid on amines is their characteristic reaction, which makes it possible to distinguish between primary, secondary and tertiary amines.
Acylation. When heated with carboxylic acids, their anhydrides, acid chlorides or esters, primary and secondary amines are acylated to form N-substituted amides. Amines are widely distributed in nature, as they are formed during the decay of living organisms. Amines are used in the preparation of drugs, dyes and starting products for organic synthesis.
5) Alcohols- organic compounds containing one or more hydroxyl groups. According to the number of hydroxyl groups contained in the molecule, alcohols are divided into monoatomic dihydric, trihydric and polyhydric. Depending on which carbon atom the hydroxyl is located on, primary, secondary, and tertiary alcohols are distinguished .Alcohol molecules are similar to the water molecule, but alcohols have significantly higher melting and boiling points. The properties characteristic of this class of compounds are due to the presence of a hydroxyl group. Alcohols interact with: alkali and alkaline earth metals, with hydrogen halides and
with organic and inorganic acids to form esters. There are also reactions of intermolecular dehydration of alcohols, dehydrogenation and oxidation reactions of alcohols. Alcohols are widely distributed in nature both in the free form and in the composition of esters. Alcohols can be derived from a wide variety of classes of compounds such as hydrocarbons, haloalkanes, amines, and carbonyl compounds. Basically, all methods are reduced to the reactions of oxidation, reduction, addition and substitution. In industry, alcohols are obtained using chemical methods or biochemical production methods. The areas of use of alcohols are numerous and varied, especially considering the widest range of compounds belonging to this class. Alcohols are used as solvents and cleaners, ethyl alcohol is the basis of alcoholic products, and is also widely used in the perfume industry and many other areas.
6) Phenols- These are organic compounds in the molecules of which the phenyl radical is bonded to one or more hydroxyl groups. According to the number of OH groups, monatomic and polyhydric phenols are distinguished. Most monohydric phenols under normal conditions are colorless crystalline substances with a low melting point and a characteristic odor. Phenols are sparingly soluble in water, readily soluble in organic solvents, toxic, and gradually darken when stored in air as a result of oxidation. Phenol has pronounced acidic properties. This is due to the fact that the free electron pair of oxygen in phenol is drawn to the nucleus. When phenol interacts with alkalis, salts are formed - phenolates. Due to the hydroxyl group, phenol will interact with alkali metals.
Substitution and addition reactions also take place with the participation of the benzene ring.
Phenols are found in significant quantities in coal tar. Phenol is also obtained by fusing the sodium salt of benzenesulfonic acid with caustic soda.
Phenol is used in the production of plastics, picric acid, dyes, insecticides. All phenols have a bactericidal effect, so they are used as disinfectants in medicine and veterinary medicine.
Aldehydes and ketones
Aldehydes- These are organic compounds whose molecules contain a carboxyl group associated with a hydrogen atom and a hydrocarbon radical.
Ketones- These are organic substances whose molecules contain a carbonyl group connected to two hydrocarbon radicals.
Since aldehydes and ketones are polar compounds, they have higher boiling points than non-polar ones, but lower than alcohols, indicating a lack of molecular association. They are highly soluble in water, but with increasing molecular size, the solubility decreases sharply. Higher aldehydes and ketones have a pleasant odor, medium homologues of a number of aldehydes have a persistent characteristic odor, lower aldehydes have a sharp unpleasant odor. Aldehydes and ketones are characterized by double bond addition reactions. In addition to the addition reaction at the carbonyl group, aldehydes are also characterized by reactions involving alpha hydrogen atoms adjacent to the carbonyl group. Their reactivity is associated with the electron-withdrawing effect of the carbonyl group, which manifests itself in increased bond polarity. This leads to the fact that aldehydes, unlike ketones, are easily oxidized. Their interaction with an ammonia solution of silver oxide is a qualitative reaction to aldehydes. A common method for obtaining aldehydes and ketones is the oxidation of alcohols on a copper catalyst. In industry, aldehydes and ketones are obtained by dehydrogenation of alcohols. In industry, ketones are used as solvents, pharmaceuticals, and for the manufacture of various polymers. Of all the aldehydes, formaldehyde is produced the most. It is mainly used in the production of resins. Also, drugs are synthesized from it and used as a preservative for biological preparations.
8) Carboxylic acids- these are organic compounds whose molecules contain a carboxyl group -COOH associated with a hydrocarbon radical. The boiling and melting points of carboxylic acids are much higher, not only than those of the corresponding hydrocarbons, but also than those of alcohols. Good solubility in water, but deteriorates with increasing hydrocarbon radical. The lower members of the homologous series under normal conditions are liquids with a characteristic pungent odor. The middle representatives of this homologous series are viscous liquids; starting from C 10 - solids. The carboxyl group is arranged in such a way that the molecule can easily split off hydrogen - exhibit the properties of an acid. Carboxylic acids react with metals and their compounds, displace weaker acids from their salts, interact with basic and amphoteric oxides and hydroxides, and also participate in the esterification reaction. Carboxylic acids are obtained by the oxidation of aldehydes and alcohols and the hydrolysis of esters. Formic acid is used in medicine, acetic acid is used in the food industry, and is also used as a solvent.
11. Makromolekulární látky vznikající polymerací, polykondenzaci a polyadicí
stavebni a strukturni jednotka
vlastnosti makromolekularnych látek
polymery, polyestery, polyamidy, fenoplasty, aminoplasty, polyuretany – příklady, použití
Hydrocarbons, in the molecules of which the atoms are connected by single bonds and which correspond to the general formula C n H 2 n +2.
In alkane molecules, all carbon atoms are in a state of sp 3 hybridization. This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed to the corners of an equilateral triangular pyramid - a tetrahedron. The angles between the orbitals are 109° 28'.
Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at carbon atoms close to tetrahedral (109 ° 28 ′), for example, in a molecule n-pentane.
It is especially worth recalling the bonds in the molecules of alkanes. All bonds in the molecules of saturated hydrocarbons are single. Overlapping occurs along the axis,
connecting the nuclei of atoms, i.e., these are σ-bonds. Carbon-carbon bonds are non-polar and poorly polarizable. The length of the C-C bond in alkanes is 0.154 nm (1.54 10 - 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e., the C-H bond is weakly polar.
The absence of polar bonds in the molecules of saturated hydrocarbons leads to the fact that they are poorly soluble in water and do not interact with charged particles (ions). The most characteristic of alkanes are reactions that involve free radicals.
Homologous series of methane
homologues- substances similar in structure and properties and differing by one or more CH 2 groups.
Isomerism and nomenclature
Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane.
Fundamentals of nomenclature
1. Selecting the main circuit. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule, which is, as it were, its basis.
2. Numbering of atoms of the main chain. The atoms of the main chain are assigned numbers. The numbering of atoms of the main chain starts from the end closest to the substituent (structures A, B). If the substituents are at an equal distance from the end of the chain, then the numbering starts from the end at which there are more of them (structure B). If different substituents are at an equal distance from the ends of the chain, then the numbering starts from the end to which the older one is closer (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins follows in the alphabet: methyl (-CH 3), then ethyl (-CH 2 -CH 3), propyl (-CH 2 -CH 2 -CH 3 ) etc.
Note that the name of the substitute is formed by replacing the suffix -an with the suffix - silt in the name of the corresponding alkane.
3. Name formation. Numbers are indicated at the beginning of the name - the numbers of carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma (2,2-). After the number, a hyphen indicates the number of substituents ( di- two, three- three, tetra- four, penta- five) and the name of the substituent (methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is referred to as a hydrocarbon - a member of the methane homologous series ( methane CH 4, ethane C 2 H 6, propane C 3 H 8, C 4 H 10, pentane C 5 H 12, hexane C 6 H 14, heptane C 7 H 16, octane C 8 H 18, nonan C 9 H 20, dean C 10 H 22).
Physical properties of alkanes
The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of "gas", having felt which, you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could smell the leak).
Hydrocarbons of composition from C 4 H 12 to C 15 H 32 - liquids; heavier hydrocarbons are solids. The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties of alkanes
substitution reactions.
The most characteristic of alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group. Let us present the equations of characteristic reactions halogenation:
In the case of an excess of halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms by chlorine:
The resulting substances are widely used as solvents and starting materials in organic synthesis.
Dehydrogenation reaction(hydrogen splitting off).
During the passage of alkanes over the catalyst (Pt, Ni, Al 2 0 3, Cr 2 0 3) at a high temperature (400-600 ° C), a hydrogen molecule is split off and an alkene is formed:
Reactions accompanied by the destruction of the carbon chain.
All saturated hydrocarbons burn with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode.
1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as a fuel:
In general, the combustion reaction of alkanes can be written as follows:
2. Thermal splitting of hydrocarbons.
The process proceeds according to the free radical mechanism. An increase in temperature leads to a homolytic rupture of the carbon-carbon bond and the formation of free radicals.
These radicals interact with each other, exchanging a hydrogen atom, with the formation of an alkane molecule and an alkene molecule:
Thermal splitting reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.
3. Pyrolysis. When methane is heated to a temperature of 1000 ° C, pyrolysis of methane begins - decomposition into simple substances:
When heated to a temperature of 1500 ° C, the formation of acetylene is possible:
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:
5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:
Alkanes enter into reactions that proceed according to the free radical mechanism, since all carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent non-polar C-C (carbon - carbon) bonds and weakly polar C-H (carbon - hydrogen) bonds. They do not have areas with high and low electron density, easily polarizable bonds, i.e., such bonds, the electron density in which can be shifted under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since bonds in alkane molecules are not broken by a heterolytic mechanism.
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Target: to acquaint with the chemical properties of saturated hydrocarbons, to teach how to write equations of chemical reactions, to indicate the conditions for their occurrence. Continue the formation of worldview concepts: about the knowability of nature, the cause-and-effect relationship between the composition, structure, properties and use of saturated hydrocarbons.
Type of lesson: learning new material.
Type of lesson: conversation, lecture.
Lesson methods:
Teaching - dialogical, demonstrative.
Teaching - informational, informing, explanatory.
Lesson equipment: computer, projector, candle, matches.
1. Update.
1. What organic substances are classified as hydrocarbons? (Hydrocarbons are organic compounds made up of two elements: carbon and hydrogen.)
2. What is the name of saturated hydrocarbons according to the international nomenclature? (Alkanes.)
3. What is the general formula of alkanes. (C n H 2n+2 .)
4. Write the formulas of alkanes containing carbon atoms: a) 16; b) 21; c) 23. (C 16 H 34, C 21 H 44, C 23 H 48.)
5. Specify the type of hybridization typical for saturated hydrocarbons. ( sp 3-G inbreeding.)
6. Name the bond angle and length characteristic of alkanes. (The angle is 109°28" and the carbon-carbon bond length is 0.154 nm.)
2. Learning new material.
Under normal conditions, alkanes are chemically inert. They are resistant to the action of many reagents: they do not interact with concentrated sulfuric and nitric acids, with concentrated and molten alkalis, they are not oxidized by strong oxidizing agents - potassium permanganate KMnO 4, etc.
The chemical stability of alkanes is due to the high strength s-C-C and C-H bonds, as well as their non-polarity. Nonpolar C-C and C-H bonds in alkanes are not prone to ionic cleavage, but are capable of cleaving homolytically under the action of active free radicals. Therefore, alkanes are characterized by radical reactions, as a result of which compounds are obtained where hydrogen atoms are replaced by other atoms or groups of atoms.
Therefore, alkanes enter into reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R (from English, substitution radicalic). According to this mechanism, hydrogen atoms are most easily replaced at tertiary, then at secondary and primary carbon atoms.
2.1. Halogenation.
When alkanes react with halogens (chlorine and bromine) under the action of UV radiation or high temperature, a mixture of products from mono- to polyhalogen-substituted alkanes is formed.
CH 4 + CL 2 ->CH 3 CL- chloromethane
CH 3 CL + CL 2 ->CH 2 CL 2 - dichloromethane
CH 2 CL 2 + CL 2 -> CHCL 3 - trichloromethane
CHCL 3 + CL 2 -> CCL 4 -tetrochloromethane
2.2. Nitration.
Despite the fact that under normal conditions, alkanes do not interact with concentrated nitric acid, when they are heated to 140 ° C with dilute (10%) nitric acid under pressure, a nitration reaction occurs - the replacement of a hydrogen atom by a nitro group (reaction of M.I. Konovalov ). All alkanes enter into the nitration reaction, but the reaction rate and yields of nitro compounds are low. The best results are observed with alkanes containing tertiary carbon atoms.
CH 3 -CH 3 + HNO 3 -> CH 3 -CH 2 -NO 2 + H 2 O.
2.3. Isomerization.
Under the influence of catalysts, when heated, hydrocarbons of a normal structure undergo isomerization - a rearrangement of the carbon skeleton with the formation of branched alkanes.
2.4. Thermal decomposition.
CH 4 ->C + 2H 2
C 2 H 2 ->2C +H 2
Cracking- at high temperature in the presence of catalysts, saturated hydrocarbons undergo splitting, which is called cracking. During cracking, a homolytic rupture of carbon-carbon bonds occurs with the formation of saturated and unsaturated hydrocarbons with shorter chains.
C 8 H 18 -> C 4 H 10 + C 4 H 8
These reactions are of great industrial importance. In this way, high-boiling oil fractions (fuel oil) are converted into gasoline, kerosene and other valuable products.
An increase in the process temperature leads to deeper decomposition of hydrocarbons and, in particular, to dehydrogenation, i.e. to the elimination of hydrogen. Thus, methane at 1500°C leads to acetylene.
2CH4 -> C2H2 + 3H2
2.5. Oxidation.
Under normal conditions, alkanes are resistant to the action of oxygen and oxidizing agents. When ignited in air, alkanes burn, turning into carbon dioxide and water and releasing a large amount of heat.
CH 4 + 2O 2 -> CO 2 + 2H 2 O
C 5 H 12 + 8O 2 -> 5CO 2 + 6H 2 O
C n H 2n +2 + (Zn + 1) / 2O 2 \u003d nCO 2 + (n + 1) H 2 O.
(Demonstration of burning a candle)
3. Application (independent work with the text of the textbook ).
The first in a series of alkanes - methane- is the main component of natural and associated gases and is widely used as industrial and household gas. It is processed in industry into acetylene, carbon black, fluorine and chlorine derivatives.
The lower members of the homologous series are used to obtain the corresponding unsaturated compounds by the dehydrogenation reaction. A mixture of propane and butane is used as a domestic fuel.
The middle members of the homologous series are used as solvents and motor fuels. Higher alkanes are used to produce higher fatty acids, synthetic fats, lubricating oils, etc.
4. Homework: paragraph 11, do exercise. 4, 5.
Video lesson 2: Cycloalkanes: Chemical Properties
Video lesson 3: Alkenes: Chemical Properties
Video lesson 4: Alkadienes (dienes): Chemical properties
Video lesson 5: Alkynes: Chemical Properties
Lecture: Characteristic chemical properties of hydrocarbons: alkanes, cycloalkanes, alkenes, dienes, alkynes, aromatic hydrocarbons
Chemical properties of alkanes and cycloalkanes
Alkanes are non-cyclic hydrocarbons. The carbon atoms in these compounds have sp 3 hybridization. In the molecules of these hydrocarbons, all carbon atoms are connected only by single non-polar and low-polar C-C bonds. The overlapping of orbitals occurs along the axis connecting the nuclei of atoms. These are σ-bonds. These organic compounds contain the maximum number of hydrogen atoms, therefore they are called limiting (saturated). Due to saturation, alkanes are unable to enter into addition reactions. Since carbon and hydrogen atoms have similar electronegativity, this factor leads to the fact that the C-H bonds in their molecules are of low polarity. Because of this, reactions involving free radicals are inherent in alkanes.
1. substitution reactions. As mentioned, these are the most characteristic reactions for alkanes. In such reactions, the carbon-hydrogen bonds are broken. Consider some types of substitution reactions:
Halogenation. Alkanes react with halogens (chlorine and bromine) when exposed to ultraviolet light or high heat. For example: CH 4 + Cl 2 → CH 3 Cl + HCl.With an excess of halogen, the reaction continues until the formation of a mixture of halogen derivatives of various degrees of substitution of hydrogen atoms: mono-, di-tri-, etc. For example, the reaction of the formation of dichloromethane (methylene chloride): CH 3 Cl + Cl 2 → HCl + CH 2 Cl 2.
Nitration (Konovalov's reaction). Under heat and pressure, alkanes react with dilute nitric acid. Subsequently, the hydrogen atom is replaced by the NO 2 nitro group and a nitroalkane is formed. General view of this reaction: R-H + HO-NO 2 → R-NO 2 + H 2 O. Where R-H is an alkane, R- NO 2 - nitroalkane.
2. Oxidation reactions. Under normal conditions, alkanes do not react with strong oxidizing agents (conc. sulfuric and nitric acids, potassium permanganate KMnO 4 and potassium dichromate K 2 Cr 2 O 7).
To obtain energy, the combustion reactions of alkanes are widely used:
a) With complete combustion with an excess of oxygen, carbon dioxide and water are formed: CH 4 + 2O 2 → CO 2 + 2H 2 O
b) Partial combustion with a lack of oxygen: CH 4 + O 2 → C + 2H 2 O. This reaction is used in industry to produce soot.
Heating alkanes with oxygen (~200 o C) using catalysts leads to the breaking of part of the C–C and C–H bonds. As a result, aldehydes, ketones, alcohols, carboxylic acids are formed. For example, with incomplete oxidation of butane, acetic acid is obtained: CH 3 -CH 2 -/-CH 2 -CH 3 + 3O 2 → 2CH 3 COOH + 2H 2 O.
Of great importance is the reaction of methane and water vapor with the formation of a mixture of gases of carbon monoxide (II) with hydrogen. It flows at t 800 0 C: CH4+ H 2 O → 3H 2 + CO. This reaction also makes it possible to obtain various hydrocarbons.
3. Thermal transformations of alkanes. Heating alkanes without access to air to high t leads to the breaking of the C-C bond. This type of reaction includes cracking and isomerization used for oil refining. Also, these reactions include dehydrogenation, which is necessary to obtain alkenes, alkadienes and aromatic hydrocarbons.
The result of cracking is a break in the carbon skeleton of alkane molecules. General view of cracking of alkanes at t 450-700 0 C: C n H 2n+2 → C n-k H 2(n-k)+2 + C k H 2k.When heated to 1000 0 C, methane decomposes into simple substances: CH 4 → C + 2 H 2 . This reaction is called methane pyrolysis.When methane is heated to 1500 0 C, acetylene is formed: 2 CH 4 → C 2 H 2 + 3 H 2 .
Isomerization. If an aluminum chloride catalyst is used in cracking, normal-chain alkanes are converted to branched-chain alkanes:
Dehydrogenation, i.e. hydrogen splitting occurs in the presence of catalysts and at t 400-600 0 C. As a result, the C-H bond is broken, an alkene is formed: CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2 or alkadiene: CH 3 -CH 2 -CH 2 -CH 3 → CH 2 \u003d CH-CH \u003d CH 2 + 2H 2.
The chemical properties of cycloalkanes with more than four carbon atoms in the cycles are practically similar to those of alkanes. However, cyclopropane and cyclobutane are characterized by addition reactions. This is due to the great tension within the cycle, which leads to the desire of cycles to break and open. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride. For example:
Chemical properties of alkenes
1. Addition reactions. Alkenes are active compounds because the double bond in their molecules consists of one strong sigma bond and one weak pi bond. Alkenes often enter into addition reactions even in the cold, in aqueous solutions and organic solvents.
Hydrogenation, i.e. hydrogen addition is possible in the presence of catalysts: CH 3 -CH \u003d CH 2 + H 2 → CH 3 -CH 2 -CH 3 . The same catalysts are used for the dehydrogenation of alkanes to alkenes. But the dehydrogenation process will take place at a higher t and lower pressure.
Halogenation. Reactions of alkenes with bromine easily occur in aqueous solution and in organic solvents. As a result, yellow solutions of bromine lose their color, that is, they become discolored: CH 2 \u003d CH 2 + Br 2 → CH 2 Br- CH 2 Br.
Hydrohalogenation. The addition of a hydrogen halide molecule to an unsymmetrical alkene molecule results in a mixture of two isomers. In the absence of specific conditions, the addition occurs selectively, according to the rule of V.V. Markovnikov. There is the following pattern of addition: hydrogen attaches to the carbon atom that has more hydrogen atoms, and halogen attaches to the carbon atom with fewer hydrogen atoms: CH 2 \u003d CH-CH 3 + HBr → CH 3 -CHBr-CH 3. 2-bromopropane was formed.
Hydration of alkenes leads to the formation of alcohols. Since the addition of water to the alkene molecule occurs according to the Markovnikov rule, the formation of primary alcohol is possible only with the hydration of ethylene: CH 2 \u003d CH 2 + H 2 O → CH 3 - CH 2 - OH.
Polymerization proceeds by a free radical mechanism: nCH 2 \u003d CH 2 → ( - CH 2 - CH 2 -) n. formed polyethylene.
2. Oxidation reactions. Alkenes to Like all other hydrocarbons, they burn in oxygen. The equation for the combustion of alkenes in excess oxygen has the form: C n H 2n+2 + O 2 → nCO 2 + (n+1)H 2 O. Carbon dioxide and water were produced.
Alkenes are fairly easy to oxidize. Under the action of an aqueous solution of KMnO 4 on alkenes, discoloration occurs.
Oxidation of alkenes with potassium permanganate in a neutral or slightly alkaline solution forms diols: C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH(cooling).
In an acidic medium, the double bond is completely broken, followed by the transformation of the carbon atoms that formed the double bond into carboxyl groups: 5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K2SO 4 + 17H 2 O(the heating).
When the double C=C bond is located at the end of the alkene molecule, carbon dioxide will act as the oxidation product of the extreme carbon atom at the double bond. This process is due to the fact that the intermediate product of oxidation, namely formic acid, is quite simply oxidized in an excess of an oxidizing agent: 5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O(the heating).
Chemical properties of alkynes
Alkynes are unsaturated hydrocarbons that enter into addition reactions.
Halogenation of alkynes leads to the addition of their molecules to both one and two halogen molecules. This is due to the presence of one strong sigma bond and two weak pi bonds in the triple bond of alkyne molecules. The addition of two halogen molecules by one alkyne molecule proceeds by the electrophilic mechanism sequentially, in two stages.
Hydrohalogenation also proceeds by an electrophilic mechanism and in two stages. In both stages, the addition of hydrogen halide molecules follows the Markovnikov rule.
Hydration takes place with the participation of mercury salts in an acidic environment and is called the Kucherov reaction:
Hydrogenation (reaction with hydrogen) of alkynes occurs in two phases. Metals such as platinum, palladium, and nickel are used as catalysts.
Trimerization of alkynes, such as acetylene. If this substance is passed over activated carbon at high t, a mixture of various products is formed, the main of which is benzene:
Alkyne dimerization proceeds in the presence of copper salts as catalysts: HC≡CH + HC≡CH → H 2 C= CH - C ≡CH
Alkyne oxidation: С n H 2n-2 + (3n+1) / 2 O 2 → nCO 2 + (n+1)H 2 O.
- Alkynes with triple C≡C at the end of the molecule interact with bases. For example, the reaction of acetylene with sodium amide in liquid ammonia: HC≡CH + NaNH 2 → NaC≡CNa + 2NH 3 . The reaction with an ammonia solution of silver oxide forms acetylenides (insoluble salt-like substances). This reaction is carried out if it is necessary to recognize alkynes with a terminal triple bond or to isolate such an alkyne from a mixture with other alkynes. All silver and copper acetylides are explosive. Acetylides are able to react with halogen derivatives. This possibility is used for the synthesis of more complex organic compounds with a triple bond: CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3; CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr.
Chemical properties of dienes
Alkadienes are chemically similar to alkenes. But there are some features:
- Halogenation. Alkadienes are able to add with hydrogen, halogens and hydrogen halides in positions of 1,2-addition: CH 2 \u003d CH -CH \u003d CH 2 + Br 2 → CH 2 \u003d CH -CH Br- CH2Br
as well as 1,4-attachments: CH 2 \u003d CH -CH \u003d CH 2 + Br 2 → Br CH 2 - CH =CH - CH2Br
- Polymerization: nCH 2 \u003d CH-CH \u003d CH 2 t, Na→ (-CH 2 -CH=CH-CH 2 -) n . This is how synthetic rubber is obtained.
Chemical properties of aromatic hydrocarbons (arenes)
Characteristic chemical properties of hydrocarbons: alkanes, alkenes, dienes, alkynes, aromatic hydrocarbons
Alkanes
Alkanes are hydrocarbons in whose molecules the atoms are linked by single bonds and which correspond to the general formula $C_(n)H_(2n+2)$.
Homologous series of methane
As you already know, homologues are substances that are similar in structure and properties and differ by one or more $CH_2$ groups.
Limit hydrocarbons make up the homologous series of methane.
Isomerism and nomenclature
Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. As you already know, the simplest alkane, which is characterized by structural isomers, is butane:
Let us consider in more detail for alkanes the basics of the IUPAC nomenclature:
1. Choice of the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.
2.
The atoms of the main chain are assigned numbers. The numbering of atoms of the main chain starts from the end closest to the substituent (structures A, B). If the substituents are at an equal distance from the end of the chain, then the numbering starts from the end at which there are more of them (structure B). If different substituents are at an equal distance from the ends of the chain, then the numbering starts from the end to which the older one is closer (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins follows in the alphabet: methyl (—$CH_3$), then propyl ($—CH_2—CH_2—CH_3$), ethyl ($—CH_2—CH_3$ ) etc.
Note that the name of the substitute is formed by replacing the suffix -an to suffix -silt in the name of the corresponding alkane.
3. Name formation.
Numbers are indicated at the beginning of the name - the numbers of carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice, separated by commas ($2.2-$). After the number, a hyphen indicates the number of substituents ( di- two, three- three, tetra- four, penta- five) and the name of the deputy ( methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is called as a hydrocarbon - a member of the homologous series of methane ( methane, ethane, propane, etc.).
The names of the substances whose structural formulas are given above are as follows:
- structure A: $2$ -methylpropane;
- Structure B: $3$ -ethylhexane;
- Structure B: $2,2,4$ -trimethylpentane;
- structure Г: $2$ -methyl$4$-ethylhexane.
Physical and chemical properties of alkanes
physical properties. The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of gas, upon smelling which you need to call $104$, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could smell the leak).
Hydrocarbons of composition from $С_5Н_(12)$ to $С_(15)Н_(32)$ are liquids; heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties.
1. substitution reactions. The most characteristic of alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most typical reactions.
Halogenation:
$CH_4+Cl_2→CH_3Cl+HCl$.
In the case of an excess of halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms by chlorine:
$CH_3Cl+Cl_2→HCl+(CH_2Cl_2)↙(\text"dichloromethane(methylene chloride)")$,
$CH_2Cl_2+Cl_2→HCl+(CHСl_3)↙(\text"trichloromethane(chloroform)")$,
$CHCl_3+Cl_2→HCl+(CCl_4)↙(\text"tetrachloromethane(carbon tetrachloride)")$.
The resulting substances are widely used as solvents and starting materials in organic synthesis.
2. Dehydrogenation (elimination of hydrogen). During the passage of alkanes over the catalyst ($Pt, Ni, Al_2O_3, Cr_2O_3$) at a high temperature ($400-600°C$), a hydrogen molecule is split off and an alkene is formed:
$CH_3—CH_3→CH_2=CH_2+H_2$
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free radical exothermic reaction, which is of great importance when using alkanes as a fuel:
$CH_4+2O_2→CO_2+2H_2O+880 kJ.$
In general, the combustion reaction of alkanes can be written as follows:
$C_(n)H_(2n+2)+((3n+1)/(2))O_2→nCO_2+(n+1)H_2O$
Thermal breakdown of hydrocarbons:
$C_(n)H_(2n+2)(→)↖(400-500°C)C_(n-k)H_(2(n-k)+2)+C_(k)H_(2k)$
The process proceeds according to the free radical mechanism. An increase in temperature leads to a homolytic rupture of the carbon-carbon bond and the formation of free radicals:
$R—CH_2CH_2:CH_2—R→R—CH_2CH_2+CH_2—R$.
These radicals interact with each other, exchanging a hydrogen atom, with the formation of an alkane molecule and an alkene molecule:
$R—CH_2CH_2+CH_2—R→R—CH=CH_2+CH_3—R$.
Thermal splitting reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.
When methane is heated to a temperature of $1000°C$, pyrolysis of methane begins - decomposition into simple substances:
$CH_4(→)↖(1000°C)C+2H_2$
When heated to a temperature of $1500°C$, the formation of acetylene is possible:
$2CH_4(→)↖(1500°C)CH=CH+3H_2$
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:
5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:
What is the reason that alkanes enter into reactions proceeding according to the free radical mechanism? All carbon atoms in alkane molecules are in the $sp^3$ hybridization state. The molecules of these substances are built using covalent nonpolar $C—C$ (carbon—carbon) bonds and weakly polar $C—H$ (carbon—hydrogen) bonds. They do not contain areas with high and low electron density, easily polarizable bonds, i.e. such bonds, the electron density in which can be shifted under the influence of external factors (electrostatic fields of ions). Therefore, alkanes will not react with charged particles, because bonds in alkane molecules are not broken by a heterolytic mechanism.
Alkenes
Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkadienes (polyenes), alkynes. Cyclic hydrocarbons containing a double bond in the cycle (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the cycle (three or four atoms) also have an unsaturated character. The property of unsaturation is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.
Alkenes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n)$.
Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from lat. oleum- oil).
Homologous series of ethene
Unbranched alkenes make up the homologous series of ethene (ethylene):
$C_2H_4$ is ethene, $C_3H_6$ is propene, $C_4H_8$ is butene, $C_5H_(10)$ is pentene, $C_6H_(12)$ is hexene, etc.
Isomerism and nomenclature
For alkenes, as well as for alkanes, structural isomerism is characteristic. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:
A special type of structural isomerism is the double bond position isomerism:
$CH_3—(CH_2)↙(butene-1)—CH=CH_2$ $CH_3—(CH=CH)↙(butene-2)—CH_3$
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.
cis- isomers are different from trance- isomers by the spatial arrangement of fragments of the molecule (in this case, methyl groups) relative to the $π$-bond plane, and, consequently, by properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
The nomenclature of alkenes developed by IUPAC is similar to the nomenclature of alkanes.
1. Choice of the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.
2. Atom numbering of the main chain.
The numbering of the atoms of the main chain starts from the end to which the double bond is closest. For example, the correct connection name is:
$5$-methylhexene-$2$, not $2$-methylhexene-$4$, as might be expected.
If it is impossible to determine the beginning of the numbering of atoms in the chain by the position of the double bond, then it is determined by the position of the substituents, just as for saturated hydrocarbons.
3. Name formation.
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix indicating that the compound belongs to the class of alkenes - -en.
For example:
Physical and chemical properties of alkenes
physical properties. The first three representatives of the homologous series of alkenes are gases; substances of the composition $C_5H_(10)$ - $C_(16)H_(32)$ are liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.
Chemical properties.
Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism
1. hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:
$CH_3—CH_2—CH=CH_2+H_2(→)↖(Pt)CH_3—CH_2—CH_2—CH_3$.
This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, because is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent ($CCl_4$) leads to a rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihalogen alkanes:
$CH_2=CH_2+Br_2→CH_2Br—CH_2Br$.
3.
$CH_3-(CH)↙(propene)=CH_2+HBr→CH_3-(CHBr)↙(2-bromopropene)-CH_3$
This reaction is subject to Markovnikov's rule:
When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen - to the less hydrogenated one.
Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:
$(CH_2)↙(ethene)=CH_2+H_2O(→)↖(t,H_3PO_4)CH_3-(CH_2OH)↙(ethanol)$
Note that a primary alcohol (with a hydroxo group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.
This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is attached to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
$nCH_2(=)↙(ethene)CH_2(→)↖(UV light,R)(...(-CH_2-CH_2-)↙(polyethylene)...)_n$
This addition reaction proceeds by a free radical mechanism.
6. Oxidation reaction.
Like any organic compounds, alkenes burn in oxygen to form $CO_2$ and $H_2O$:
$CH_2=CH_2+3O_2→2CO_2+2H_2O$.
In general:
$C_(n)H_(2n)+(3n)/(2)O_2→nCO_2+nH_2O$
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:
Alkadienes (diene hydrocarbons)
Alkadienes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, two double bonds between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Depending on the mutual arrangement of double bonds, there are three types of dienes:
- alkadienes with cumulated arrangement of double bonds:
- alkadienes with conjugated double bonds;
$CH_2=CH—CH=CH_2$;
- alkadienes with isolated double bonds
$CH_2=CH—CH_2—CH=CH_2$.
All three types of alkadienes differ significantly from each other in structure and properties. The central carbon atom (an atom that forms two double bonds) in alkadienes with cumulated bonds is in the $sp$-hybridization state. It forms two $σ$-bonds lying on the same straight line and directed in opposite directions, and two $π$-bonds lying in perpendicular planes. $π$-bonds are formed due to unhybridized p-orbitals of each carbon atom. The properties of alkadienes with isolated double bonds are very specific, because conjugated $π$-bonds significantly affect each other.
p-Orbitals forming conjugated $π$-bonds make up practically a single system (it is called a $π$-system), because p-orbitals of neighboring $π$-bonds partially overlap.
Isomerism and nomenclature
Alkadienes are characterized by both structural isomerism and cis- and trans-isomerism.
Structural isomerism.
— isomerism of the carbon skeleton:
— isomerism of the position of multiple bonds:
$(CH_2=CH—CH=CH_2)↙(butadiene-1,3)$ $(CH_2=C=CH—CH_3)↙(butadiene-1,2)$
cis-, trans- isomerism (spatial and geometric)
For example:
Alkadienes are isomeric compounds of the classes of alkynes and cycloalkenes.
When forming the name of the alkadiene, the numbers of double bonds are indicated. The main chain must necessarily contain two multiple bonds.
For example:
Physical and chemical properties of alkadienes
physical properties.
Under normal conditions, propandien-1,2, butadiene-1,3 are gases, 2-methylbutadiene-1,3 is a volatile liquid. Alkadienes with isolated double bonds (the simplest of them is pentadiene-1,4) are liquids. Higher dienes are solids.
Chemical properties.
The chemical properties of alkadienes with isolated double bonds differ little from those of alkenes. Alkadienes with conjugated bonds have some special features.
1. Addition reactions. Alkadienes are capable of adding hydrogen, halogens, and hydrogen halides.
A feature of addition to alkadienes with conjugated bonds is the ability to attach molecules both in positions 1 and 2, and in positions 1 and 4.
The ratio of the products depends on the conditions and method of carrying out the corresponding reactions.
2.polymerization reaction. The most important property of dienes is the ability to polymerize under the influence of cations or free radicals. The polymerization of these compounds is the basis of synthetic rubbers:
$nCH_2=(CH—CH=CH_2)↙(butadiene-1,3)→((... —CH_2—CH=CH—CH_2— ...)_n)↙(\text"synthetic butadiene rubber")$ .
The polymerization of conjugated dienes proceeds as 1,4-addition.
In this case, the double bond turns out to be central in the link, and the elementary link, in turn, can take both cis-, and trance- configuration.
Alkynes
Alkynes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one triple bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Homologous series of ethine
Unbranched alkynes make up the homologous series of ethyne (acetylene):
$C_2H_2$ - ethyne, $C_3H_4$ - propyne, $C_4H_6$ - butyne, $C_5H_8$ - pentine, $C_6H_(10)$ - hexine, etc.
Isomerism and nomenclature
For alkynes, as well as for alkenes, structural isomerism is characteristic: isomerism of the carbon skeleton and isomerism of the position of the multiple bond. The simplest alkyne, which is characterized by structural isomers of the multiple bond position of the alkyne class, is butyne:
$CH_3—(CH_2)↙(butyn-1)—C≡CH$ $CH_3—(C≡C)↙(butyn-2)—CH_3$
The isomerism of the carbon skeleton in alkynes is possible, starting from pentyn:
Since the triple bond assumes a linear structure of the carbon chain, the geometric ( cis-, trans-) isomerism is not possible for alkynes.
The presence of a triple bond in hydrocarbon molecules of this class is reflected by the suffix -in, and its position in the chain - the number of the carbon atom.
For example:
Alkynes are isomeric compounds of some other classes. So, hexine (alkyne), hexadiene (alkadiene) and cyclohexene (cycloalkene) have the chemical formula $С_6Н_(10)$:
Physical and chemical properties of alkynes
physical properties. The boiling and melting points of alkynes, as well as alkenes, naturally increase with an increase in the molecular weight of the compounds.
Alkynes have a specific smell. They are more soluble in water than alkanes and alkenes.
Chemical properties.
Addition reactions. Alkynes are unsaturated compounds and enter into addition reactions. Basically, these are reactions. electrophilic addition.
1. Halogenation (addition of a halogen molecule). Alkyne is able to attach two halogen molecules (chlorine, bromine):
$CH≡CH+Br_2→(CHBr=CHBr)↙(1,2-dibromoethane),$
$CHBr=CHBr+Br_2→(CHBr_2-CHBr_2)↙(1,1,2,2-tetrabromoethane)$
2. Hydrohalogenation (addition of hydrogen halide). The addition reaction of hydrogen halide, proceeding according to the electrophilic mechanism, also proceeds in two stages, and at both stages the Markovnikov rule is fulfilled:
$CH_3-C≡CH+Br→(CH_3-CBr=CH_2)↙(2-bromopropene),$
$CH_3-CBr=CH_2+HBr→(CH_3-CHBr_2-CH_3)↙(2,2-dibromopropane)$
3. Hydration (addition of water). Of great importance for the industrial synthesis of ketones and aldehydes is the water addition reaction (hydration), which is called Kucherov's reaction:
4. hydrogenation of alkynes. Alkynes add hydrogen in the presence of metal catalysts ($Pt, Pd, Ni$):
$R-C≡C-R+H_2(→)↖(Pt)R-CH=CH-R,$
$R-CH=CH-R+H_2(→)↖(Pt)R-CH_2-CH_2-R$
Since the triple bond contains two reactive $π$ bonds, alkanes add hydrogen in steps:
1) trimerization.
When ethyne is passed over activated carbon, a mixture of products is formed, one of which is benzene:
2) dimerization.
In addition to trimerization of acetylene, its dimerization is also possible. Under the action of monovalent copper salts, vinylacetylene is formed:
$2HC≡CH→(HC≡C-CH=CH_2)↙(\text"butene-1-yn-3(vinylacetylene)")$
This substance is used to produce chloroprene:
$HC≡C-CH=CH_2+HCl(→)↖(CaCl)H_2C=(CCl-CH)↙(chloroprene)=CH_2$
polymerization of which produces chloroprene rubber:
$nH_2C=CCl-CH=CH_2→(...-H_2C-CCl=CH-CH_2-...)_n$
Alkyne oxidation.
Ethine (acetylene) burns in oxygen with the release of a very large amount of heat:
$2C_2H_2+5O_2→4CO_2+2H_2O+2600kJ$ This reaction is based on the action of an oxy-acetylene torch, the flame of which has a very high temperature (more than $3000°C$), which makes it possible to use it for cutting and welding metals.
In air, acetylene burns with a smoky flame, because. the carbon content in its molecule is higher than in the molecules of ethane and ethene.
Alkynes, like alkenes, decolorize acidified solutions of potassium permanganate; in this case, the destruction of the multiple bond occurs.
Reactions characterizing the main methods for obtaining oxygen-containing compounds
1. Hydrolysis of haloalkanes. You already know that the formation of halokenalkanes in the interaction of alcohols with hydrogen halides is a reversible reaction. Therefore, it is clear that alcohols can be obtained by hydrolysis of haloalkanes- reactions of these compounds with water:
$R-Cl+NaOH(→)↖(H_2O)R-OH+NaCl+H_2O$
Polyhydric alcohols can be obtained by hydrolysis of haloalkanes containing more than one halogen atom in the molecule. For example:
2. Hydration of alkenes- the addition of water to the $π$-bond of the alkene molecule - is already familiar to you, for example:
$(CH_2=CH_2)↙(ethene)+H_2O(→)↖(H^(+))(C_2H_5OH)↙(ethanol)$
Hydration of propene leads, in accordance with Markovnikov's rule, to the formation of a secondary alcohol - propanol-2:
3. Hydrogenation of aldehydes and ketones. You already know that the oxidation of alcohols under mild conditions leads to the formation of aldehydes or ketones. Obviously, alcohols can be obtained by hydrogenation (hydrogen reduction, hydrogen addition) of aldehydes and ketones:
4. Alkene oxidation. Glycols, as already noted, can be obtained by oxidizing alkenes with an aqueous solution of potassium permanganate. For example, ethylene glycol (ethanediol-1,2) is formed during the oxidation of ethylene (ethene):
$CH_2=CH_2+[O]+H_2O(→)↖(KMnO_4)HO-CH_2-CH_2-OH$
5. Specific methods for obtaining alcohols. Some alcohols are obtained in ways characteristic only of them. So, methanol is produced in industry by the interaction of hydrogen with carbon monoxide (II) (carbon monoxide) at elevated pressure and high temperature on the surface of the catalyst (zinc oxide):
$CO+2H_2(→)↖(t,p,ZnO)CH_3-OH$
The mixture of carbon monoxide and hydrogen required for this reaction, also called synthesis gas ($CO + nH_2O$), is obtained by passing water vapor over hot coal:
$C+H_2O(→)↖(t)CO+H_2-Q$
6. Fermentation of glucose. This method of obtaining ethyl (wine) alcohol has been known to man since ancient times:
$(C_6H_(12)O_6)↙(glucose)(→)↖(yeast)2C_2H_5OH+2CO_2$
Methods for obtaining aldehydes and ketones
Aldehydes and ketones can be obtained oxidation or alcohol dehydrogenation. Once again, we note that aldehydes can be obtained during the oxidation or dehydrogenation of primary alcohols, and ketones can be obtained from secondary alcohols:
Kucherov's reaction. From acetylene, as a result of the hydration reaction, acetaldehyde is obtained, from acetylene homologs - ketones:
When heated calcium or barium salts carboxylic acids form a ketone and a metal carbonate:
Methods for obtaining carboxylic acids
Carboxylic acids can be obtained by oxidation of primary alcohols of aldehydes:
Aromatic carboxylic acids are formed during the oxidation of benzene homologues:
Hydrolysis of various carboxylic acid derivatives also results in acids. So, during the hydrolysis of an ester, an alcohol and a carboxylic acid are formed. As mentioned above, acid-catalyzed esterification and hydrolysis reactions are reversible:
The hydrolysis of an ester under the action of an aqueous solution of alkali proceeds irreversibly, in this case, not an acid, but its salt is formed from the ester.