Alkanes have bonds. Alkanes - nomenclature, preparation, chemical properties. Industrial methods of production
Saturated hydrocarbons, or paraffins, are those biocompounds in whose molecules the carbon atoms are connected by a simple (single) bond, and all other valency units are saturated with hydrogen atoms.
Alkanes: physical properties
The abstraction of hydrogen from an alkane molecule, or dehydrogenation, in the presence of catalysts and upon heating (up to 460 °C) allows one to obtain the necessary alkenes. Methods have been developed for the oxidation of alkanes at low temperatures in the presence of catalysts (magnesium salts). This allows you to specifically influence the course of the reaction and obtain the necessary oxidation products in the process of chemical synthesis. For example, the oxidation of higher alkanes produces various higher alcohols or higher fatty acids.
The splitting of alkanes also occurs under other conditions (combustion, cracking). Saturated hydrocarbons burn with a blue flame, releasing enormous amounts of heat. These properties make it possible to use them as a high-calorie fuel both in everyday life and in industry.
DEFINITION
Alkanes– saturated (aliphatic) hydrocarbons, the composition of which is expressed by the formula C n H 2 n +2.
Alkanes form a homologous series, each chemical compound of which differs in composition from the next and previous ones by the same number of carbon and hydrogen atoms - CH 2, and the substances included in the homologous series are called homologues. The homologous series of alkanes is presented in Table 1.
Table 1. Homologous series of alkanes.
In alkane molecules, primary (i.e. connected by one bond), secondary (i.e. connected by two bonds), tertiary (i.e. connected by three bonds) and quaternary (i.e. connected by four bonds) carbon atoms are distinguished.
C 1 H3 – C 2 H 2 – C 1 H 3 (1 – primary, 2 – secondary carbon atoms)
CH 3 –C 3 H(CH 3) – CH 3 (3-tertiary carbon atom)
CH 3 – C 4 (CH 3) 3 – CH 3 (4-quaternary carbon atom)
Alkanes are characterized by structural isomerism (carbon skeleton isomerism). Thus, pentane has the following isomers:
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 (pentane)
CH 3 –CH(CH 3)-CH 2 -CH 3 (2-methylbutane)
CH 3 -C(CH 3) 2 -CH 3 (2,2 – dimethylpropane)
Alkanes, starting with heptane, are characterized by optical isomerism.
The carbon atoms in saturated hydrocarbons are in sp 3 hybridization. The angles between bonds in alkane molecules are 109.5.
Chemical properties of alkanes
Under normal conditions, alkanes are chemically inert - they do not react with either acids or alkalis. This is explained by the high strength of C-C and C-H bonds. Non-polar C-C and C-H bonds can only be cleaved homolytically under the influence of active free radicals. Therefore, alkanes enter into reactions that proceed by the radical substitution mechanism. In radical reactions, hydrogen atoms are first replaced at tertiary carbon atoms, then at secondary and primary carbon atoms.
Radical substitution reactions have a chain nature. The main stages: nucleation (initiation) of the chain (1) - occurs under the influence of UV radiation and leads to the formation of free radicals, chain growth (2) - occurs due to the abstraction of a hydrogen atom from the alkane molecule; chain termination (3) – occurs when two identical or different radicals collide.
X:X → 2X . (1)
R:H+X . → HX + R . (2)
R . + X:X → R:X + X . (2)
R . + R . → R:R (3)
R . +X . → R:X (3)
X . +X . → X:X (3)
Halogenation. When alkanes interact with 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 3 Cl +Cl 2 = CH 2 Cl 2 + HCl (dichloromethane)
CH 2 Cl 2 + Cl 2 = CHCl 3 + HCl (trichloromethane)
CHCl 3 +Cl 2 = CCl 4 + HCl (carbon tetrachloride)
Nitration (Konovalov reaction). When dilute nitric acid acts on alkanes at 140C and low pressure, a radical reaction occurs:
CH 3 -CH 3 +HNO 3 = CH 3 -CH 2 -NO 2 (nitroethane) + H 2 O
Sulfochlorination and sulfoxidation. Direct sulfonation of alkanes is difficult and is most often accompanied by oxidation, resulting in the formation of alkanesulfonyl chlorides:
R-H + SO 2 + Cl 2 → R-SO 3 Cl + HCl
The sulfonic oxidation reaction proceeds similarly, only in this case alkanesulfonic acids are formed:
R-H + SO 2 + ½ O 2 → R-SO 3 H
Cracking– radical cleavage of C-C bonds. Occurs when heated and in the presence of catalysts. When higher alkanes are cracked, alkenes are formed; when methane and ethane are cracked, acetylene is formed:
C 8 H 18 = C 4 H 10 (butane) + C 3 H 8 (propane)
2CH 4 = C 2 H 2 (acetylene) + 3H 2
Oxidation. The mild oxidation of methane with atmospheric oxygen can produce methanol, formic aldehyde or formic acid. In air, alkanes burn to carbon dioxide and water:
C n H 2 n +2 + (3n+1)/2 O 2 = nCO 2 + (n+1)H 2 O
Physical properties of alkanes
Under normal conditions, C 1 -C 4 are gases, C 5 -C 17 are liquids, and starting from C 18 are solids. Alkanes are practically insoluble in water, but are highly soluble in non-polar solvents, such as benzene. Thus, methane CH 4 (swamp, mine gas) is a colorless and odorless gas, highly soluble in ethanol, ether, hydrocarbons, but poorly soluble in water. Methane is used as a high-calorie fuel in natural gas, as a raw material for the production of hydrogen, acetylene, chloroform and other organic substances on an industrial scale.
Propane C 3 H 8 and butane C 4 H 10 are gases used in everyday life as bottled gases due to their easy liquefaction. Propane is used as a car fuel because it is more environmentally friendly than gasoline. Butane is the raw material for the production of 1,3-butadiene, which is used in the production of synthetic rubber.
Preparation of alkanes
Alkanes are obtained from natural sources - natural gas (80-90% - methane, 2-3% - ethane and other saturated hydrocarbons), coal, peat, wood, oil and rock wax.
There are laboratory and industrial methods for producing alkanes. In industry, alkanes are obtained from bituminous coal (1) or by the Fischer-Tropsch reaction (2):
nC + (n+1)H 2 = C n H 2 n +2 (1)
nCO + (2n+1)H 2 = C n H 2 n +2 + H 2 O (2)
Laboratory methods for producing alkanes include: hydrogenation of unsaturated hydrocarbons by heating and in the presence of catalysts (Ni, Pt, Pd) (1), the interaction of water with organometallic compounds (2), electrolysis of carboxylic acids (3), by decarboxylation reactions (4) and Wurtz (5) and in other ways.
R 1 -C≡C-R 2 (alkyne) → R 1 -CH = CH-R 2 (alkene) → R 1 -CH 2 – CH 2 -R 2 (alkane) (1)
R-Cl + Mg → R-Mg-Cl + H 2 O → R-H (alkane) + Mg(OH)Cl (2)
CH 3 COONa↔ CH 3 COO — + Na +
2CH 3 COO - → 2CO 2 + C 2 H 6 (ethane) (3)
CH 3 COONa + NaOH → CH 4 + Na 2 CO 3 (4)
R 1 -Cl +2Na +Cl-R 2 →2NaCl + R 1 -R 2 (5)
Examples of problem solving
EXAMPLE 1
| Exercise | Determine the mass of chlorine required for the first stage chlorination of 11.2 liters of methane. |
| Solution | Let us write the reaction equation for the first stage of methane chlorination (i.e., in the halogenation reaction, only one hydrogen atom is replaced, resulting in the formation of a monochlorine derivative): CH 4 + Cl 2 = CH 3 Cl + HCl (methane chloride) Let's find the amount of methane substance: v(CH 4) = V(CH 4)/V m v(CH 4) = 11.2/22.4 = 0.5 mol According to the reaction equation, the number of moles of chlorine and the number of moles of methane are equal to 1 mol, therefore, the practical number of moles of chlorine and methane will also be the same and will be equal to: v(Cl 2) = v(CH 4) = 0.5 mol Knowing the amount of chlorine substance, you can find its mass (which is what is posed in the problem question). The mass of chlorine is calculated as the product of the amount of chlorine substance and its molar mass (molecular mass of 1 mole of chlorine; molecular mass is calculated using the table of chemical elements by D.I. Mendeleev). The mass of chlorine will be equal to: m(Cl 2) = v(Cl 2)×M(Cl 2) m(Cl 2) = 0.5 × 71 = 35.5 g |
| Answer | The mass of chlorine is 35.5 g |
DEFINITION
Alkanes are called saturated hydrocarbons, the molecules of which consist of carbon and hydrogen atoms connected to each other only by σ bonds.
Under normal conditions (at 25 o C and atmospheric pressure), the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes. The structure of the alkane molecule using methane as an example is shown in Fig. 1.
Rice. 1. The structure of the methane molecule.
Alkanes are practically insoluble in water, since their molecules are low-polar and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, diethyl ether, etc.
Preparation of alkanes
The main sources of various saturated hydrocarbons containing up to 40 carbon atoms are oil and natural gas. Alkanes with a small number of carbon atoms (1 - 10) can be isolated by fractional distillation of natural gas or the gasoline fraction of oil.
There are industrial (I) and laboratory (II) methods for producing alkanes.
C + H 2 → CH 4 (kat = Ni, t 0);
CO + 3H 2 → CH 4 + H 2 O (kat = Ni, t 0 = 200 - 300);
CO 2 + 4H 2 → CH 4 + 2H 2 O (kat, t 0).
— hydrogenation of unsaturated hydrocarbons
CH 3 -CH=CH 2 + H 2 →CH 3 -CH 2 -CH 3 (kat = Ni, t 0);
- reduction of haloalkanes
C 2 H 5 I + HI →C 2 H 6 + I 2 (t 0);
- alkaline melting reactions of salts of monobasic organic acids
C 2 H 5 -COONa + NaOH → C 2 H 6 + Na 2 CO 3 (t 0);
— interaction of haloalkanes with sodium metal (Wurtz reaction)
2C 2 H 5 Br + 2Na → CH 3 -CH 2 -CH 2 -CH 3 + 2NaBr;
— electrolysis of salts of monobasic organic acids
2C 2 H 5 COONa + 2H 2 O → H 2 + 2NaOH + C 4 H 10 + 2CO 2 ;
K(-): 2H 2 O + 2e → H 2 + 2OH - ;
A(+):2C 2 H 5 COO — -2e → 2C 2 H 5 COO + → 2C 2 H 5 + + 2CO 2 .
Chemical properties of alkanes
Alkanes are among the least reactive organic compounds, which is explained by their structure.
Alkanes under normal conditions do not react with concentrated acids, molten and concentrated alkalis, alkali metals, halogens (except fluorine), potassium permanganate and potassium dichromate in an acidic environment.
For alkanes, the most typical reactions are those that proceed by a radical mechanism. Homolytic cleavage of C-H and C-C bonds is energetically more favorable than their heterolytic cleavage.
Radical substitution reactions most easily occur at the tertiary carbon atom, then at the secondary carbon atom, and lastly at the primary carbon atom.
All chemical transformations of alkanes proceed with splitting:
1) C-H bonds
— halogenation (S R)
CH 4 + Cl 2 → CH 3 Cl + HCl ( hv);
CH 3 -CH 2 -CH 3 + Br 2 → CH 3 -CHBr-CH 3 + HBr ( hv).
- nitration (S R)
CH 3 -C(CH 3)H-CH 3 + HONO 2 (dilute) → CH 3 -C(NO 2)H-CH 3 + H 2 O (t 0).
— sulfochlorination (S R)
R-H + SO 2 + Cl 2 → RSO 2 Cl + HCl ( hv).
- dehydrogenation
CH 3 -CH 3 → CH 2 =CH 2 + H 2 (kat = Ni, t 0).
- dehydrocyclization
CH 3 (CH 2) 4 CH 3 → C 6 H 6 + 4H 2 (kat = Cr 2 O 3, t 0).
2) C-H and C-C bonds
- isomerization (intramolecular rearrangement)
CH 3 -CH 2 -CH 2 -CH 3 →CH 3 -C(CH 3)H-CH 3 (kat=AlCl 3, t 0).
- oxidation
2CH 3 -CH 2 -CH 2 -CH 3 + 5O 2 → 4CH 3 COOH + 2H 2 O (t 0 , p);
C n H 2n+2 + (1.5n + 0.5) O 2 → nCO 2 + (n+1) H 2 O (t 0).
Applications of alkanes
Alkanes have found application in various industries. Let us consider in more detail, using the example of some representatives of the homologous series, as well as mixtures of alkanes.
Methane forms the raw material basis for the most important chemical industrial processes for the production of carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids. Propane is used as automobile fuel. Butane is used to produce butadiene, which is a raw material for the production of synthetic rubber.
A mixture of liquid and solid alkanes up to C 25, called Vaseline, is used in medicine as the basis of ointments. A mixture of solid alkanes C 18 - C 25 (paraffin) is used to impregnate various materials (paper, fabrics, wood) to give them hydrophobic properties, i.e. non-wetting with water. In medicine it is used for physiotherapeutic procedures (paraffin treatment).
Examples of problem solving
EXAMPLE 1
| Exercise | When chlorinating methane, 1.54 g of a compound was obtained, the vapor density of which in air is 5.31. Calculate the mass of manganese dioxide MnO 2 that will be required to produce chlorine if the ratio of the volumes of methane and chlorine introduced into the reaction is 1:2. |
| Solution | The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure is called the relative density of the first gas to the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular mass of air” is used conditionally, since air is a mixture of gases. Let's find the molar mass of the gas formed during the chlorination of methane: M gas = 29 ×D air (gas) = 29 × 5.31 = 154 g/mol. This is carbon tetrachloride - CCl 4. Let's write the reaction equation and arrange the stoichiometric coefficients: CH 4 + 4Cl 2 = CCl 4 + 4HCl. Let's calculate the amount of carbon tetrachloride substance: n(CCl 4) = m(CCl 4) / M(CCl 4); n(CCl 4) = 1.54 / 154 = 0.01 mol. According to the reaction equation n(CCl 4) : n(CH 4) = 1: 1, which means n(CH 4) = n(CCl 4) = 0.01 mol. Then, the amount of chlorine substance should be equal to n(Cl 2) = 2 × 4 n(CH 4), i.e. n(Cl 2) = 8 × 0.01 = 0.08 mol. Let us write the reaction equation for the production of chlorine: MnO 2 + 4HCl = MnCl 2 + Cl 2 + 2H 2 O. The number of moles of manganese dioxide is 0.08 mol, because n(Cl 2) : n(MnO 2) = 1: 1. Find the mass of manganese dioxide: m(MnO 2) = n(MnO 2) × M(MnO 2); M(MnO 2) = Ar(Mn) + 2×Ar(O) = 55 + 2×16 = 87 g/mol; m(MnO 2) = 0.08 × 87 = 10.4 g. |
| Answer | The mass of manganese dioxide is 10.4 g. |
EXAMPLE 2
| Exercise | Determine the molecular formula of trichloroalkane, the mass fraction of chlorine in which is 72.20%. Draw up the structural formulas of all possible isomers and give the names of the substances according to the IUPAC substitutive nomenclature. |
| Answer | Let's write down the general formula of trichloroalkean: C n H 2 n -1 Cl 3 . According to the formula ω(Cl) = 3×Ar(Cl) / Mr(C n H 2 n -1 Cl 3) × 100% Let's calculate the molecular weight of trichloroalkane: Mr(C n H 2 n -1 Cl 3) = 3 × 35.5 / 72.20 × 100% = 147.5. Let's find the value of n: 12n + 2n - 1 + 35.5×3 = 147.5; Therefore, the formula of trichloroalkane is C 3 H 5 Cl 3. Let's compose the structural formulas of the isomers: 1,2,3-trichloropropane (1), 1,1,2-trichloropropane (2), 1,1,3-trichloropropane (3), 1,1,1-trichloropropane (4) and 1 ,2,2-trichloropropane (5). CH 2 Cl-CHCl-CH 2 Cl (1); CHCl 2 -CHCl-CH 3 (2); CHCl 2 -CH 2 -CH 2 Cl (3); CCl 3 -CH 2 -CH 3 (4); Alkanes are saturated hydrocarbons. In their molecules, the atoms have single bonds. The structure is determined by the formula CnH2n+2. Let's consider alkanes: chemical properties, types, applications. Connection structureIn the structure of carbon, there are four orbits in which the atoms rotate. Orbitals have the same shape and energy.
A single carbon bond allows the alkane molecules to rotate freely, causing the structures to take on different shapes, forming vertices at the carbon atoms. All alkane compounds are divided into two main groups:
Types of alkanesThere are several types of alkane compounds, each of which has its own formula, structure, chemical properties and alkyl substituent. The table contains a homological series
Name of alkanes The general formula of saturated hydrocarbons is CnH2n+2. By changing the value of n, a compound with a simple interatomic bond is obtained. Useful video: alkanes - molecular structure, physical propertiesTypes of alkanes, reaction optionsUnder natural conditions, alkanes are chemically inert compounds. Hydrocarbons do not react to contact with nitric and sulfuric acid concentrate, alkali and potassium permanganate. Single molecular bonds determine the reactions characteristic of alkanes. Alkane chains are characterized by nonpolar and weakly polarizable bonds. It is slightly longer than S-N.
General formula of alkanes Substitution reactionParaffin substances are characterized by insignificant chemical activity. This is explained by the increased strength of the chain connection, which is not easy to break. For destruction, a homological mechanism is used, in which free radicals take part. For alkanes, substitution reactions are more natural. They do not react to water molecules and charged ions. During substitution, hydrogen particles are replaced by halogen and other active elements. Among such processes are halogenation, nitridation and sulfochlorination. Such reactions are used to form alkane derivatives. Free radical replacement occurs in three main stages:
HalogenationThe process is carried out according to the radical type. Halogenation occurs under the influence of ultraviolet radiation and thermal heating of the hydrocarbon and halogen mixture. The whole process follows Markovnikov's rule. Its essence lies in the fact that the hydrogen atom belonging to the hydrogenated carbon is the first to undergo halogenation. The process begins with a tertiary atom and ends with a primary carbon. SulfochlorinationAnother name is the Reed reaction. It is carried out by the method of free radical substitution. Thus, alkanes react to the combination of sulfur dioxide and chlorine under the influence of ultraviolet radiation. The reaction begins with the activation of a chain mechanism. At this time, two radicals are released from chlorine. The action of one is directed towards the alkane, resulting in the formation of a hydrogen chloride molecule and an alkyl element. Another radical combines with sulfur dioxide, creating a complex combination. To achieve equilibrium, one chlorine atom is removed from another molecule. The result is alkane sulfonyl chloride. This substance is used to produce surfactants.
Sulfochlorination NitrationThe nitration process involves the combination of saturated carbons with gaseous tetravalent nitrogen oxide and nitric acid, brought to a 10% solution. The reaction will require a low level of pressure and high temperature, approximately 104 degrees. As a result of nitration, nitroalkanes are obtained. Splitting offDehydrogenation reactions are carried out by separating atoms. The molecular particle of methane completely decomposes under the influence of temperature. DehydrogenationIf a hydrogen atom is separated from the carbon lattice of paraffin (except methane), unsaturated compounds are formed. These reactions are carried out under conditions of significant temperature conditions (400-600 degrees). Various metal catalysts are also used. Alkanes are obtained by hydrogenation of unsaturated hydrocarbons. Decomposition processUnder the influence of temperatures during alkane reactions, molecular bonds can be broken and active radicals can be released. These processes are known as pyrolysis and cracking. When the reaction component is heated to 500 degrees, the molecules begin to decompose, and in their place complex radical alkyl mixtures are formed. Alkanes and alkenes are prepared industrially in this way. OxidationThese are chemical reactions based on the donation of electrons. Paraffins are characterized by auto-oxidation. The process uses the oxidation of saturated hydrocarbons by free radicals. Alkane compounds in the liquid state are converted into hydroperoxide. First, paraffin reacts with oxygen. Active radicals are formed. Then the alkyl species reacts with a second oxygen molecule. A peroxide radical is formed, which subsequently interacts with the alkane molecule. As a result of the process, hydroperoxide is released.
Alkanes oxidation reaction Applications of alkanesCarbon compounds are widely used in almost all major areas of human life. Some types of compounds are indispensable for certain industries and the comfortable existence of modern man. Gaseous alkanes are the basis of valuable fuels. The main component of most gases is methane. Methane has the ability to create and release large amounts of heat. Therefore, it is used in significant quantities in industry and for domestic consumption. By mixing butane and propane, a good household fuel is obtained. Methane is used in the production of the following products:
Application of methane Liquid hydrocarbons are intended to create fuel for engines and rockets, and solvents. Higher hydrocarbons, where the number of carbon atoms exceeds 20, are involved in the production of lubricants, paints and varnishes, soaps and detergents. A combination of fatty hydrocarbons with less than 15 H atoms is vaseline oil. This tasteless, transparent liquid is used in cosmetics, in the creation of perfumes, and for medical purposes. Vaseline is the result of a combination of solid and fatty alkanes with less than 25 carbon atoms. The substance is involved in the creation of medical ointments. Paraffin, obtained by combining solid alkanes, is a solid, tasteless mass, white in color and without aroma. The substance is used to make candles, an impregnating substance for wrapping paper and matches. Paraffin is also popular for thermal procedures in cosmetology and medicine.
Halogenated alkane compounds function as solvents, refrigerants, and also as the main substance for further synthesis. Useful video: alkanes - chemical propertiesConclusionAlkanes are acyclic hydrocarbon compounds with a linear or branched structure. A single bond is established between the atoms, which cannot be broken. Reactions of alkanes based on the substitution of molecules characteristic of this type of compound. The homologous series has the general structural formula CnH2n+2. Hydrocarbons belong to the saturated class because they contain the maximum permissible number of hydrogen atoms. In contact with Saturated hydrocarbons are compounds that are molecules consisting of carbon atoms in a state of sp 3 hybridization. They are connected to each other exclusively by covalent sigma bonds. The name "saturated" or "saturated" hydrocarbons comes from the fact that these compounds do not have the ability to attach any atoms. They are extreme, completely saturated. The exception is cycloalkanes. What are alkanes?Alkanes are saturated hydrocarbons, and their carbon chain is open and consists of carbon atoms connected to each other using single bonds. It does not contain other (that is, double, like alkenes, or triple, like alkyls) bonds. Alkanes are also called paraffins. They received this name because well-known paraffins are a mixture of predominantly these saturated hydrocarbons C 18 -C 35 with particular inertness. General information about alkanes and their radicalsTheir formula: C n P 2 n +2, here n is greater than or equal to 1. The molar mass is calculated using the formula: M = 14n + 2. Characteristic feature: the endings in their names are “-an”. The residues of their molecules, which are formed as a result of the replacement of hydrogen atoms with other atoms, are called aliphatic radicals, or alkyls. They are designated by the letter R. The general formula of monovalent aliphatic radicals: C n P 2 n +1, here n is greater than or equal to 1. The molar mass of aliphatic radicals is calculated by the formula: M = 14n + 1. A characteristic feature of aliphatic radicals: endings in the names “- silt." Alkane molecules have their own structural features:
Alkanes begin the homologous series: methane, ethane, propane, butane, and so on. Physical properties of alkanesAlkanes are substances that are colorless and insoluble in water. The temperature at which alkanes begin to melt and the temperature at which they boil increase in accordance with the increase in molecular weight and hydrocarbon chain length. From less branched to more branched alkanes, the boiling and melting points decrease. Gaseous alkanes can burn with a pale blue or colorless flame and produce quite a lot of heat. CH 4 -C 4 H 10 are gases that also have no odor. C 5 H 12 -C 15 H 32 are liquids that have a specific odor. C 15 H 32 and so on are solids that are also odorless. Chemical properties of alkanesThese compounds are chemically inactive, which can be explained by the strength of difficult-to-break sigma bonds - C-C and C-H. It is also worth considering that C-C bonds are non-polar, and C-H bonds are low-polar. These are low-polarized types of bonds belonging to the sigma type and, accordingly, they are most likely to be broken by a homolytic mechanism, as a result of which radicals will be formed. Thus, the chemical properties of alkanes are mainly limited to radical substitution reactions.
Nitration reactionsAlkanes react only with nitric acid with a concentration of 10% or with tetravalent nitrogen oxide in a gaseous environment at a temperature of 140°C. The nitration reaction of alkanes is called the Konovalov reaction. As a result, nitro compounds and water are formed: CH 4 + nitric acid (diluted) = CH 3 - NO 2 (nitromethane) + water. Combustion reactionsSaturated hydrocarbons are very often used as fuel, which is justified by their ability to burn: C n P 2n+2 + ((3n+1)/2) O 2 = (n+1) H 2 O + n CO 2. Oxidation reactionsThe chemical properties of alkanes also include their ability to oxidize. Depending on what conditions accompany the reaction and how they are changed, different end products can be obtained from the same substance. Mild oxidation of methane with oxygen in the presence of a catalyst accelerating the reaction and a temperature of about 200 ° C can result in the following substances: 1) 2CH 4 (oxidation with oxygen) = 2CH 3 OH (alcohol - methanol). 2) CH 4 (oxidation with oxygen) = CH 2 O (aldehyde - methanal or formaldehyde) + H 2 O. 3) 2CH 4 (oxidation with oxygen) = 2HCOOH (carboxylic acid - methane or formic) + 2H 2 O.
Also, the oxidation of alkanes can be carried out in a gaseous or liquid medium with air. Such reactions lead to the formation of higher fatty alcohols and corresponding acids. Relation to heatAt temperatures not exceeding +150-250°C, always in the presence of a catalyst, a structural rearrangement of organic substances occurs, which consists of a change in the order of connection of atoms. This process is called isomerization, and the substances resulting from the reaction are called isomers. Thus, from normal butane, its isomer is obtained - isobutane. At temperatures of 300-600°C and the presence of a catalyst, C-H bonds are broken with the formation of hydrogen molecules (dehydrogenation reactions), hydrogen molecules with the closure of the carbon chain into a cycle (cyclization or aromatization reactions of alkanes): 1) 2CH 4 = C 2 H 4 (ethene) + 2H 2. 2) 2CH 4 = C 2 H 2 (ethyne) + 3H 2. 3) C 7 H 16 (normal heptane) = C 6 H 5 - CH 3 (toluene) + 4 H 2.
Halogenation reactionsSuch reactions involve the introduction of halogens (their atoms) into the molecule of an organic substance, resulting in the formation of a C-halogen bond. When alkanes react with halogens, halogen derivatives are formed. This reaction has specific features. It proceeds according to a radical mechanism, and in order to initiate it, it is necessary to expose the mixture of halogens and alkanes to ultraviolet radiation or simply heat it. The properties of alkanes allow the halogenation reaction to proceed until complete replacement with halogen atoms is achieved. That is, the chlorination of methane will not end in one stage and the production of methyl chloride. The reaction will go further, all possible substitution products will be formed, starting with chloromethane and ending with carbon tetrachloride. Exposure of other alkanes to chlorine under these conditions will result in the formation of various products resulting from the substitution of hydrogen at different carbon atoms. The temperature at which the reaction occurs will determine the ratio of the final products and the rate of their formation. The longer the hydrocarbon chain of the alkane, the easier the reaction will be. During halogenation, the least hydrogenated (tertiary) carbon atom will be replaced first. The primary one will react after all the others. The halogenation reaction will occur in stages. In the first stage, only one hydrogen atom is replaced. Alkanes do not interact with halogen solutions (chlorine and bromine water).
Sulfochlorination reactionsThe chemical properties of alkanes are also complemented by the sulfochlorination reaction (called the Reed reaction). When exposed to ultraviolet radiation, alkanes are able to react with a mixture of chlorine and sulfur dioxide. As a result, hydrogen chloride is formed, as well as an alkyl radical, which adds sulfur dioxide. The result is a complex compound that becomes stable due to the capture of a chlorine atom and the destruction of its next molecule: R-H + SO 2 + Cl 2 + ultraviolet radiation = R-SO 2 Cl + HCl. The sulfonyl chlorides formed as a result of the reaction are widely used in the production of surfactants. |
