Alkanes are saturated hydrocarbons. In their molecules, atoms have single bonds. The structure is determined by the formula CnH2n+2. Consider alkanes: Chemical properties, types, application.
In the structure of carbon, there are four orbits along which atoms rotate. Orbitals have the same shape, energy.
Note! The angles between them are 109 degrees and 28 minutes, they are directed to the vertices of the tetrahedron.
A simple carbon bond allows alkane molecules to rotate freely, as a result of which the structures take on various forms, forming vertices at carbon atoms.
All alkane compounds are divided into two main groups:
- Hydrocarbons of an aliphatic compound. Such structures have a linear connection. The general formula looks like this: CnH2n+2. The value of n is equal to or more than one, means the number of carbon atoms.
- Cycloalkanes of cyclic structure. The chemical properties of cyclic alkanes differ significantly from those of linear compounds. The formula of cycloalkanes to some extent makes them similar to hydrocarbons that have a triple atomic bond, that is, to alkynes.
Types of alkanes
There are several types of alkane compounds, each of which has its own formula, structure, chemical properties and alkyl substituent. Table contains homologous series
Name of alkanes
The general formula for 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 properties
Varieties of alkanes, reaction options
Under natural conditions, alkanes are chemically inert compounds. Hydrocarbons do not react to contact with a concentrate of nitric and sulfuric acid, alkali and potassium permanganate.
Single molecular bonds determine the reactions characteristic of alkanes. Alkane chains are characterized by a non-polar and weakly polarizable bond. It is somewhat longer than S-N.
General formula of alkanes
substitution reaction
Paraffin substances differ in insignificant chemical activity. This is explained by the increased strength of the chain bond, 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 these processes are halogenation, nitration and sulfochlorination. Such reactions are used to form alkane derivatives.
Free radical substitution occurs in three main steps:
- The appearance of a chain on the basis of which free radicals are created. Heating and ultraviolet light are used as catalysts.
- The development of a chain in the structure of which interactions of active and inactive particles take place. This is how molecules and radical particles are formed.
- At the end, the chain is terminated. Active elements create new combinations or disappear altogether. The chain reaction ends.
Halogenation
The process is radical. Halogenation occurs under the influence of ultraviolet radiation and thermal heating of the hydrocarbon and halogen mixture.
The whole process occurs according to Markovnikov's rule. Its essence lies in the fact that the hydrogen atom belonging to hydrogenated carbon is the first to be halogenated. The process starts with a tertiary atom and ends with primary carbon.
Sulfochlorination
Another name is the Reed reaction. It is carried out by the method of free radical substitution. Thus, alkanes react to the action of a combination of sulfur dioxide and chlorine under the influence of ultraviolet radiation.
The reaction begins with the activation of the chain mechanism. At this time, two radicals are released from chlorine. The action of one is directed to the alkane, resulting in the formation of a molecule of hydrogen chloride and an alkyl element. Another radical combines with sulfur dioxide, creating a complex combination. For equilibrium, one chlorine atom is taken from another molecule. The result is an alkane sulfonyl chloride. This substance is used to produce surface-active components.
Sulfochlorination
Nitration
The nitration process involves the combination of saturated carbons with gaseous tetravalent nitrogen oxide and nitric acid, brought to a 10% solution. For the reaction to take place, low level pressure and high temperature, about 104 degrees. As a result of nitration, nitroalkanes are obtained.
splitting off
By separating the atoms, dehydrogenation reactions are carried out. The molecular particle of methane completely decomposes under the influence of temperature.
Dehydrogenation
If 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 produced by hydrogenation saturated hydrocarbons.
decomposition process
Under the influence of temperatures during alkane reactions, ruptures of molecular bonds and the release of active radicals can occur. These processes are known as pyrolysis and cracking.
When the reaction component is heated to 500 degrees, the molecules begin to decompose, and complex radical alkyl mixtures are formed in their place. In this way, alkanes and alkenes are obtained in industry.
Oxidation
This chemical reactions based on electron donation. Paraffins are characterized by autoxidation. The process uses the oxidation of saturated hydrocarbons by free radicals. Alkane compounds in the liquid state are converted to hydroperoxide. First, the paraffin reacts with oxygen. Active radicals are formed. Then the alkyl particle 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.
Alkane oxidation reaction
Application of alkanes
Carbon compounds have wide application in almost every major area of human life. Some of the types of compounds are indispensable for certain industries and the comfortable existence of modern man.
Gaseous alkanes are the basis of valuable fuel. 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 volumes in industry, for consumption at home. When mixing butane and propane, a good household fuel is obtained.
Methane is used in the production of such products:
- methanol;
- solvents;
- freon;
- ink;
- fuel;
- synthesis gas;
- acetylene;
- formaldehyde;
- formic acid;
- plastic.
Methane application
Liquid hydrocarbons are designed to create fuel for engines and rockets, 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 paraffin oil. This tasteless transparent liquid is used in cosmetics, in the creation of perfumes, and for medical purposes.
Vaseline is the result of the 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 color and without fragrance. The substance is used to produce candles, an impregnating substance for wrapping paper and matches. Paraffin is also popular in the implementation of thermal procedures in cosmetology and medicine.
Note! Synthetic fibers, plastics, detergent chemicals and rubber are also made from alkane mixtures.
Halogenated alkane compounds act as solvents, refrigerants, and also as the main substance for further synthesis.
Useful video: alkanes - chemical properties
Output
Alkanes are acyclic hydrocarbon compounds with a linear or branched structure. A single bond is established between the atoms, which is indestructible. Reactions of alkanes based on the substitution of molecules, characteristic of this type of compounds. The homologous series has the general structural formula CnH2n+2. Hydrocarbons belong to the saturated class because they contain the maximum allowable number of hydrogen atoms.
In contact with
ALKANE (saturated hydrocarbons, paraffins)
- Alkanes are aliphatic (acyclic) saturated hydrocarbons in which carbon atoms are linked together by simple (single) bonds into unbranched or branched chains.
Alkanes- the name of saturated hydrocarbons according to the international nomenclature.
Paraffins- a historically established name reflecting the properties of these compounds (from lat. parrum affinis- having little affinity, inactive).
limiting, or rich, these hydrocarbons are named in connection with the complete saturation of the carbon chain with hydrogen atoms.
The simplest representatives of alkanes:
Molecule Models:
When comparing these compounds, it is clear that they differ from each other by a group -CH 2 - (methylene). Adding another group to propane -CH 2 -, we get butane C 4 H 10, then alkanes C 5 H 12, C 6 H 14 etc.
Now you can derive the general formula for alkanes. The number of carbon atoms in the series of alkanes will be taken as n
, then the number of hydrogen atoms will be 2n+2
. Therefore, the composition of alkanes corresponds to the general formula C n H 2n+2.
Therefore, the following definition is often used:
Alkanes- hydrocarbons, the composition of which is expressed by the general formula C n H 2n+2, where n is the number of carbon atoms.
The structure of alkanes
Chemical structure(the order of connection of atoms in molecules) of the simplest alkanes - methane, ethane and propane - show their structural formulas given in section 2. From these formulas it can be seen that there are two types of chemical bonds in alkanes:
S–S And S–N.
The C–C bond is covalent nonpolar. The C–H bond is covalent, weakly polar, because carbon and hydrogen are close in electronegativity (2.5 for carbon and 2.1 for hydrogen). The formation of covalent bonds in alkanes due to the common electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:
Electronic and structural formulas reflect chemical structure, but give no idea of spatial structure of molecules, which significantly affects the properties of the substance.
Spatial structure, i.e. mutual arrangement atoms of a molecule in space depends on the direction of the atomic orbitals (AO) of these atoms. in hydrocarbons leading role the spatial orientation of the atomic orbitals of carbon plays a role, since the spherical 1s-AO of the hydrogen atom is devoid of a definite directionality.
Spatial arrangement The AO of carbon, in turn, depends on the type of its hybridization (Part I, Section 4.3). The saturated carbon atom in alkanes is bonded to four other atoms. Therefore, its state corresponds to sp 3 hybridization (Part I, Section 4.3.1). In this case, each of the four sp 3 -hybrid carbon AOs participates in axial (σ-) overlap with the s-AO of hydrogen or with the sp 3 -AO of another carbon atom, forming σ - C-H connections or S-S.
Four σ-bonds of carbon are directed in space at an angle of 109 about 28 ", which corresponds to the smallest repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH 4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices - hydrogen atoms:
Valence angle H-C-H equals 109 o 28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.
For recording, it is convenient to use the spatial (stereochemical) formula.
In the molecule of the next homologue - ethane C 2 H 6 - two tetrahedral sp 3 carbon atoms form a more complex spatial structure:
Alkanes containing more than 2 carbon atoms are characterized by curved shapes. This can be shown with an example n-butane (VRML model) or n-pentane:
Isomerism of alkanes
- Isomerism is the phenomenon of the existence of compounds that have the same composition (the same molecular formula), but a different structure. Such connections are called isomers.
Differences in the order of connection of atoms in molecules (i.e. in the chemical structure) lead to structural isomerism. The structure of structural isomers is reflected by structural formulas. In the alkane series structural isomerism manifests itself when the chain contains 4 or more carbon atoms, i.e. starting with butane C 4 H 10 .
If in molecules the same composition and the same chemical structure, a different mutual arrangement of atoms in space is possible, then there is spatial isomerism (stereoisomerism). In this case, the use of structural formulas is not enough, and molecular models or special formulas - stereochemical (spatial) or projection - should be used.
Alkanes, starting from ethane H 3 C–CH 3, exist in various spatial forms ( conformations) caused by intramolecular rotation along the C–C σ-bonds and exhibit the so-called rotational (conformational) isomerism.
In addition, if there is a carbon atom in the molecule associated with 4 different substituents, another type of spatial isomerism is possible, when two stereoisomers relate to each other as an object and its mirror image (similar to how left hand refers to the right). Such differences in the structure of molecules are called optical isomerism.
Structural isomerism of alkanes
- Structural isomers - compounds of the same composition, differing in the order of binding atoms, i.e. chemical structure molecules.
The reason for the manifestation of structural isomerism in the alkane series is the ability of carbon atoms to form chains of various structures. This type of structural isomerism is called isomerism of the carbon skeleton.
For example, an alkane of composition C 4 H 10 can exist in the form two structural isomers:
and alkane C 5 H 12 - in the form three structural isomers that differ in the structure of the carbon chain:
With an increase in the number of carbon atoms in the composition of molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms.
Structural isomers are different physical properties. Alkanes with a branched structure, due to a less dense packing of molecules and, accordingly, smaller intermolecular interactions, boil at a lower temperature than their unbranched isomers.
When deriving the structural formulas of isomers, the following methods are used.
Alkanes are saturated hydrocarbons, in the molecules of which all carbon atoms are occupied by hydrogen atoms through simple bonds. Therefore, the structural isomerism of alkanes is characteristic of the homologues of the methane series.
Isomerism of the carbon skeleton
Homologs with four or more carbon atoms are characterized by structural isomerism in terms of changes in the carbon skeleton. Methyl groups -CH 2 can attach to any carbon of the chain, forming new substances. The more carbon atoms in the chain, the more isomers homologues can form. The theoretical number of homologs is calculated mathematically.
Rice. 1. Approximate number of isomers of methane homologues.
In addition to methyl groups, long carbon chains can be attached to carbon atoms, forming complex branched substances.
Examples of isomerism of alkanes:
- normal butane or n-butane (CH 3 -CH 2 -CH 2 -CH 3) and 2-methylpropane (CH 3 -CH(CH 3) -CH 3);
- n-pentane (CH 3 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylbutane (CH 3 -CH 2 -CH (CH 3) -CH 3), 2,2-dimethylpropane (CH 3 -C (CH 3) 2 -CH 3);
- n-hexane (CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylpentane (CH 3 -CH (CH 3) -CH 2 -CH 2 -CH 3), 3-methylpentane ( CH 3 -CH 2 -CH (CH 3) -CH 2 -CH 3), 2,3-dimethylbutane (CH 3 -CH (CH 3) -CH (CH 3) -CH 3), 2,2-dimethylbutane ( CH 3 -C(CH 3) 2 -CH 2 -CH 3).
Rice. 2. Examples of structural isomers.
Branched isomers differ from linear molecules in their physical properties. Branched alkanes melt and boil at lower temperatures than their linear counterparts.
Nomenclature
The international IUPAC nomenclature has established rules for naming branched chains. To name a structural isomer, one should:
- find the longest chain and name it;
- number the carbon atoms, starting from the end, where there are the most substituents;
- indicate the number of identical substituents with numerical prefixes;
- name substitutes.
The name consists of four parts, going one after another:
- numbers denoting chain atoms that have substituents;
- numerical prefixes;
- the name of the substitute;
- the name of the main circuit.
For example, in a CH 3 -CH (CH 3) -CH 2 -C (CH 3) 2 -CH 3 molecule, the main chain has five carbon atoms. So it's pentane. The right end has more branches, so the numbering of atoms starts from here. In this case, the second atom has two identical substituents, which is also reflected in the name. It turns out that this substance has the name 2,2,4-trimethylpentane.
Various substituents (methyl, ethyl, propyl) are listed alphabetically in the name: 4,4-dimethyl-3-ethylheptane, 3-methyl-3-ethyloctane.
Usually, numerical prefixes from two to four are used: di- (two), tri- (three), tetra- (four).
What have we learned?
Alkanes are characterized by structural isomerism. Structural isomers are common to all homologues, starting with butane. In structural isomerism, substituents are attached to carbon atoms in the carbon chain, forming complex branched chains. The name of the isomer consists of the names of the main chain, substituents, the verbal designation of the number of substituents, the digital designation of the carbon atoms to which the substituents are attached.
Alkanes are acyclic hydrocarbons of a linear or branched structure, containing only simple connections and forming a homologous series with the general formula C n H 2n+2 (CH 4 , C 2 C 6 , ...). Alkanes are also called paraffins. Each carbon atom in an alkane molecule has the maximum number of other atoms associated with it, that is, four, which is why such hydrocarbons are called saturated.
Connections
The electronic configuration of a carbon atom with an atomic number of 6, 1s 2 2s 2 2p 2, cannot form four bonds, but only two, therefore sp 3 hybridization took place here, that is, the redistribution of four electrons from two different energy levels for one. The formed bonds of carbon electrons (from the sp 3 orbital) and hydrogen (s orbital) form a very strong bond σ. Due to the strength of the bond, saturated hydrocarbons have low reactivity.
Geometry
The presence of four orbitals at the carbon atom creates the shape of a regular tetrahedron and all angles between the orbitals are 109 ° 28 ". The bond length between carbon and hydrogen atoms is 0.109 nm, between two carbon atoms - 0.154 nm.
Reactions
Atoms in alkane molecules are connected by a strong σ-bond. In reaction C-C connections and C-H are equally likely to break down to form a new compound, so a reaction always results in a complex mixture of products. Under normal conditions, alkanes do not react with acids, bases, or strong oxidizing agents.
When a bond is broken in alkanes, two scenarios are possible: bond breaking with the formation of two radicals, A: B → A + B. Such a break is called homolytic (homo - the same). In another case, a gap occurs with the formation of ions, when a common pair of electrons goes to one of the atoms: A: B → A + : B, such a gap is called heterolytic. Accordingly, the types of reaction of alkanes are called: homolytic and heterolytic reactions.
At the moment, two types of reactions of alkanes are known in which C-C bonds are not broken - these are halogenation and nitration. Below are examples of methane reactions.
Halogenation
The halogenation reaction takes place at a temperature of 300-400°C or under the influence of ultraviolet rays. During the reaction, haloalkanes are formed. Most often, reactions with chromium and bromine occur, reactions with fluorine are dangerous because of the possibility of an explosion, and the reaction does not work with iodine.
The halogenation process consists of three stages: initiation, chain growth and chain termination.
1. Initiation - homolytic splitting of a halogen into two radicals:
Cl 2 → 2Cl (exposure to light energy, hν)
2. Chain development - free radicals interact with molecules and two reactions are possible:
(1) Cl + CH 4 → HCl + CH 3
(2) Cl + CH 4 → CH 3 Cl + H
The energy of atomic hydrogen is much higher than that of the methyl radical CH 3 , so reaction (2) does not proceed.
3. Chain termination - radicals react with each other and form products:
Cl + Cl → Cl2
CH 3 + CH 3 → 2CH 3
CH 3 + Cl → CH 3 Cl
Combustion
The main use of alkanes is as a fuel, so the combustion reaction can be called the most popular for saturated hydrocarbons. In a combustion reaction, alkanes are converted to water and carbon dioxide. The combustion reaction is exothermic and requires a large number energy, such as a spark or fire. The general combustion reaction of alkanes:
R + O 2 → CO 2 + H 2 O + heat
2C n H 2n+2 + (3n+1)O 2 → 2nCO 2 + (2n+2)H 2 O + heat
Combustion reaction of methane
CH 4 + 2O 2 → CO 2 + 2H 2 O + 212 kcal / mol
Nitriding
At a temperature of 140 ° C, with increasing pressure, alkanes react with nitric acid, the hydrogen atom is replaced by the nitric acid residue NO 2, the reaction products are called nitro compounds:
CH 4 + HO-NO 2 → CH 3 -NO 2 + H 2 O (140°C, p)
Synthesis
Wurtz synthesis
In 1855, Adolf Wurtz discovered that the reaction of metallic sodium with a haloalkane produces a sodium salt:
2CH 3 I + 2Na → 2Na + I - + CH 3 CH 3
The haloalkane free radicals react with each other to form longer compounds. General Equation reactions looks like:
Recovery of haloalkyls
Most haloalkyls react with zinc and hydrogen cations (or Brønsted-Lowry acid) to form alkanes. In such a reaction, zinc is a reducing agent and allows you to replace the halogen with hydrogen:
2C 4 H 9 Br (2-bromobutane) + H + (acid) + Zn → 2C 4 H 10 (butane) + ZnBr 2
Grynyard reagents
Grignard reagents are organic compounds in which a metal-carbon bond is present. Such reagents are formed as a result of the reaction of haloalkyl with magnesium in a solution of diethyl ether:
R-X + Mg → RMgX (in diethyl ether solution)
The reaction also takes place with chlorides, bromides and alkyl iodides. In the process of hydrolysis, Grignard reagents are converted to alkanes:
CH 3 MgI + H 2 O → CH 4 + HO-Mg-I
C 2 H 5 MgBr + H 2 O → C 2 H 6 + HO-Mg-Br
Getting and using
Alkanes are obtained either by synthesis or from natural sources(natural gas, oil, coal). The use of saturated hydrocarbons is very extensive, alkanes are used as gas, gasoline, diesel and rocket fuel. Vaseline, solvents and paraffin are also the merit of alkanes.
Properties of alkanes
sp 3 hybridization makes alkanes the least polar of all organic compounds, which implies that they are poorly soluble in polar solutions, so the boiling and melting points will mainly depend only on the molecular weight, on average, the boiling point of saturated hydrocarbons increases by 25-30 degrees for each carbon atom after pentane. Branched alkanes have more low temperature boiling, since more branched molecules have a smaller surface area, so intermolecular bonds are weaker and boil earlier.
The viscosity of a substance depends on the size of the molecule, so the more carbon atoms in the molecule, the larger it is and the more likely the interaction of molecules and, as a result, the greater the viscosity. Alkanes with carbon numbers from 20 to 35 are the main component for lubricants.
Nomenclature
The name of alkanes consists of two parts: the prefix indicates the number of carbon atoms, the suffix -an is attached to it, which indicates the type of compound, i.e. alkane
| Number of carbons | Name | Structural formula |
|---|---|---|
| 1 | Methane | CH4 |
| 2 | Ethane | CH 3 -CH 3 |
| 3 | Propane | CH 3 -CH 2 -CH 3 |
| 4 | Butane | CH 3 -(CH 2) 2 -CH 3 |
| 5 | Pentane | CH 3 -(CH 2) 3 -CH 3 |
| 6 | Hexane | CH 3 -(CH 2) 4 -CH 3 |
| 7 | Heptane | CH 3 -(CH 2) 5 -CH 3 |
| 8 | Octane | CH 3 -(CH 2) 6 -CH 3 |
| 9 | Nonan | CH 3 -(CH 2) 7 -CH 3 |
| 10 | Dean | CH 3 -(CH 2) 8 -CH 3 |
| 11 | Undecan | CH 3 -(CH 2) 9 -CH 3 |
| 12 | Dodecan | CH 3 -(CH 2) 10 -CH 3 |
| 13 | Tridecan | CH 3 -(CH 2) 11 -CH 3 |
| 14 | Tetradecane | CH 3 -(CH 2) 12 -CH 3 |
| 15 | Pentadecan | CH 3 -(CH 2) 13 -CH 3 |
| 16 | Hexadecane | CH 3 -(CH 2) 14 -CH 3 |
| 17 | Heptadecane | CH 3 -(CH 2) 15 -CH 3 |
| 18 | Octadecan | CH 3 -(CH 2) 16 -CH 3 |
| 19 | Nonadecan | CH 3 -(CH 2) 17 -CH 3 |
| 20 | Eikozan | CH 3 -(CH 2) 18 -CH 3 |
| Table 1. Alkanes nomenclature | ||
Alkanes or aliphatic saturated hydrocarbons are compounds with an open (non-cyclic) chain, in the molecules of which carbon atoms are interconnected by a σ-bond. The carbon atom in alkanes is in a state of sp 3 hybridization.
Alkanes form a homologous series in which each member differs by a constant structural unit-CH 2 -, which is called the homologous difference. The simplest representative is methane CH 4 .
- General formula of alkanes: C n H 2n+2
For alkanes, in addition to structural isomerism, there is conformational isomerism and, starting with heptane, enantiomerism:
IUPAC nomenclature Prefixes are used in the names of alkanes n-, second-, iso, tert-, neo:
- n- means the normal (nezagaluzhenu) structure of the hydrocarbon chain;
- second- applies only to recycled butyl;
- tert- means alkyl tertiary structure;
- iso branches at the end of the chain;
- neo used for alkyl with a quaternary carbon atom.
The nomenclature of branched alkanes is based on the following basic rules:
- To build a name, a long chain of carbon atoms is chosen and numbered with Arabic numerals (locants), starting from the end closest to which the substituent is located, for example:
- If the same alkyl group occurs more than once, then multiplying prefixes are placed in front of it in the name di-(before a vowel di-), three-, tetra- etc. and designate each alkyl separately with a number, for example:
It should be noted that for complex residues (groups) multiplying prefixes like bis-, tris-, tetrakis- other.
- If different alkyl substituents are placed in the side branches of the main chain, then they are reordered alphabetically (while multiplying the prefixes di-, tetra- etc., as well as prefixes n-, second-, tert- ignored), for example:
- If two or more variants of the longest chain are possible, then choose the one that has the maximum number of side branches.
- The names of complex alkyl groups are built on the same principles as the names of alkanes, but the numbering of the alkyl chain is always autonomous and starts from that carbon atom that has a free valency, for example:
- When used in the name of such a group, it is taken in brackets and the first letter of the name of the whole is taken into account in alphabetical order:
Industrial mining methods 1. Alkane gas extraction. Natural gas consists mainly of methane and minor impurities of ethane, propane, butane. The gas under pressure at reduced at reduced temperatures is separated into the appropriate fractions.
2. Extraction of alkanes from oil. Crude oil is purified and subjected to processing (distillation, fractionation, cracking). Mixtures or individual compounds are obtained from processed products.
3. Hydrogenation of coal (method of F. Bergius, 1925). Hard or brown coal in autoclaves at 30 MPa in the presence of catalysts (oxides and sulfides of Fe, Mo, W, Ni) in a hydrocarbon medium is hydrogenated and converted into alkanes, the so-called motor fuel:
nC + (n+1)H 2 = C n H 2n+2
4. Oxosynthesis of alkanes (method of F. Fischer - G. Tropsch, 1922). According to the Fischer-Tropsch method, alkanes are obtained from synthesis gas. Synthesis gas is a mixture of CO and H 2 with different ratios. It is obtained from methane of one of the reactions that occur at 800-900 ° C in the presence of nickel oxide NiO deposited on Al 2 O 3:
CH 4 + H 2 O ⇄ CO + 3H 2
CH 4 + CO 2 ⇄ 2CO + 2H 2
2CH 4 + O 2 ⇄ 2CO + 4H 2
Alkanes are obtained by the reaction (temperature about 300°C, Fe-Co catalyst):
nCO + (2n+1)H 2 → C n H 2n+2 + nH 2 O
The resulting mixture of hydrocarbons, consisting mainly of structure alkanes (n=12-18), is called "synthine".
5. Dry distillation. In relatively small quantities, alkanes are obtained by dry distillation or heating of coal, shale, wood, peat without air. The approximate composition of the resulting mixture is 60% hydrogen, 25% methane and 3-5% ethylene.
Laboratory mining methods 1. Preparation from haloalkyls
1.1. Interaction with metallic sodium (Wurz, 1855). The reaction consists in the interaction of an alkali metal with a haloalkyl and is used for the synthesis of higher symmetrical alkanes:
2CH 3 -I + 2Na ⇄ CH 3 -CH 3 + 2NaI
In the case of participation in the reaction of two different haloalkyls, a mixture of alkanes is formed:
3CH 3 -I + 3CH 3 CH 2 -I + 6Na → CH 3 -CH 3 + CH 3 CH 2 CH 3 + CH 3 CH 2 CH 2 CH 3 + 6NaI
1.2 Interaction with lithium dialkyl cuprates. The method (sometimes called the reaction of E. Kore - H. House) consists in the interaction of reactive lithium dialkyl cuprates R 2 CuLi with haloalkyls. First, metal lithium interacts with a haloalkane in an ether medium. Further, the corresponding alkyl lithium reacts with copper(I) halide to form soluble lithium dialkyl cuprate:
CH 3 Cl + 2Li → CH 3 Li + LiCl
2CH 3 Li + CuI → (CH 3 ) 2 CuLi + LiI
When such lithium dialkyl cuprate reacts with the corresponding haloalkyl, the final compound is formed:
(CH 3 ) 2 CuLi + 2CH 3 (CH 2 ) 6 CH 2 -I → 2CH 3 (CH 2 ) 6 CH 2 -CH 3 + LiI + CuI
The method makes it possible to achieve almost 100% yield of alkanes when using primary haloalkyls. With their secondary or tertiary structure, the yield is 30-55%. The nature of the alkyl component in lithium dialkyl cuprate has little effect on the alkane yield.
1.3 Restoration of haloalkyls. It is possible to reduce haloalkyls with catalytically excited molecular hydrogen, atomic hydrogen, iodine, etc.:
CH 3 I + H 2 → CH 4 + HI (Pd catalyst)
CH 3 CH 2 I + 2H → CH 3 CH 3 + HI
CH 3 I + HI → CH 4 + I 2
The method has a preparative value, a strong reducing agent is often used - iodine water.
2. Preparation from salts carboxylic acids.
2.1 Electrolysis of salts (Kolbe, 1849). The Kolbe reaction is electrolysis aqueous solutions salts of carboxylic acids:
R-COONa ⇄ R-COO - + Na +
At the anode, the carboxylic acid anion is oxidized, forming a free radical, and it is easy to decarboxylate or eliminate CO 2 . Alkyl radicals are further converted into alkanes due to recombination:
R-COO - → R-COO . +e-
R-COO. →R. +CO2
R. +R. → R-R
Kolbe's preparative method is considered effective in the presence of the appropriate carboxylic acids and the inability to apply other methods of synthesis.
2.2 Fusion of salts of carboxylic acids with alkali. salt alkali metals carboxylic acids when mixed with alkali form alkanes:
CH 3 CH 2 COONa + NaOH → Na 2 CO 3 + CH 3 CH 3
3. Reduction of oxygen-containing compounds(alcohols, ketones, carboxylic acids) . The aforementioned compounds act as reducing agents. Most often, iodine water is used, which is able to restore even ketones: The first four representatives of alkanes from methane to butane (C 1 -C 4) are gases, from pentane to pentadecane (C 5 -C 15 - liquids, from hexadecane (C 16) - solids. Increases them molecular weights leads to an increase in boiling and melting points, while branched-chain alkanes boil at a lower temperature than normal alkanes. This is due to the smaller van der Waals interaction between the molecules of branched hydrocarbons in the liquid state. The melting temperature of even homologues is higher compared to the temperature of odd homologues, respectively.
Alkanes are much easier for water, non-polar and difficult to polarize, however, they are soluble in most non-polar solvents, due to which they themselves can be a solvent for many organic compounds.
