ALKANE (saturated hydrocarbons, paraffins)

• Alkanes are aliphatic (acyclic) saturated hydrocarbons in which the carbon atoms are linked together by simple (single) bonds in straight 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, low activity).
Limit, or saturated, these hydrocarbons are named due to 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 we can derive the general formula of alkanes. The number of carbon atoms in the series of alkanes is taken to be 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 – number of carbon atoms.

Structure of alkanes

Chemical structure(the order of connection of atoms in molecules) of the simplest alkanes - methane, ethane and propane - is shown by their structural formulas given in section 2. From these formulas it is clear that there are two types of chemical bonds in alkanes:

S–S And S–H.

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 shared electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:

Electronic and structural formulas reflect chemical structure, but do not give an idea about spatial structure of molecules, which significantly affects the properties of the substance.

Spatial structure, i.e. the relative arrangement of the atoms of a molecule in space depends on the direction of the atomic orbitals (AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of the atomic orbitals of carbon, since the spherical 1s-AO of the hydrogen atom lacks a specific orientation.

The spatial arrangement of carbon AO, 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-CH or C-C bonds.

The four σ-bonds of carbon are directed in space at an angle of 109 about 28", which corresponds to the least repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices there are hydrogen atoms:

The H-C-H bond angle is 109°28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.

For recording, it is convenient to use a 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:

Alkane molecules 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 (same molecular formula), but different structures. Such connections are called isomers.

Differences in the order in which atoms are combined in molecules (i.e., chemical structure) lead to structural isomerism. The structure of structural isomers is reflected by structural formulas. In the series of alkanes, 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 of the same composition and the same chemical structure different relative positions of atoms in space are possible, then we observe 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 with ethane H 3 C–CH 3, exist in various spatial forms ( conformations), caused by intramolecular rotation along C–C σ bonds, and exhibit the so-called rotational (conformational) isomerism.

In addition, if a molecule contains a carbon atom bonded to 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 the left hand relates to the right). Such differences in the structure of molecules are called optical isomerism.

Structural isomerism of alkanes

• Structural isomers are compounds of the same composition that differ in the order of bonding of atoms, i.e. chemical structure of molecules.

The reason for the manifestation of structural isomerism in the series of alkanes is the ability of carbon atoms to form chains of different structures. This type of structural isomerism is called carbon skeleton isomerism.

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, differing in the structure of the carbon chain:

With an increase in the number of carbon atoms in the molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms.

Structural isomers differ in physical properties. Alkanes with a branched structure, due to the 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 techniques are used.

Alkanes are saturated hydrocarbons in the molecules of which all carbon atoms are occupied through simple bonds by hydrogen atoms. Therefore, homologues of the methane series are characterized by structural isomerism of alkanes.

Isomerism of the carbon skeleton

Homologues with four or more carbon atoms are characterized by structural isomerism due to 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 homologues 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 physical properties. Branched alkanes melt and boil at lower temperatures than their linear counterparts.

Nomenclature

The IUPAC international nomenclature has established rules for naming branched chains. To name a structural isomer:

• find the longest chain and name it;
• number the carbon atoms starting from the end with the most substituents;
• indicate the number of identical substituents using numerical prefixes;
• give names to the substitutes.

The name consists of four parts, following each other:

• numbers indicating the atoms of the chain that have substituents;
• numeric prefixes;
• name of the deputy;
• name of the main circuit.

For example, in the 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 is called 2,2,4-trimethylpentane.

The various substituents (methyl, ethyl, propyl) are listed in the name alphabetically: 4,4-dimethyl-3-ethylheptane, 3-methyl-3-ethyloctane.

Typically, number 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 characteristic of all homologues, starting with butane. In structural isomerism, substituents attach 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, a verbal designation of the number of substituents, and a digital designation of the carbon atoms to which the substituents are attached.

Heating the sodium salt of acetic acid (sodium acetate) with an excess of alkali leads to the elimination of the carboxyl group and the formation of methane:

CH3CONa + NaOH CH4 + Na2C03

If you take sodium propionate instead of sodium acetate, then ethane is formed, from sodium butanoate - propane, etc.

RCH2CONa + NaOH -> RCH3 + Na2C03

5. Wurtz synthesis. When haloalkanes interact with the alkali metal sodium, saturated hydrocarbons and an alkali metal halide are formed, for example:

The action of an alkali metal on a mixture of halocarbons (eg bromoethane and bromomethane) will result in the formation of a mixture of alkanes (ethane, propane and butane).

The reaction on which the Wurtz synthesis is based proceeds well only with haloalkanes in the molecules of which a halogen atom is attached to a primary carbon atom.

6. Hydrolysis of carbides. When some carbides containing carbon in the -4 oxidation state (for example, aluminum carbide) are treated with water, methane is formed:

Al4C3 + 12H20 = 3CH4 + 4Al(OH)3 Physical properties

The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a gas without color, taste and smell (the smell of “gas”, 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, for so that people nearby can detect a leak by smell).

Hydrocarbons of composition from C5H12 to C15H32 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 reactions for 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 characteristic reactions.

Halogenation:

СН4 + С12 -> СН3Сl + HCl

In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:

СН3Сl + С12 -> HCl + СН2Сl2
dichloromethane methylene chloride

СН2Сl2 + Сl2 -> HCl + CHCl3
trichloromethane chloroform

СНСl3 + Сl2 -> HCl + СCl4
carbon tetrachloride carbon tetrachloride

The resulting substances are widely used as solvents and starting materials in organic syntheses.

2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst (Pt, Ni, Al2O3, Cr2O3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed:

CH3-CH3 -> CH2=CH2 + H2

3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form 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 very important when using alkanes as fuel.

CH4 + 2O2 -> C02 + 2H2O + 880kJ

In general, the combustion reaction of alkanes can be written as follows:

Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining.

When methane is heated to a temperature of 1000 ° C, methane pyrolysis 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. Flavoring. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:

What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. 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 increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by a heterolytic mechanism.

The most characteristic reactions of alkanes are free radical substitution reactions. During these reactions, a hydrogen atom is replaced by a halogen atom or some group.

The kinetics and mechanism of free radical chain reactions, i.e. reactions occurring under the influence of free radicals - particles with unpaired electrons - were studied by the remarkable Russian chemist N. N. Semenov. It was for these studies that he was awarded the Nobel Prize in Chemistry.

Typically, the mechanism of free radical substitution reactions is represented by three main stages:

1. Initiation (nucleation of a chain, formation of free radicals under the influence of an energy source - ultraviolet light, heating).

2. Chain development (a chain of sequential interactions of free radicals and inactive molecules, as a result of which new radicals and new molecules are formed).

3. Chain termination (combination of free radicals into inactive molecules (recombination), “death” of radicals, cessation of the development of a chain of reactions).

Scientific research by N.N. Semenov

Semenov Nikolay Nikolaevich

(1896 - 1986)

Soviet physicist and physical chemist, academician. Nobel Prize winner (1956). Scientific research relates to the study of chemical processes, catalysis, chain reactions, the theory of thermal explosion and the combustion of gas mixtures.

Let's consider this mechanism using the example of the methane chlorination reaction:

CH4 + Cl2 -> CH3Cl + HCl

Chain initiation occurs as a result of the fact that under the influence of ultraviolet irradiation or heating, homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule disintegrates into atoms:

Сl: Сl -> Сl· + Сl·

The resulting free radicals attack methane molecules, tearing off their hydrogen atom:

CH4 + Cl· -> CH3· + HCl

and transforming into CH3· radicals, which, in turn, colliding with chlorine molecules, destroy them with the formation of new radicals:

CH3 + Cl2 -> CH3Cl + Cl etc.

The chain develops.

Along with the formation of radicals, their “death” occurs as a result of the process of recombination - the formation of an inactive molecule from two radicals:

СН3+ Сl -> СН3Сl

Сl· + Сl· -> Сl2

CH3 + CH3 -> CH3-CH3

It is interesting to note that during recombination, only as much energy is released as is necessary to break the newly formed bond. In this regard, recombination is possible only if a third particle (another molecule, the wall of the reaction vessel) participates in the collision of two radicals, which absorbs excess energy. This makes it possible to regulate and even stop free radical chain reactions.

Note the last example of a recombination reaction - the formation of an ethane molecule. This example shows that a reaction involving organic compounds is a rather complex process, as a result of which, along with the main reaction product, by-products are very often formed, which leads to the need to develop complex and expensive methods for the purification and isolation of target substances.

The reaction mixture obtained from the chlorination of methane, along with chloromethane (CH3Cl) and hydrogen chloride, will contain: dichloromethane (CH2Cl2), trichloromethane (CHCl3), carbon tetrachloride (CCl4), ethane and its chlorination products.

Now let's try to consider the halogenation reaction (for example, bromination) of a more complex organic compound - propane.

If in the case of methane chlorination only one monochloro derivative is possible, then in this reaction two monobromo derivatives can be formed:

It can be seen that in the first case, the hydrogen atom is replaced at the primary carbon atom, and in the second case, at the secondary one. Are the rates of these reactions the same? It turns out that the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates in the final mixture, i.e. 2-bromopropane (CH3-CHBg-CH3). Let's try to explain this.

In order to do this, we will have to use the idea of ​​​​the stability of intermediate particles. Did you notice that when describing the mechanism of the methane chlorination reaction we mentioned the methyl radical - CH3·? This radical is an intermediate particle between methane CH4 and chloromethane CH3Cl. The intermediate particle between propane and 1-bromopropane is a radical with an unpaired electron at the primary carbon, and between propane and 2-bromopropane at the secondary carbon.

A radical with an unpaired electron at the secondary carbon atom (b) is more stable compared to a free radical with an unpaired electron at the primary carbon atom (a). It is formed in greater quantities. For this reason, the main product of the propane bromination reaction is 2-bromopropane, a compound whose formation occurs through a more stable intermediate species.

Here are some examples of free radical reactions:

Nitration reaction (Konovalov reaction)

The reaction is used to obtain nitro compounds - solvents, starting materials for many syntheses.

Catalytic oxidation of alkanes with oxygen

These reactions are the basis of the most important industrial processes for the production of aldehydes, ketones, and alcohols directly from saturated hydrocarbons, for example:

CH4 + [O] -> CH3OH

Application

Saturated hydrocarbons, especially methane, are widely used in industry (Scheme 2). They are a simple and fairly cheap fuel, a raw material for the production of a large number of important compounds.

Compounds obtained from methane, the cheapest hydrocarbon raw material, are used to produce many other substances and materials. Methane is used as a source of hydrogen in the synthesis of ammonia, as well as to produce synthesis gas (a mixture of CO and H2), used for the industrial synthesis of hydrocarbons, alcohols, aldehydes and other organic compounds.

Hydrocarbons of higher boiling oil fractions are used as fuel for diesel and turbojet engines, as the basis of lubricating oils, as raw materials for the production of synthetic fats, etc.

Here are several industrially significant reactions that occur with the participation of methane. Methane is used to produce chloroform, nitromethane, and oxygen-containing derivatives. Alcohols, aldehydes, carboxylic acids can be formed by the direct interaction of alkanes with oxygen, depending on the reaction conditions (catalyst, temperature, pressure):

As you already know, hydrocarbons of the composition from C5H12 to C11H24 are included in the gasoline fraction of oil and are used mainly as fuel for internal combustion engines. It is known that the most valuable components of gasoline are isomeric hydrocarbons, since they have maximum detonation resistance.

When hydrocarbons come into contact with atmospheric oxygen, they slowly form compounds with it - peroxides. This is a slowly occurring free radical reaction, initiated by an oxygen molecule:

Please note that the hydroperoxide group is formed at secondary carbon atoms, which are most abundant in linear, or normal, hydrocarbons.

With a sharp increase in pressure and temperature that occurs at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation of a large number of free radicals, which “trigger” the free radical combustion chain reaction earlier than necessary. The piston still goes up, and the combustion products of gasoline, which have already formed as a result of premature ignition of the mixture, push it down. This leads to a sharp decrease in engine power and wear.

Thus, the main cause of detonation is the presence of peroxide compounds, the ability to form which is maximum in linear hydrocarbons.

C-heptane has the lowest detonation resistance among the hydrocarbons of the gasoline fraction (C5H14 - C11H24). The most stable (i.e., forms peroxides to the least extent) is the so-called isooctane (2,2,4-trimethylpentane).

A generally accepted characteristic of the knock resistance of gasoline is the octane number. An octane number of 92 (for example, A-92 gasoline) means that this gasoline has the same properties as a mixture consisting of 92% isooctane and 8% heptane.

In conclusion, we can add that the use of high-octane gasoline makes it possible to increase the compression ratio (pressure at the end of the compression stroke), which leads to increased power and efficiency of the internal combustion engine.

Being in nature and receiving

In today's lesson, you became acquainted with the concept of alkanes, and also learned about its chemical composition and methods of preparation. Therefore, let's now dwell in more detail on the topic of the presence of alkanes in nature and find out how and where alkanes have found application.

The main sources for the production of alkanes are natural gas and oil. They make up the bulk of oil refining products. Methane, common in sedimentary rock deposits, is also a gas hydrate of alkanes.

The main component of natural gas is methane, but it also contains a small proportion of ethane, propane and butane. Methane can be found in emissions from coal seams, swamps and associated petroleum gases.

Ankans can also be obtained by coking coal. In nature, there are also so-called solid alkanes - ozokerites, which are presented in the form of deposits of mountain wax. Ozokerite can be found in the waxy coatings of plants or their seeds, as well as in beeswax.

The industrial isolation of alkanes is taken from natural sources, which, fortunately, are still inexhaustible. They are obtained by the catalytic hydrogenation of carbon oxides. Methane can also be produced in the laboratory using the method of heating sodium acetate with solid alkali or hydrolysis of certain carbides. But alkanes can also be obtained by decarboxylation of carboxylic acids and by their electrolysis.

Applications of alkanes

Alkanes at the household level are widely used in many areas of human activity. After all, it is very difficult to imagine our life without natural gas. And it will not be a secret to anyone that the basis of natural gas is methane, from which carbon black is produced, which is used in the production of topographic paints and tires. The refrigerator that everyone has in their home also works thanks to alkane compounds used as refrigerants. Acetylene obtained from methane is used for welding and cutting metals.

Now you already know that alkanes are used as fuel. They are present in gasoline, kerosene, diesel oil and fuel oil. In addition, they are also found in lubricating oils, petroleum jelly and paraffin.

Cyclohexane has found wide use as a solvent and for the synthesis of various polymers. Cyclopropane is used in anesthesia. Squalane, as a high-quality lubricating oil, is a component of many pharmaceutical and cosmetic preparations. Alkanes are the raw materials used to produce organic compounds such as alcohol, aldehydes and acids.

Paraffin is a mixture of higher alkanes, and since it is non-toxic, it is widely used in the food industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture of chewing gum. And heated paraffin is used in medicine for paraffin treatment.

In addition to the above, the heads of matches are impregnated with paraffin for better burning, pencils, and candles are made from it.

By oxidizing paraffin, oxygen-containing products, mainly organic acids, are obtained. When liquid hydrocarbons with a certain number of carbon atoms are mixed, Vaseline is obtained, which is widely used in perfumery and cosmetology, as well as in medicine. It is used to prepare various ointments, creams and gels. They are also used for thermal procedures in medicine.

Practical tasks

1. Write down the general formula of hydrocarbons of the homologous series of alkanes.

2. Write the formulas of possible isomers of hexane and name them according to systematic nomenclature.

3. What is cracking? What types of cracking do you know?

4. Write the formulas of possible products of hexane cracking.

5. Decipher the following chain of transformations. Name the compounds A, B and C.

6. Give the structural formula of the hydrocarbon C5H12, which forms only one monobromine derivative upon bromination.

7. For the complete combustion of 0.1 mol of an alkane of unknown structure, 11.2 liters of oxygen were consumed (at ambient conditions). What is the structural formula of an alkane?

8. What is the structural formula of a gaseous saturated hydrocarbon if 11 g of this gas occupy a volume of 5.6 liters (at standard conditions)?

9. Recall what you know about the use of methane and explain why a domestic gas leak can be detected by smell, although its components are odorless.

10*. What compounds can be obtained by catalytic oxidation of methane under various conditions? Write the equations for the corresponding reactions.

eleven*. Products of complete combustion (in excess oxygen) 10.08 liters (N.S.) of a mixture of ethane and propane were passed through an excess of lime water. In this case, 120 g of sediment was formed. Determine the volumetric composition of the initial mixture.

12*. The ethane density of a mixture of two alkanes is 1.808. Upon bromination of this mixture, only two pairs of isomeric monobromoalkanes were isolated. The total mass of lighter isomers in the reaction products is equal to the total mass of heavier isomers. Determine the volume fraction of the heavier alkane in the initial mixture.

Isomerism

The ability of carbon atoms to form four covalent bonds, including with other carbon atoms, opens up the possibility of the existence of several compounds of the same elemental composition - isomers.

All isomers are divided into two large classes - structural isomers and spatial isomers. Structural isomers are those that correspond to different structural formulas of organic compounds (with different orders of atoms). Spatial isomers have the same substituents on each carbon atom and differ only in their relative location in space. Structural isomers. In accordance with the above classification of organic compounds by type, three groups are distinguished among structural isomers:

1) compounds containing various functional groups and belonging to different classes of organic compounds, for example:

CH3-CH2-NO2 HOOC-CH2-NH2

nitroethane amioacetic acid (glycine)

2) compounds that differ in carbon skeletons:

butane 2-methylpropane (isobutane)

3) compounds that differ in the position of the substituent or multiple bond in the molecule:

CH3-CH=CH-CH3 CH3-CH2-CH=CH2

butene-2 ​​butene-1

propanol-2 propanol-1

Spatial isomers (stereoisomers). Stereoisomers can be divided into two types: geometric isomers, optical isomers.

Geometric isomerism is characteristic of compounds containing a double bond or ring. In such molecules it is often possible to draw a conventional plane in such a way that the substituents on different carbon atoms can be on the same side (cis-) or on opposite sides (trans-) of this plane.

If a change in the orientation of these substituents relative to the plane is possible only due to the breaking of one of the chemical bonds, then they speak of the presence of geometric isomers. Geometric isomers differ in their physical and chemical properties.

trans-1,2- cis-1,2- cis-butene-2 ​​trans-6utene-2

dimethyl-dimethyl-

cyclopentane cyclopentane

Optical isomers are molecules whose mirror images are incompatible with each other.

This property is possessed by molecules that have an asymmetric center - a carbon atom connected to four different substituents. For example, in the form of two optical isomers, there is a molecule of lactic acid CH 3-CH(OH)-COOH, containing one asymmetric center:

Alkanes. general characteristics

Hydrocarbons are the simplest organic compounds consisting of two elements: carbon and hydrogen. Saturated hydrocarbons, or alkanes (international name), are compounds whose composition is expressed by the general formula C n H2n+2, where n is the number of carbon atoms. In the molecules of saturated hydrocarbons, carbon atoms are connected to each other by a simple (single) bond, and all other valences are saturated with hydrogen atoms. Alkanes are also called saturated hydrocarbons or paraffins (the term "paraffins" means "low affinity").

The first member of the homologous series of alkanes is methane CH4. The ending -an is typical for the names of saturated hydrocarbons. This is followed by ethane C2H6, propane C3H8, butane C4H10. Starting with the fifth hydrocarbon, the name is formed from the Greek numeral, indicating the number of carbon atoms in the molecule, and the ending -an. These are pentane C5H12 hexane C6H14, heptane C7H16, octane C8H18, nonane C9H20, decane C10H22, etc.

In the homologous series, a gradual change in the physical properties of hydrocarbons is observed: boiling and melting points increase, density increases. Under normal conditions (temperature ~ 22°C), the first four members of the series (methane, ethane, propane, butane) are gases, from C5H12 to C16H34 are liquids, and from C17H36 are solids.

Alkanes, starting from the fourth member of the series (butane), have isomers.

All alkanes are saturated with hydrogen to the limit (maximum). Their carbon atoms are in a state of sp 3 hybridization, which means they have simple (single) bonds.

Nomenclature:

The names of the first ten members of the series of saturated hydrocarbons have already been given. To emphasize that an alkane has a straight carbon chain, the word normal (n-) is often added to the name, for example:

CH3--CH2--CH2--CH3 CH3--CH2--CH2--CH2--CH2--CH 2 --CH3

n-butane n-heptane

(normal butane) (normal heptane)

When a hydrogen atom is removed from an alkane molecule, single-valent particles are formed called hydrocarbon radicals (abbreviated as R). The names of monovalent radicals are derived from the names of the corresponding hydrocarbons with the ending -an replaced by -yl. Here are relevant examples:

Hydrocarbons

Hexane C6H14

Ethane C2H6

Heptane C 7 H 16

Propane C 3 H 8

Octane C 8 H 18

Butane C4H10

Nonane C 9 H 20

Pentane C5H12

Dean C 10 H 22

Monovalent radicals

Methyl CH 3 -

Hexyl C 6 H 13 -

Ethyl C2H5 -

Heptyl C 7 H 15 -

Cut C 3 H 7 -

Octyl C 8 H 17 -

Butyl C4H9 -

Nonyl C 9 H 19 -

Pentyl (amyl) C 5 H 11 -

Decyl C 10 H 21 -

Radicals are formed not only by organic, but also by inorganic compounds. So, if you subtract the hydroxyl group OH from nitric acid, you get a monovalent radical - NO2, called a nitro group, etc.

When two hydrogen atoms are removed from a hydrocarbon molecule, divalent radicals are obtained. Their names are also derived from the names of the corresponding saturated hydrocarbons with the ending -ane replaced by -ylidene (if the hydrogen atoms are separated from one carbon atom) or -ylene (if the hydrogen atoms are removed from two adjacent carbon atoms). The CH2= radical is called methylene.

The names of radicals are used in the nomenclature of many hydrocarbon derivatives. For example: CH3I - methyl iodide, C4H9Cl - butyl chloride, CH 2Cl 2 - methylene chloride, C2H4Br 2 - ethylene bromide (if bromine atoms are bonded to different carbon atoms) or ethylidene bromide (if bromine atoms are bonded to one carbon atom).

To name isomers, two nomenclatures are widely used: old - rational and modern - substitutive, which is also called systematic or international (proposed by the International Union of Pure and Applied Chemistry IUPAC).

According to rational nomenclature, hydrocarbons are considered to be derivatives of methane, in which one or more hydrogen atoms are replaced by radicals. If the same radicals are repeated several times in a formula, then they are indicated by Greek numerals: di - two, three - three, tetra - four, penta - five, hexa - six, etc. For example:

Rational nomenclature is convenient for not very complex connections.

According to substitutive nomenclature, the name is based on one carbon chain, and all other fragments of the molecule are considered as substituents. In this case, the longest chain of carbon atoms is selected and the atoms of the chain are numbered from the end to which the hydrocarbon radical is closest. Then they call: 1) the number of the carbon atom to which the radical is associated (starting with the simplest radical); 2) a hydrocarbon that has a long chain. If the formula contains several identical radicals, then before their names indicate the number in words (di-, tri-, tetra-, etc.), and the numbers of the radicals are separated by commas. Here is how hexane isomers should be called according to this nomenclature:

Both substitutive and rational nomenclature are used not only for hydrocarbons, but also for other classes of organic compounds. For some organic compounds, historically established (empirical) or so-called trivial names are used (formic acid, sulfuric ether, urea, etc.).

When writing the formulas of isomers, it is easy to notice that the carbon atoms occupy different positions in them. A carbon atom that is bonded to only one carbon atom in the chain is called primary, to two is called secondary, to three is tertiary, and to four is quaternary. So, for example, in the last example, carbon atoms 1 and 7 are primary, 4 and 6 are secondary, 2 and 3 are tertiary, 5 is quaternary. The properties of hydrogen atoms, other atoms, and functional groups depend on whether they are bonded to a primary, secondary, or tertiary carbon atom. This should always be taken into account.

Hydrocarbons in whose molecules 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 the carbon atoms close to tetrahedral (109° 28′), for example, in the molecule n-pentane.

It is especially worth recalling the bonds in alkane molecules. All bonds in the molecules of saturated hydrocarbons are single. The overlap 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 reactions for alkanes are those involving free radicals.

Homologous series of methane

Homologs- substances that are similar in structure and properties and differ by one or more CH 2 groups.

Isomerism and nomenclature

Alkanes are characterized by 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.

Nomenclature Basics

1. Selection 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. Numbering of atoms of the main chain. The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (-CH 3), then ethyl (-CH 2 -CH 3), propyl (-CH 2 -CH 2 -CH 3 ) etc.
Please note that the name of the substituent is formed by replacing the suffix -an with the suffix - silt in the name of the corresponding alkane.
3. Formation of the name. At the beginning of the name, numbers are indicated - the numbers of the 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, the number of substituents is indicated with a hyphen ( di- two, three- three, tetra- four, penta- five) and the name of the substituent (methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane ( 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 S 9 N 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”, when you smell it, 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 , located next to them, could detect the leak by smell).
Hydrocarbons of composition from C 4 H 12 to C 15 H 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 of alkanes

Substitution reactions.
The most characteristic reactions for 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 case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:

The resulting substances are widely used as solvents and starting materials in organic syntheses.
Dehydrogenation reaction(hydrogen abstraction).
When alkanes are passed over a catalyst (Pt, Ni, Al 2 0 3, Cr 2 0 3) at high temperatures (400-600 ° C), a hydrogen molecule is eliminated and an alkene is formed:


Reactions accompanied by the destruction of the carbon chain.
All saturated hydrocarbons burn to form 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 fuel:

In general, the combustion reaction of alkanes can be written as follows:

2. Thermal splitting of hydrocarbons.

The process occurs via a free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals.

These radicals interact with each other, exchanging a hydrogen atom, to form an alkane molecule and an alkene molecule:

Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining.

3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis 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 cyclize in the presence of a catalyst 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 nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by the heterolytic mechanism.