How to distinguish alkanes. Structural isomerism of alkanes. What are isomers

Alkanes or aliphatic saturated hydrocarbons are compounds with an open (non-cyclic) chain, in the molecules of which the carbon atoms are connected to each other 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 a homological difference. The simplest representative is methane CH4.

• General formula of alkanes: C n H 2n+2

Isomerism Starting from butane C 4 H 10, alkanes are characterized by structural isomerism. The number of structural isomers increases with the number of carbon atoms in the alkane molecule. Thus, for pentane C 5 H 12 three isomers are known, for octane C 8 H 18 - 18, for decane C 10 H 22 - 75.

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 normal (uncorroded) structure of the hydrocarbon chain;
• second- applies only to recycled butyl;
• tert- means alkyl of tertiary structure;
• iso branches at the end of the chain;
• neo used for alkyl with a quaternary carbon atom.

Prefixes iso And neo are written together, and n-, second-, tert- hyphenated

The nomenclature of branched alkanes is based on the following basic rules:

• To construct a name, a long chain of carbon atoms is selected and numbered with Arabic numerals (locants), starting from the end closer 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 various alkyl substituents are placed in the side branches of the main chain, then they are rearranged alphabetically (with multiplying prefixes di-, tetra- etc., as well as prefixes n-, second-, tert- are not taken into account), for example:

• If two or more options for the longest chain are possible, then choose the one that has the maximum number of side branches.
• The names of complex alkyl groups are constructed according to the same principles as the names of alkanes, but the numbering of the alkyl chain is always autonomous and begins with the carbon atom having free valency, for example:

• When used in the name of such a group, it is put in brackets and the first letter of the name of the entire group is taken into account in alphabetical order:

Industrial extraction methods 1. Extraction of alkanes gas. Natural gas consists mainly of methane and small admixtures of ethane, propane, and butane. Gas under pressure at low temperatures is divided into appropriate fractions.

2. Extraction of alkanes from oil. Crude oil is purified and processed (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 environment 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). Using 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 by one of the reactions that occur at 800-900°C in the presence of nickel oxide NiO supported 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 alkanes of the structure (n = 12-18), is called “syntin”.

5. Dry distillation. Alkanes are obtained in relatively small quantities by dry distillation or heating of coal, shale, wood, and peat without access to air. The approximate composition of the resulting mixture is 60% hydrogen, 25% methane and 3-5% ethylene.

Laboratory extraction methods 1. Preparation from haloalkyls

1.1. Reaction with metallic sodium (Wurz, 1855). The reaction consists of 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

If two different haloalkyls participate in the reaction, 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 E. Core - H. House reaction) involves the interaction of reactive lithium dialkyl cuprates R 2 CuLi with haloalkyls. First, lithium metal reacts with a haloalkane in an ether environment. Next, the corresponding alkyl lithium reacts with copper(I) halide to form a soluble lithium dialkyl cuprate:

CH 3 Cl + 2Li → CH 3 Li + LiCl

2CH 3 Li + CuI → (CH 3 ) 2 CuLi + LiI

When such a 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 a yield of alkanes of almost 100% 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 yield of the alkane.

1.3 Reduction 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 no preparative value; a strong reducing agent, iodine water, is often used.

2. Preparation from salts of carboxylic acids.
2.1 Electrolysis of salts (Kolbe, 1849). The Kolbe reaction involves the electrolysis of aqueous solutions of carboxylic acid salts:

R-COONa ⇄ R-COO - + Na +

At the anode, the carboxylic acid anion is oxidized, forming a free radical, and is easily decarboxylated or eliminated by 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 corresponding carboxylic acids and the impossibility of using other synthesis methods.

2.2 Fusion of salts of carboxylic acids with alkali. Alkali metal salts of carboxylic acids, when combined 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 reducing agents are the above-mentioned compounds. Most often, iodine is used, which is capable of reducing 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 An increase in their molecular weights leads to an increase in boiling and melting points, with branched-chain alkanes boiling at a lower temperature than normal-structure alkanes. This is explained by the lower van der Waals interaction between the molecules of branched hydrocarbons in the liquid state. The melting point of even-numbered homologues is higher. compared with the temperature, respectively, for odd ones.

Alkanes are much lighter than water, non-polar and difficult to polarize, but they are soluble in most non-polar solvents, due to which they themselves can be a solvent for many organic compounds.

alkane or paraffin(historical name, which also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane is made up of hydrogen and carbon atoms arranged in a tree-like structure in which all carbon-carbon bonds are single.

Alkanes have a general chemical formula CnH2n+2. Alkanes range in complexity from the simplest case of methane, CH 4, where n = 1 (sometimes called the original molecule), to arbitrarily large molecules.

Chemical structure of methane, the simplest alkane

Apart from this standard definition called by the International Union of Pure and Applied Chemistry, in the use of some authors the term alkane applies to any saturated hydrocarbon, including those that are either monocyclic (i.e. cycloalkanes) or polycyclic.

In an alkane, each carbon atom has 4 bonds (either C-C or C-H), and each hydrogen atom is attached to one of the carbon atoms (as in a C-H bond). The longest series of bonded carbon atoms in a molecule is known as its carbon skeleton or carbon backbone. The number of carbon atoms can be thought of as the size of the alkane.

One group of higher alkanes are waxes, solids at standard ambient temperature and pressure (STAP) for which the number of carbon atoms in the carbon chain is greater, about 17 times.

With repeated -CH 2 — alkanes constitute a homologous series of organic compounds in which the groups differ in molecular weight by a multiple of 14.03 μm (the total mass of each such methylene unit, which contains a single carbon atom with a mass of 12.01 μm and two hydrogen atoms with a mass of ~ 1.01 μm every).

Alkanes are not very reactive and have little biological activity. They can be thought of as molecular trees on which the more active/reactive functional groups of biological molecules can be suspended.

Alkanes have two main sources: petroleum (crude oil) and natural gas.

An alkyl group, usually abbreviated as R, is a functional group that, like an alkane, consists solely of bonded acyclic carbon and hydrogen atoms, such as a methyl or ethyl group.

Classification structure

Saturated hydrocarbons are hydrocarbons that have only single covalent bonds between their carbon atoms. They may represent:

• Linear (general formula C n H 2n + 2), in which the carbon atoms are connected in a snake-like structure.
• Branched (general formula C n H2 n + 2, n> 2), where the carbon skeleton is split off in one or more directions.
• Cyclic (general formula C n H 2n, n> 3), where the carbon chain is connected to form a loop.

Isobutane for 2-methylpropane
Isopentane for 2-methylbutane
Neopentane for 2,2-dimethylpropane.

Chemical properties of alkanes

- you can study this, in a complete, understandable presentation.

Physical properties of alkanes

All alkanes are colorless and odorless.

Table of alkanes.

Alkane	Formula	Boiling point [°C]	Melting point [°C]	Density [g cm-3] (at 20 °C)
Methane	CH 4	−162	−182	Gas
Ethane	C2H6	-89	−183	Gas
Propane	C 3 H 8	−42	−188	Gas
Butane	C4H10	0	−138	0.626
Pentane	C5H12	36	−130	0.659
Hexane	C6H14	69	−95	0.684
Heptane	C 7 H 16	98	−91	0.684
Octane	C 8 H 18	126	−57	0.718
Nonan	C 9 H 20	151	−54	0.730
Dean	C 10 H 22	174	−30	0.740
Undekan	C 11 H 24	196	-26	0.749
Dodecan	C 12 H 26	216	−10	0.769
Pentadecane	C 15 H 32	270	10-17	0.773
Hexadecane	C 16 H 34	287	18	Solid
Eikosan	C 20 H 42	343	37	Solid
Tricontan	C 30 H 62	450	66	Solid
Tetrocontan	C 40 H 82	525	82	Solid
Pentocontan	C 50 H 102	575	91	Solid
Hexocontane	C 60 H 122	625	100	Solid

Boiling point

Alkanes experience intermolecular van der Waals forces. Stronger intermolecular van der Waals forces cause higher boiling points of alkanes.

There are two determinants for the strength of Van Der Waals forces:

• The number of electrons surrounding the molecule, which increases with the molecular weight of the alkane
• Molecule surface area

Under standard conditions from CH 4 to C 4 H 10, alkanes are gaseous; From C5H12 to C17H36 they are liquids; And after C 18 H 38 they are solid. Since the boiling points of alkanes are primarily determined by weight, it should not be surprising that the boiling point has an almost linear relationship with the size (molecular weight) of the molecule. Typically, the boiling point increases by 20-30 °C for each carbon added to the chain. This rule also applies to other homologous series.

In physical chemistry, van der Waals forces (or van der Waals interactions), named after the Dutch scientist Johannes Diederik van der Waals, are residual forces of attraction or repulsion between molecules or atomic groups that do not arise from covalent bonds. It can be shown that van der Waals forces have the same origin as the Casimir effect, due to quantum interactions with the zero point field. The resulting van der Waals forces can be attractive or repulsive.

A straight-chain alkane will have a higher boiling point than a branched-chain alkane due to the greater surface area in contact, thus greater van der Waals forces between adjacent molecules. For example, compare isobutane (2-methylpropane) and n-butane (butane), which boil at -12 and 0 °C, and 2,2-dimethylbutane and 2,3-dimethylbutane, which boil at 50 and 58 °C, respectively . In the latter case, two molecules of 2,3-dimethylbutane can "click" together better than the cross-shaped 2,2-dimethylbutane, hence the large van der Waals forces

On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which provide a plane of intermolecular contact.

Melting points

The melting points of alkanes have a similar trend to their boiling points for the same reason as above. That is, (other things being equal) the larger the molecule, the higher the melting point. There is one significant difference between boiling points and melting points. Solids have a more rigid and fixed structure than liquids. This rigid structure requires energy to break down. Thus, for better bonding of solid structures, more energy will be required to break. For alkanes, this can be seen in the graph above (i.e. the green line). Odd-numbered alkanes have a lower tendency to melt than even-numbered alkanes. This is explained by the fact that even numbered alkanes pack well in the solid phase, forming a well-organized structure that requires more energy to break. Odd-numbered alkanes pack less well, and therefore an organized compaction structure with a looser one requires less energy to break.

The melting points of branched-chain alkanes can be either higher or lower than the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to stack well in the solid phase: this is especially true for isoalkanes (2-methyl isomers), which often have melting temperatures higher than those of their linear analogues.

Conductivity and solubility

Alkanes do not conduct electricity and are not polarized by an electric field. For this reason, they do not form hydrogen bonds and are insoluble in polar solvents such as water. Because the hydrogen bonds between individual water molecules are aligned away from the alkane molecule, the coexistence of the alkane and water results in increased molecular order (decreased entropy). Since there is no significant cohesion between water molecules and alkane molecules, the second law of thermodynamics suggests that this decrease in entropy should be minimized by minimizing contact between the alkane and water: alkanes are said to be hydrophobic in the sense that they repel water.

Their solubility in non-polar solvents is relatively good, a property called lipophilicity. Various alkanes, for example, are mixed in all proportions with each other.

The density of alkanes generally increases with the number of carbon atoms, but remains less than that of water. Therefore, the alkanes form the top layer as an alkane-water mixture.

Molecular geometry

The molecular structure of alkanes directly affects their physical and chemical characteristics. It is derived from the electron configuration of carbon, which has four valence electrons. The carbon atoms in alkanes are always sp 3 hybridized, that is, the valence electrons are said to be in four equivalent orbitals, derived from a combination of 2 s orbitals and three 2p orbitals. These orbitals, having the same energies, are spatially arranged in the form of a tetrahedron, the angle between them cos -1 (- 1/3) ≈ 109.47 °.

Bond lengths and bond angles

An alkane molecule has only C-H and C-C single bonds. The former are a consequence of the overlap of the sp 3 orbital of carbon with the 1s orbital of hydrogen; The latter is due to the overlap of two sp 3 orbitals on different carbon atoms. The bond lengths are 1.09 × 10 -10 m for the C-H bond and 1.54 × 10 -10 μm for the C-C bond.

The spatial arrangement of the bonds is similar to that of the four sp3 orbitals—they are arranged tetrahedrally with an angle of 109.47° between them. Structural formulas that present bonds as being at right angles to each other, while general and useful, are not true.

Conformation

The structural formula and bond angles are usually insufficient to fully describe the geometry of a molecule. There is one more degree of freedom for each carbon-carbon bond: the torsion angle between the atoms or groups bonded to the atoms at each end of the bond. The spatial arrangement described by the angles of torsion of a molecule is known as its shape.

Ethane forms the simplest case for studying the conformation of alkanes, since there is only one C-C bond. If you look down the C-C bond axis, you will see what is called the Newman projection. The hydrogen atoms on both the front and back carbon atoms have an angle of 120° between them, which is due to the projection of the base of the tetrahedron onto a flat plane. However, the angle of torsion between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the back carbon can be freely varied from 0° to 360°. This is a consequence of free rotation around a simple carbon-carbon bond. Despite this apparent freedom, only two extreme conformations are important: the eclipsing conformation and the step conformation.

Ball and twin screw models of two ethane rotamers

The two conformations, also known as rotamers, differ in energy: the staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (least stable).

This difference in energy between the two conformations, called torsional energy, is small compared to the thermal energy of an ethane molecule at ambient temperature. Constant rotation around the C-C bond. The time required for the transition of an ethane molecule from one staggered conformation to another, which is equivalent to the rotation of one CH3 group by 120 ° relative to the other, is on the order of 10 -11 s.

Projections of two conformations of ethane: eclipsed on the left side, checkerboard on the right.

Higher alkanes are more complex, but based on similar principles, with the antiperiplanar conformation always favored around each carbon-carbon bond. For this reason, alkanes are usually shown in a zigzag pattern in diagrams and models. The actual structure will always differ somewhat from these idealized forms, since the differences in energy between conformations are small compared to the thermal energy of the molecules, since alkane molecules do not have a fixed structural form, no matter what the model may show.

Spectroscopic properties

Almost all organic compounds contain carbon-carbon and carbon-hydrogen bonds and therefore show some of the features of alkanes in their spectra. Alkanes are distinguished by the absence of other groups and, therefore, the absence of other characteristic spectroscopic features of various functional groups, such as -OH, -CHO, -COOH, etc.

Infrared spectroscopy

The carbon-hydrogen stretching mode gives strong absorption between 2850 and 2960 cm -1, while the carbon-carbon stretching mode absorbs from 800 to 1300 cm -1. Carbon-hydrogen bending methods depend on the nature of the group: methyl groups show bands at 1450 cm -1 and 1375 cm -1 , while methylene groups show bands at 1465 cm -1 and 1450 cm -1 . Carbon chains with more than four carbon atoms show weak absorption at about 725 cm -1.

NMR spectroscopy

Proton resonances of alkanes are usually found at δH = 0.5-1.5. Resonances of carbon 13 depend on the number of hydrogen atoms bonded to carbon: δ C = 8-30 (primary, methyl, -CH 3), 15-55 (secondary, methylene, -CH 2 -), 20-60 (tertiary, Metin , C-H) and quaternary. Carbon-13 resonance of quaternary carbon atoms is characterized by weakness due to the absence of the nuclear Overhauser effect and long relaxation times, and can be missed in weak samples or samples that have not been processed for a sufficiently long time.

Mass spectrometry

Alkanes have high ionization energy, while molecular ions usually have weak ionization energies. Fragmentation fragmentation can be difficult to interpret, but in the case of branched alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting free radicals. The fragment resulting from the loss of one methyl group (M-15) is often missing, and the other fragment is often separated by intervals of fourteen mass units, corresponding to the sequential loss of CH 2 groups.

Methods for producing alkanes

You can also learn and study about methods for obtaining alkanes in this article.

Structure of alkanes

Alkanes are hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to the general formula C n H 2n+2. In alkane molecules, all carbon atoms are in the state sp 3 -hybridization.

This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed towards the corners of an equilateral triangular pyramid - 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 n-pentane molecule.

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. it 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.

Homologous series of methane

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

Saturated hydrocarbons constitute the homologous series of methane.

Isomerism and nomenclature of alkanes

Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane.

Let us consider in more detail the basic nomenclature for alkanes IUPAC.

1. Main circuit selection. 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 main chain atoms. 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 propyl (-CH 2 -CH 2 -CH 3), ethyl (-CH 2 -CH 3 ) etc.

Please note that the name of the substituent is formed by replacing the suffix -ane with the suffix -yl 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, a hyphen indicates the number of substituents (di - two, three - three, tetra - four, penta - five) and the name of the substituent (methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane (methane, ethane, propane, etc.).

The names of substances whose structural formulas are given above are as follows:

Structure A: 2-methylpropane;

Structure B: 3-ethylhexane;

Structure B: 2,2,4-trimethylpentane;

Structure D: 2-methyl 4-ethylhexane.

Absence of saturated hydrocarbons in molecules polar bonds leads to them poorly soluble in water, do not interact with charged particles (ions). The most characteristic reactions for alkanes are those involving free radicals.

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 WITH 5 N 12 before WITH 15 N 32 - liquids; heavier hydrocarbons are solids. The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.

Chemical properties of alkanes

Substitution reactions.

The most characteristic reactions for alkanes are free radical substitution, during which a hydrogen atom is replaced by a halogen atom or some group.

Let us present the characteristic equations halogenation reactions:

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 O 3, Cr 2 O 3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and a alkene:

Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode.

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 proceeds according to 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 a molecule alkane and alkene molecule:

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

3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis- decomposition into simple substances:

When heated to a temperature of 1500 °C, the formation of acetylene:

4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with branched carbon skeleton:

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.

It would be useful to start with a definition of the concept of alkanes. These are saturated or saturated. We can also say that these are carbons in which the connection of C atoms is carried out through simple bonds. The general formula is: CnH₂n+ 2.

It is known that the ratio of the number of H and C atoms in their molecules is maximum when compared with other classes. Due to the fact that all valences are occupied by either C or H, the chemical properties of alkanes are not clearly expressed, so their second name is the phrase saturated or saturated hydrocarbons.

There is also an older name that best reflects their relative chemical inertness - paraffins, which means “devoid of affinity.”

So, the topic of our conversation today is: “Alkanes: homological series, nomenclature, structure, isomerism.” Data regarding their physical properties will also be presented.

Alkanes: structure, nomenclature

In them, the C atoms are in a state called sp3 hybridization. In this regard, the alkane molecule can be demonstrated as a set of tetrahedral C structures that are connected not only to each other, but also to H.

Between the C and H atoms there are strong, very low-polar s-bonds. Atoms always rotate around simple bonds, which is why alkane molecules take on various shapes, and the bond length and the angle between them are constant values. Shapes that transform into each other due to the rotation of the molecule around σ bonds are usually called conformations.

In the process of abstraction of an H atom from the molecule in question, 1-valent species called hydrocarbon radicals are formed. They appear as a result of not only but also inorganic compounds. If you subtract 2 hydrogen atoms from a saturated hydrocarbon molecule, you get 2-valent radicals.

Thus, the nomenclature of alkanes can be:

• radial (old version);
• substitution (international, systematic). It was proposed by IUPAC.

Features of radial nomenclature

In the first case, the nomenclature of alkanes is characterized as follows:

1. Consideration of hydrocarbons as derivatives of methane, in which 1 or several H atoms are replaced by radicals.
2. High degree of convenience in the case of not very complex connections.

Features of substitution nomenclature

The substitutive nomenclature of alkanes has the following features:

1. The basis for the name is 1 carbon chain, while the remaining molecular fragments are considered as substituents.
2. If there are several identical radicals, the number is indicated before their name (strictly in words), and the radical numbers are separated by commas.

Chemistry: nomenclature of alkanes

For convenience, the information is presented in table form.

Substance name	The basis of the name (root)	Molecular formula	Name of carbon substituent	Carbon Substituent Formula

The above nomenclature of alkanes includes names that have developed historically (the first 4 members of the series of saturated hydrocarbons).

The names of unexpanded alkanes with 5 or more C atoms are derived from Greek numerals that reflect the given number of C atoms. Thus, the suffix -an indicates that the substance is from a series of saturated compounds.

When composing the names of unfolded alkanes, the main chain is the one that contains the maximum number of C atoms. It is numbered so that the substituents have the lowest number. In the case of two or more chains of the same length, the main one becomes the one that contains the largest number of substituents.

Isomerism of alkanes

The parent hydrocarbon of their series is methane CH₄. With each subsequent representative of the methane series, a difference from the previous one is observed in the methylene group - CH₂. This pattern can be traced throughout the entire series of alkanes.

The German scientist Schiel put forward a proposal to call this series homological. Translated from Greek it means “similar, similar.”

Thus, a homologous series is a set of related organic compounds that have the same structure and similar chemical properties. Homologues are members of a given series. Homologous difference is a methylene group in which 2 neighboring homologues differ.

As mentioned earlier, the composition of any saturated hydrocarbon can be expressed using the general formula CnH₂n + 2. Thus, the next member of the homologous series after methane is ethane - C₂H₆. To convert its structure from methane, it is necessary to replace 1 H atom with CH₃ (figure below).

The structure of each subsequent homolog can be deduced from the previous one in the same way. As a result, propane is formed from ethane - C₃H₈.

What are isomers?

These are substances that have an identical qualitative and quantitative molecular composition (identical molecular formula), but a different chemical structure, and also have different chemical properties.

The hydrocarbons discussed above differ in such a parameter as boiling point: -0.5° - butane, -10° - isobutane. This type of isomerism is called carbon skeleton isomerism; it belongs to the structural type.

The number of structural isomers increases rapidly as the number of carbon atoms increases. Thus, C₁₀H₂₂ will correspond to 75 isomers (not including spatial ones), and for C₁₅H₃₂ 4347 isomers are already known, for C₂₀H₄₂ - 366,319.

So, it has already become clear what alkanes are, homologous series, isomerism, nomenclature. Now it’s worth moving on to the rules for compiling names according to IUPAC.

IUPAC nomenclature: rules for the formation of names

First, it is necessary to find in the hydrocarbon structure the carbon chain that is longest and contains the maximum number of substituents. Then you need to number the C atoms of the chain, starting from the end to which the substituent is closest.

Secondly, the base is the name of an unbranched saturated hydrocarbon, which, in terms of the number of C atoms, corresponds to the main chain.

Thirdly, before the base it is necessary to indicate the numbers of the locants near which the substituents are located. The names of the substituents are written after them with a hyphen.

Fourthly, in the case of the presence of identical substituents at different C atoms, the locants are combined, and a multiplying prefix appears before the name: di - for two identical substituents, three - for three, tetra - four, penta - for five, etc. Numbers must be separated from each other by a comma, and from words by a hyphen.

If the same C atom contains two substituents at once, the locant is also written twice.

According to these rules, the international nomenclature of alkanes is formed.

Newman projections

This American scientist proposed special projection formulas for graphical demonstration of conformations - Newman projections. They correspond to forms A and B and are presented in the figure below.

In the first case, this is an A-occluded conformation, and in the second, it is a B-inhibited conformation. In position A, the H atoms are located at a minimum distance from each other. This form corresponds to the highest energy value, due to the fact that the repulsion between them is greatest. This is an energetically unfavorable state, as a result of which the molecule tends to leave it and move to a more stable position B. Here the H atoms are as far apart as possible from each other. Thus, the energy difference between these positions is 12 kJ/mol, due to which the free rotation around the axis in the ethane molecule, which connects the methyl groups, is uneven. After entering an energetically favorable position, the molecule lingers there, in other words, “slows down.” That is why it is called inhibited. The result is that 10 thousand ethane molecules are in the inhibited form of conformation at room temperature. Only one has a different shape - obscured.

Obtaining saturated hydrocarbons

From the article it has already become known that these are alkanes (their structure and nomenclature were described in detail earlier). It would be useful to consider ways to obtain them. They are released from natural sources such as oil, natural, and coal. Synthetic methods are also used. For example, H₂ 2H₂:

1. Hydrogenation process CnH₂n (alkenes)→ CnH₂n+2 (alkanes)← CnH₂n-2 (alkynes).
2. From a mixture of C and H monoxide - synthesis gas: nCO+(2n+1)H₂→ CnH₂n+2+nH₂O.
3. From carboxylic acids (their salts): electrolysis at the anode, at the cathode:

• Kolbe electrolysis: 2RCOONa+2H₂O→R-R+2CO₂+H₂+2NaOH;
• Dumas reaction (alloy with alkali): CH₃COONa+NaOH (t)→CH₄+Na₂CO₃.

1. Oil cracking: CnH₂n+2 (450-700°)→ CmH₂m+2+ Cn-mH₂(n-m).
2. Gasification of fuel (solid): C+2H₂→CH₄.
3. Synthesis of complex alkanes (halogen derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) +2Na →CH₃- CH₃ (ethane) +2NaCl.
4. Decomposition of methanides (metal carbides) by water: Al₄C₃+12H₂O→4Al(OH₃)↓+3CH₄.

Physical properties of saturated hydrocarbons

For convenience, the data is grouped into a table.

Formula	Alkane	Melting point in °C	Boiling point in °C	Density, g/ml
				0.415 at t = -165°С
				0.561 at t= -100°C
				0.583 at t = -45°C
				0.579 at t =0°C
	2-Methylpropane			0.557 at t = -25°C
	2,2-Dimethylpropane
	2-Methylbutane
	2-Methylpentane
	2,2,3,3-Tetra-methylbutane
	2,2,4-Trimethylpentane
n-C₁₀H₂₂
n-C₁₁H₂₄	n-Undecane
n-C₁₂H₂₆	n-Dodecane
n-C₁₃H₂₈	n-Tridecan
n-C₁₄H₃₀	n-Tetradecane
n-C₁₅H₃₂	n-Pentadecan
n-C₁₆H₃₄	n-Hexadecane
n-C₂₀H₄₂	n-Eicosane
n-C₃₀H₆₂	n-Triacontan		1 mmHg st
n-C₄₀H₈₂	n-Tetracontane		3 mmHg Art.
n-C₅₀H₁₀₂	n-Pentacontan		15 mmHg Art.
n-C₆₀H₁₂₂	n-Hexacontane
n-C₇₀H₁₄₂	n-Heptacontane
n-C₁₀₀H₂₀₂

Conclusion

The article examined such a concept as alkanes (structure, nomenclature, isomerism, homologous series, etc.). A little is said about the features of radial and substitutive nomenclatures. Methods for obtaining alkanes are described.

In addition, the article lists in detail the entire nomenclature of alkanes (the test can help you assimilate the information received).

Physical properties of alkanes

Under normal conditions, 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 number of carbon atoms in the chain increases, i.e. 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.

Alkanes are practically insoluble in water, because 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, etc.

Structure

The molecule of the simplest alkane - methane - has the shape of a regular tetrahedron, in the center of which there is a carbon atom, and at the vertices there are hydrogen atoms. The angles between the axes of the C-H bonds are 109°28" (Fig. 29).

In molecules of other saturated hydrocarbons, the angles between bonds (both C-H and C-C) have the same meaning. To describe the shape of molecules is used concept of hybridization of atomic orbitals(see Part I, §6).

In alkanes, all carbon atoms are in the state sp 3 - hybridization (Fig. 30).

Thus, the carbon atoms in the carbon chain are not in a straight line. The distance between neighboring carbon atoms (between the nuclei of atoms) is strictly fixed - this is chemical bond length(0.154 nm). Distance C 1 - C 3, C 2 - C 4, etc. (through one atom) are also constant, because the angle between the bonds is constant - bond angle.

The distances between more distant carbon atoms can change (within certain limits) as a result of rotation around s-bonds. This rotation does not disrupt the overlap of the orbitals that form the s-bond, since this bond has axial symmetry.

Different spatial forms of one molecule formed by the rotation of groups of atoms around s-bonds are called conformations(Fig. 31).

Conformations differ in energy, but this difference is small (12-15 kJ/mol). Conformations of alkanes in which the atoms are located as far apart as possible are more stable (repulsion of electron shells). The transition from one conformation to another is carried out due to the energy of thermal motion. To depict the conformation, special spatial formulas (Newman's formulas) are used.

Don't be confused!

It is necessary to distinguish between the concepts conformation and configuration.

Different conformations can transform into each other without breaking chemical bonds. To transform a molecule with one configuration into a molecule with another configuration requires the breaking of chemical bonds.

Of four types isomerism Alkanes are characterized by two: isomerism of the carbon skeleton and optical isomerism (see part

Chemical bonds in alkanes, their breaking and formation determine the chemical properties of alkanes. C-C and C-H bonds are covalent, simple (s-bonds), practically non-polar, quite strong, therefore:

1) alkanes most often enter into reactions that involve hemolytic cleavage of bonds;

2) compared to organic compounds of other classes, alkanes have low reactivity (for this they are called paraffins- “devoid of properties”). Thus, alkanes are resistant to the action of aqueous solutions of acids, alkalis and oxidizing agents (for example, potassium permanganate) even when boiled.

Alkanes do not react with the addition of other molecules to them, because Alkanes do not have multiple bonds in their molecules.

Alkanes undergo decomposition under strong heating in the presence of catalysts in the form of platinum or nickel, and hydrogen is eliminated from the alkanes.

Alkanes can undergo isomerization reactions. Their typical reaction is substitution reaction, proceeding through a radical mechanism.

Chemical properties

Radical displacement reactions

As an example, consider interaction of alkanes with halogens. Fluorine reacts very vigorously (usually with an explosion) - in this case, all C-H and C-C bonds are broken, and as a result, CF 4 and HF compounds are formed. The reaction has no practical significance. Iodine does not interact with alkanes. Reactions with chlorine or bromine occur either with light or with strong heat; in this case, the formation of mono- to polyhalogen-substituted alkanes occurs, for example:

CH 3 -CH 3 +Cl 2 ® hv CH 3 -CH 2 -Cl + HCl

The formation of methane halogen derivatives proceeds through a chain free radical mechanism. When exposed to light, chlorine molecules break down into inorganic radicals:

Inorganic radical Cl. abstracts a hydrogen atom with one electron from a methane molecule, forming HC1 and the free radical CH3

The free radical interacts with the Cl 2 chlorine molecule, forming a halogen derivative and a chlorine radical.

The oxidation reaction begins with the abstraction of a hydrogen atom by an oxygen molecule (which is a diradical) and then proceeds as a branched chain reaction. The number of radicals increases during the reaction. The process is accompanied

by releasing a large amount of heat, not only the C-H bonds, but also the C-C bonds are broken, so that as a result, carbon monoxide (IV) and water are formed. The reaction may proceed as combustion or lead to an explosion.

2С n Н2 n+2 +(3n+1)О 2 ®2nСО 2 +(2n+2)Н 2 O

At ordinary temperatures, the oxidation reaction does not occur; it can be initiated either by ignition or by electrical discharge.

With strong heating (over 1000°C), alkanes completely decompose into carbon and hydrogen. This reaction is called pyrolysis.

CH 4 ® 1200° C+2H 2

By mild oxidation of alkanes, in particular methane, with atmospheric oxygen in the presence of various catalysts, methyl alcohol, formaldehyde, and formic acid can be obtained.

If methane is passed through a heated zone very quickly and then immediately cooled with water, the result is acetylene.

This reaction is the basis of an industrial synthesis called cracking(incomplete decomposition) of methane.

Cracking of methane homologues is carried out at a lower temperature (about 600°C). For example, propane cracking includes the following stages:

So, cracking of alkanes leads to the formation of a mixture of alkanes and alkenes of lower molecular weight.

Heating alkanes to 300-350°C (cracking has not yet occurred) in the presence of a catalyst (Pt or Ni) leads to dehydrogenation- removal of hydrogen.

When dilute nitric acid acts on alkanes at 140°C and low pressure, a radical reaction occurs:

CH 3 -CH 3 + HNO 3 ®CH 3 -CH 2 -NO 2 + H 2 O Isomerization

Under certain conditions, normal alkanes can transform into branched-chain alkanes.

Preparation of alkanes

Let's consider the production of alkanes using the example of methane production. Methane is widespread in nature. It is the main component of many flammable gases, both natural (90-98%) and artificial, released during the dry distillation of wood, peat, coal, as well as during oil cracking. Natural gases, especially associated gases from oil fields, contain ethane, propane, butane and pentane in addition to methane.

Methane is released from the bottom of swamps and from coal seams in mines, where it is formed during the slow decomposition of plant debris without access to air. Therefore, methane is often called swamp gas or firedamp.

In the laboratory, methane is produced by heating a mixture of sodium acetate and sodium hydroxide:

CH 3 COONa+NaOH® 200 ° Na 2 CO 3 +CH 4

or when aluminum carbide interacts with water: Al 4 Cl 3 +12H 2 O®4Al(OH) 3 +3CH 4

In the latter case, the methane turns out to be very pure.

Methane can be produced from simple substances by heating in the presence of a catalyst:

C+2H 2 ® Ni CH 4 8 also by synthesis based on water gas

CO+3H 2 ® Ni CH 4 +H 2 O

This method is of industrial importance. However, methane from natural gases or gases formed during the coking of coal and during oil refining are usually used.

Homologues of methane, like methane, are obtained in laboratory conditions by calcination of salts of the corresponding organic acids with alkalis. Another method is the Wurtz reaction, i.e. heating monohalogen derivatives with sodium metal, for example:

C 2 H 5 Br + 2Na + BrC 2 H 6 ® C 2 H 5 -C 2 H 5 + 2NaBr

In technology, synthesis is used to produce technical gasoline (a mixture of hydrocarbons containing 6-10 carbon atoms).

from carbon monoxide (II) and hydrogen in the presence of a catalyst (cobalt compound) and at elevated pressure. Process

can be expressed by the equation

nСО+(2n+1)Н 2 ® 200° C n H 2n+2 +nН 2 O

I So, the main sources of alkanes are natural gas and oil. However, some saturated hydrocarbons are synthesized from other compounds.

Applications of alkanes

Most alkanes are used as fuel. Cracking and

Their dehydrogenation leads to unsaturated hydrocarbons, which

from which many other organic substances are obtained.

Methane is the main part of natural gases (60-99%). Part

natural gases include propane and butane. Liquid hydrocarbons

used as fuel in internal combustion engines, cars, airplanes, etc. A purified mixture of liquid

and solid alkanes forms Vaseline. Higher alkanes are

starting materials for the production of synthetic detergents. Alkanes obtained by isomerization are used in the production of high-quality gasoline and rubber. Below is a diagram of the use of methane

Cycloalkanes

Structure

Cycloalkanes are saturated hydrocarbons whose molecules contain a closed ring of carbon atoms.

Cycloalkanes (cycloparaffins) form a homologous series with the general formula C n H 2 n, in which the first member is

cyclopropane C 3 H 6, because To form a ring, at least three carbon atoms must be present.

Cycloalkanes have several names: cycloparaffins, naphthenes, cyclanes, polymethylenes. Examples of some connections:

The formula C n H 2 n is characteristic of cycloparaffins, and exactly the same formula describes the homologous series of alkenes (unsaturated hydrocarbons having one multiple bond). From this we can conclude that each cycloalkane is isomeric with a corresponding alkene - this is an example of “interclass” isomerism.

Cycloalkanes are divided into a number of groups based on ring size, of which we will consider two: small (C 3, C 4) and ordinary (C 5 -C 7) cycles.

The names of cycloalkanes are constructed by adding the prefix cyclo- to the name of the alkane with the corresponding number of carbon atoms. The numbering in the cycle is carried out so that the substituents receive the lowest numbers.

Structural formulas of cycloalkanes are usually written in abbreviated form, using the geometric shape of the ring and omitting the symbols for the carbon and hydrogen atoms. For example:

The structural isomerism of cycloalkanes is determined by the size of the ring (cyclobutane and methylcyclopropane are isomers) and the position of the substituents in the ring (for example, 1,1- and 1,2-dimethylbutane), as well as their structure.

Spatial isomerism is also characteristic of cycloalkanes, because it is associated with different arrangements of substituents relative to the ring plane. When substituents are located on one side of the ring plane, cis-isomers are obtained, and trans-isomers are obtained on opposite sides.