Alkanes are characterized by the presence of Alkanes - definition, structure, physical and chemical properties. – Hydrogenation of unsaturated hydrocarbons

From a chemical point of view, alkanes are hydrocarbons, that is, the general formula of alkanes includes only carbon and hydrogen atoms. In addition to the fact that these compounds do not contain any functional groups, they are formed only by single bonds. Such hydrocarbons are called saturated.

Types of alkanes

All alkanes can be divided into two large groups:

• aliphatic compounds. Their structure has the form of a linear chain, the general formula of aliphatic alkanes is C n H 2n+2, where n is the number of carbon atoms in the chain.
• Cycloalkanes. These compounds have a cyclic structure, which causes a significant difference in their chemical properties from linear compounds. In particular, the structural formula of this type of alkanes determines the similarity of their properties with alkynes, that is, hydrocarbons with a triple bond between carbon atoms.

Electronic structure of aliphatic compounds

This group of alkanes may have either a linear or branched hydrocarbon chain. Their chemical activity is low compared to other organic compounds, since all bonds within the molecule are saturated.

The molecular formula of alkanes of the aliphatic type indicates that their chemical bond has sp 3 hybridization. This means that all four covalent bonds around the carbon atom are absolutely equal in terms of their characteristics (geometric and energy). With this type of hybridization, the electron shells of the s and p levels of carbon atoms have the same shape of an elongated dumbbell.

Between carbon atoms, the bond in the chain is covalent, and between carbon and hydrogen it is partially polarized, while the electron density is drawn to carbon, as to an element more electronegative.

It follows that only C-C and C-H bonds exist in their molecules. The former are formed as a result of the overlap of two electronic hybridized sp 3 orbitals of two carbon atoms, and the latter are formed as a result of the overlap of the s orbital of hydrogen and the sp 3 orbital of carbon. The C-C bond length is 1.54 angstroms and the C-H bond length is 1.09 angstroms.

Geometry of the methane molecule

Methane is the simplest alkane, consisting of only one carbon atom and four hydrogen atoms.

Due to the energy equality of its three 2p and one 2s orbitals, obtained as a result of sp 3 hybridization, all orbitals in space are located at the same angle to each other. It is equal to 109.47°. As a result of such a molecular structure, a similarity of a triangular equilateral pyramid is formed in space.

Simple alkanes

The simplest alkane is methane, which consists of one carbon atom and four hydrogen atoms. Following methane in the alkane series, propane, ethane and butane are formed by three, two and four carbon atoms, respectively. Starting with five carbons in the chain, compounds are named according to the IUPAC nomenclature.

A table with the formulas of alkanes and their names is given below:

With the loss of one hydrogen atom, an alkane molecule forms an active radical, the end of which changes from "an" to "yl", for example, ethane C 2 H 6 - ethyl C 2 H 5. The structural formula of ethane alkane is shown in the photo.

Nomenclature of organic compounds

The rules for determining the names of alkanes and compounds based on them are established by the international IUPAC nomenclature. For organic compounds, the following rules apply:

1. The name of a chemical compound is based on the name of its longest chain of carbon atoms.
2. The numbering of carbon atoms should start from the end, closer to which the chain branching begins.
3. If there are two or more carbon chains of the same length in the compound, then the one that has the fewest radicals is chosen as the main one, and they have a simpler structure.
4. If there are two or more identical groups of radicals in the molecule, then the corresponding prefixes are used in the name of the compound, which double, triple, and so on, the names of these radicals. For example, "3,5-dimethyl" is used instead of "3-methyl-5-methyl".
5. All radicals are written in alphabetical order in the common name of the compound, and prefixes are not taken into account. The last radical is written together with the name of the chain itself.
6. The numbers reflecting the numbers of radicals in the chain are separated from the names by a hyphen, and the numbers themselves are written separated by commas.

Compliance with the rules of IUPAC nomenclature makes it easy to determine the molecular formula of an alkane by, for example, 2,3-dimethylbutane has the following form.

Physical Properties

The physical properties of alkanes largely depend on the length of the carbon chain forming a particular compound. The main properties are the following:

• The first four representatives, according to the general formula of alkanes, are in a gaseous state under normal conditions, that is, they are butane, methane, propane and ethane. As for pentane and hexane, they already exist in the form of liquids, and starting from seven carbon atoms, alkanes are solids.
• With an increase in the length of the carbon chain, the density of the compound also increases, as well as its first-order phase transition temperatures, that is, the melting and boiling points.
• Since the polarity of the chemical bond in the formula of the substance of alkanes is insignificant, they do not dissolve in polar liquids, for example, in water.
• Accordingly, they can be used as good solvents for compounds such as non-polar fats, oils and waxes.
• The home gas stove uses a mixture of alkanes rich in the third member of the chemical series - propane.
• Oxygen combustion of alkanes releases a large amount of energy in the form of heat, so these compounds are used as a combustible fuel.

Chemical properties

Due to the presence of stable bonds in the molecules of alkanes, their reactivity in comparison with other organic compounds is low.

Alkanes practically do not react with ionic and polar chemical compounds. They behave inertly in solutions of acids and bases. Alkanes react only with oxygen and halogens: in the first case, we are talking about oxidation processes, in the second, about substitution processes. They also show some chemical activity in reactions with transition metals.

In all these chemical reactions, the branching of the carbon chain of alkanes, that is, the presence of radical groups in them, plays an important role. The more of them, the stronger the change in the ideal angle between bonds 109.47° in the spatial structure of the molecule, which leads to the creation of stresses inside it and, as a result, increases the chemical activity of such a compound.

The reaction of simple alkanes with oxygen occurs according to the following scheme: C n H 2n+2 + (1.5n+0.5)O 2 → (n+1)H 2O+ nCO 2 .

An example of a reaction with chlorine is shown in the photo below.

The danger of alkanes for nature and man

Heptane, pentane and hexane are highly flammable liquids and are hazardous to both the environment and human health because they are toxic.

The table shows some representatives of a number of alkanes and their radicals.

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

Isomerism of the carbon skeleton

Homologs with four or more carbon atoms are characterized by structural isomerism in terms of changes in the carbon skeleton. Methyl groups -CH 2 can attach to any carbon of the chain, forming new substances. The more carbon atoms in the chain, the more isomers homologues can form. The theoretical number of homologs is calculated mathematically.

Rice. 1. Approximate number of isomers of methane homologues.

In addition to methyl groups, long carbon chains can be attached to carbon atoms, forming complex branched substances.

Examples of isomerism of alkanes:

• normal butane or n-butane (CH 3 -CH 2 -CH 2 -CH 3) and 2-methylpropane (CH 3 -CH(CH 3) -CH 3);
• n-pentane (CH 3 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylbutane (CH 3 -CH 2 -CH (CH 3) -CH 3), 2,2-dimethylpropane (CH 3 -C (CH 3) 2 -CH 3);
• n-hexane (CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3), 2-methylpentane (CH 3 -CH (CH 3) -CH 2 -CH 2 -CH 3), 3-methylpentane ( CH 3 -CH 2 -CH (CH 3) -CH 2 -CH 3), 2,3-dimethylbutane (CH 3 -CH (CH 3) -CH (CH 3) -CH 3), 2,2-dimethylbutane ( CH 3 -C(CH 3) 2 -CH 2 -CH 3).

Rice. 2. Examples of structural isomers.

Branched isomers differ from linear molecules in their physical properties. Branched alkanes melt and boil at lower temperatures than their linear counterparts.

Nomenclature

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

• find the longest chain and name it;
• number the carbon atoms, starting from the end, where there are the most substituents;
• indicate the number of identical substituents with numerical prefixes;
• name substitutes.

The name consists of four parts, going one after another:

• numbers denoting chain atoms that have substituents;
• numerical prefixes;
• the name of the substitute;
• the name of the main circuit.

For example, in a CH 3 -CH (CH 3) -CH 2 -C (CH 3) 2 -CH 3 molecule, the main chain has five carbon atoms. So it's pentane. The right end has more branches, so the numbering of atoms starts from here. In this case, the second atom has two identical substituents, which is also reflected in the name. It turns out that this substance has the name 2,2,4-trimethylpentane.

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

Usually, numerical prefixes from two to four are used: di- (two), tri- (three), tetra- (four).

What have we learned?

Alkanes are characterized by structural isomerism. Structural isomers are common to all homologues, starting with butane. In structural isomerism, substituents are attached to carbon atoms in the carbon chain, forming complex branched chains. The name of the isomer consists of the names of the main chain, substituents, the verbal designation of the number of substituents, the digital designation of the carbon atoms to which the substituents are attached.

Alkanes have a common chemical formula C n H 2n + 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.

Other than this standard definition given by the International Union of Pure and Applied Chemistry, in some authors' usage the term alkane is applied 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 (STiDOS(Standard ambient temperature and pressure)), for which the number of carbon atoms in the carbon chain is greater, which is about 17 times.

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

Alkanes are not very reactive and have little biological activity. They can be thought of as molecular trees from 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 which, like an alkane, consists solely of bonded atoms of acyclically bonded carbon and hydrogen atoms, such as a methyl or ethyl group.

Classification structure

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

• Linear (general formula C n H 2n + 2), in which 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 according to this, in a full, understandable presentation.

Physical properties of alkanes

All alkanes are colorless and odorless.

Table of alkanes.

Boiling point

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

For the strength of van der Waals forces, there are two determinants:

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

Under standard conditions CH 4 to C 4 H 10 alkanes are gaseous; From C 5 H 12 to C 17 H 36 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 come as a surprise that the boiling point has an almost linear relationship with the size (molecular weight) of the molecule. Typically, the boiling point rises by 20-30°C for each carbon added to the chain. This rule applies to other homologous series as well.

In physical chemistry, van der Waals forces (or van der Waals interaction), 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 the 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 either 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 the 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 "snap" together better than cruciform 2,2-dimethylbutane, so 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 give an intermolecular contact plane.

melting points

The melting points of alkanes have a similar tendency to boiling points for the same reason as above. That is, (ceteris paribus) 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. Thus, better bonding of solid structures will require more energy to break. For alkanes, this can be seen in the graph above (i.e. the green line). Odd alkanes have a lower melting tendency than even alkanes. This is because even numbered alkanes fit well in the solid phase, forming a well-organized structure that requires more energy to break. Odd-numbered alkanes stack worse, and therefore an organized compaction structure with more "loose" 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 fit well in the solid phase: this is especially true for isoalkanes (2-methyl isomers), which are often have melting points higher than those of their linear counterparts.

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 the individual water molecules are aligned away from the alkane molecule, the coexistence of the alkane and water results in an increase in molecular order (decreasing entropy). Since there is no significant bonding between water molecules and alkane molecules, the second law of thermodynamics suggests that this decrease in entropy should be minimized by minimizing contact between 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, 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, meaning that the valence electrons are said to be in four equivalent orbitals, derived from a combination of a 2 s orbital and three 2p orbitals. These orbitals, having the same energy, are located spatially in the form of a tetrahedron, the angle between them is cos -1 (- 1/3) ≈ 109.47 °.

Bond lengths and bond angles

The alkane molecule has only C-H and C-C single bonds. The former result from 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 the spatial arrangement of the four sp3 orbitals - they are arranged tetrahedrally with an angle of 109.47° between them. Structural formulas that represent a bond as being at right angles to each other, while both general and useful, are not true.

Conformation

The structural formula and bond angles are usually insufficient to fully describe the geometry of the molecule. There is one more degree of freedom for every 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 twist angles of a molecule is known as its shape.

Ethane forms the simplest case for studying the conformation of alkanes, as there is only one C-C bond. If you look down the axis of the C-C relationship, you will see the so-called Newman projection. The hydrogen atoms on both the front and back carbon atom 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 twist between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the back carbon can freely change from 0° to 360°. This is a consequence of free rotation around a simple carbon-carbon bond. Despite this apparent freedom, only two limiting conformations are important: the eclipsed conformation and the stepped conformation.

The two conformations, also known as rotamers, differ in energy: the chess 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 the 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 another, is on the order of 10 -11 s.

Higher alkanes are more complex but based on similar principles, with the antiperiplanar conformation always being most favorable around each carbon-carbon bond. For this reason, alkanes are usually shown in a zigzag pattern on 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 molecules, since alkane molecules do not have a fixed structural shape, no matter what the model may show.

Spectroscopic properties

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

infrared spectroscopy

The carbon-hydrogen stretching method gives a strong absorption between 2850 and 2960 cm -1 while the carbon-carbon stretching mode absorbs from 800 to 1300 cm -1 . The 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 exhibit weak absorption at about 725 cm -1 .

NMR spectroscopy

Proton resonances of alkanes are usually found at δH = 0.5-1.5. The resonances of carbon 13 depend on the number of hydrogen atoms associated with carbon: δ C \u003d 8-30 (primary, methyl, -CH 3), 15-55 (secondary, methylene, -CH 2 -), 20-60 (tertiary, Metin , C-H) and Quaternary. The carbon-13 resonance of quaternary carbons is weak due to the absence of a nuclear Overhauser effect and a long relaxation time, and can be missed in weak samples or samples that have not been worked for a sufficiently long time.

Mass spectrometry

Alkanes have a high ionization energy, while a molecular ion is usually weak. Fragmentation fragmentation can be difficult to interpret, but in the case of branched alkanes, the carbon chain preferentially cleaves 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 absent, and the other fragment is often separated by intervals of fourteen mass units, which corresponds to the successive loss of CH 2 groups.

Methods for obtaining alkanes

About methods for obtaining alkanes, you can also learn and study on this.

In chemistry, alkanes are called saturated hydrocarbons, in which the carbon chain is open and consists of carbon linked to each other by means of single bonds. Also, a characteristic feature of alkanes is that they do not contain double or triple bonds at all. Sometimes alkanes are called paraffins, the fact is that paraffins are actually a mixture of saturated carbons, that is, alkanes.

Formula of alkanes

The formula for an alkane can be written as:

In this case, n is greater than or equal to 1.

Alkanes are characterized by isomerism of the carbon skeleton. In this case, the connections can take on different geometric shapes, as shown in the picture below.

Isomerism of the carbon skeleton of alkanes

With an increase in the growth of the carbon chain, the number of isomers also increases. For example, butane has two isomers.

Obtaining alkanes

Alkane is usually obtained by various synthetic methods. Let's say one of the ways to get an alkane involves the "hydrogenation" reaction, when alkanes are extracted from unsaturated carbohydrates under the influence of a catalyst and at a temperature.

Physical properties of alkanes

Alkanes differ from other substances in the complete absence of color, and they are also insoluble in water. The melting point of alkanes rises with an increase in their molecular weight and the length of the hydrocarbon chain. That is, the more branched an alkane is, the higher its combustion and melting temperature. Gaseous alkanes burn with a pale blue or colorless flame, while releasing a lot of heat.

Chemical properties of alkanes

Alkanes are chemically inactive substances, due to the strength of strong sigma bonds C-C and C-H. In this case, C-C bonds are non-polar, and C-H are slightly polar. And since all these are low-polarizable types of bonds that belong to the sigma type, they will break according to the homolytic mechanism, resulting in the formation of radicals. And as a consequence, the chemical properties of alkanes are mainly radical substitution reactions.

This is how the formula for radical substitution of alkanes (halogenation of alkanes) looks like.

In addition, it is also possible to distinguish such chemical reactions as the nitration of alkanes (the Konovalov reaction).

This reaction proceeds at a temperature of 140 C, and it is best with a tertiary carbon atom.

Cracking of alkanes - this reaction proceeds under the action of high temperatures and catalysts. Then conditions are created when higher alkanes can break their bonds forming lower-order alkanes.