Saturated (saturated) hydrocarbons hydrocarbons are called hydrocarbons in whose molecules carbon atoms are connected to each other by a simple bond, and all valency units not spent on the bond between carbon atoms are saturated with hydrogen atoms.

Representatives of saturated hydrocarbons are methane CH 4 ; ethane C 2 H 6 ; propane C 3 H 8; butane C4H10; pentane C5H12; hexane C 6 H 14 . However, this series can be continued. There are carbohydrates C 30 H 62, C 50 H 102, C 70 H 142, C 100 H 202.

If we consider the hydrocarbons of the methane series, it is easy to notice that each subsequent hydrocarbon can be produced from the corresponding previous one by replacing one hydrogen atom with a CH 3 (methyl) group. Thus, the composition of the subsequent hydrocarbon molecule is increased by the CH 2 group.

A series of chemical compounds of the same structural type, differing from each other by one or more structural units (usually the CH 2 group), called homological series and each of the carbohydrates – member of a homologous series or homologue. If we arrange the homologues in increasing order of their relative molecular weight, they form a homologous series.

The CH 2 group is called homologous difference or homologous difference. The general formula of saturated hydrocarbons is C n H 2 n + 2, where n – number of carbon atoms in a molecule.

If a hydrogen atom is removed from a hydrocarbon molecule, the remainder of the molecule with an open bond is called a hydrocarbon radical (denoted by the letter R). Due to their high reactivity, radicals do not exist in their free form.

Homology phenomenon– the existence of series of organic compounds in which the formula of any two neighbors of the series differs by the same group (most often CH 2). The physicochemical properties of compounds change along the homologous series. In organic chemistry, the concept of homology is based on the fundamental idea that the chemical and physical properties of a compound are determined by the structure of its molecules: these properties are determined by both the functional groups of the compound and its carbon skeleton.

The entire complex of chemical properties and, therefore, the assignment of a compound to a certain class is determined precisely by the functional groups, but the degree of manifestation of chemical or physical properties depends on the carbon skeleton of the molecule.

In the absence of isomerism, in the case of similarity of carbon skeletons of compounds, the formula of homologous compounds can be written as X – (CH 2)n – Y, compounds with different numbers n of methylene units are homologues and belong to the same class of compounds. So, homologous compounds belong to the same class of compounds, and the properties of the closest homologues are the closest.

In homologous series There is a certain regular change in properties from the younger members of the series to the older ones, but this pattern is not always observed, in some cases it may be violated. Most often this occurs at the beginning of the series, because hydrogen bonds are formed in the presence of functional groups capable of forming them.

An example of a homologous series is a series of saturated hydrocarbons (alkanes). Its simplest representative – methane CH4. Homologues of methane are: ethane C 2 H 6 ; propane C 3 H 8; butane C4H10; pentane C5H12; hexane C 6 H 14, heptane C 7 H 16, octane – C 8 H 18, nonane – C 9 H 20, decane – C 10 H 22, undecane – C 11 H 24, nodecane – C12H26, tridecane – C13H28, tetradecane – C 14 H 30, pentadecane – C 15 H 32, eicosane – C 20 H 42, pentacosane – C 25 H 52, triacontane – C 30 H 62, tetracontane – C 40 H 82, hectane – C 100 H 202.

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In this article, the reader will find information about homologous compounds and find out what they are. General properties, formulas of substances and their names, characteristics will be considered. In addition, not only the chemical understanding of homologs will be affected, but also the biological one.

What is a homologous series

Homologous series are chemical compounds that have a similar structural type, but differ in the number of repetitions of elementary units of the substance. The difference in structural components, namely identical units, is called homological difference. Homologues are substances that are in the same homologous series.

Examples of homologues include alcohols, alkanes, alkynes, and ketones. If we consider a homologous series using the example of alkanes - the simplest representatives (characteristic formula: C n H 2 n + 2), we see similarities in the structure of a number of representatives of this type of substance: methane CH4, ethane C2H6, propane C3H8 and so on; CH2 methylene units are a homologous difference in a number of these substances.

General ideas about the structure and homology of compounds

The idea of ​​homology of substances in organic chemistry is based on the understanding that both physical and chemical qualitative characteristics of substances can be determined by their molecular structure. The properties of homologous compounds may depend on the structure of the carbon skeleton and the functional group of a particular compound.

It is possible to determine the chemical properties and, therefore, whether a homologue belongs to a specific class by its functional group. As an example, we can pay attention to the carboxyl group, which is responsible for the manifestation of acidic properties and the substance’s belonging to carboxylic acids. However, the level of manifestation of chemical or physical qualities can be determined by studying not only the functional group, but also the carbon molecular skeleton.

There are compounds in which the carbon skeletons are similar, in other words, there is no isomerism in them. Such homologues are written as follows: X - (CH 2) n - Y. The number of methylene n-unit units is homologous and belongs to a class of compounds of the same type. Similar types of homologs are the closest.

The homologous series of substances has some general patterns of changes in properties from younger to older representatives. This phenomenon can be disrupted, which is associated with the formation of a hydrogen bond in the presence of a group that can form them.

Aldehyde homology

Aldehydes are a series of organic compounds containing an aldehyde group - COH. In substances of this type, the carboxyl group is interconnected with a hydrogen atom and one radical group.

The homologous series of aldehydes has the general formula R-COH. One of the elementary representatives is formaldehyde (H-COH), in which the aldehyde group is bonded to H. In other, limiting representatives of this series of compounds, the hydrogen atom is replaced by an alkyne. General formula: C n C 2 n+1 -COH.

Aldehydes are considered as substances that result from the replacement of the H atom in a paraffinic hydrocarbon by an aldehyde group. For such chemical compounds, isomerism and homology are similar to other derivatives of saturated monosubstituted hydrocarbons.

The name of aldehydes is based on the name of the acid with the same number of carbon atoms in the molecule, for example: CH3-CHO - acetaldehyde, CH3CH2-CHO - propionic aldehyde, (CH3)2CH-CHO - isobutyraldehyde, etc.

Alkyne homology

Alkynes are hydrocarbon chemical compounds that contain triple bonds between C atoms. They form a series of homologues with the characteristic formula C n H 2 n-2. A common feature of the position of the carbon atom with a triple number of bonds is the state of sp-hybridization.

Homologous series of alkynes: ethyn (C2H2), propyne (C3H4), butyne (C4H6), pentine (C5H8), hexine (C6H10), heptine (C7H12), octine (C8H14), nonine (C9H16), decine (C10H18).

The physical properties of alkynes are determined in a similar way to alkenes. For example, boiling and melting points gradually increase with increasing carbon chain length and molecular weight. Chemical properties include halogenation, hydrohalogenation, hydration, and polymerization reactions. Alkynes are also characterized by substitution reactions.

Homology in biology

The homologous series is used in biology, but is of a slightly different nature. N.I. Vavilov discovered the law according to which the origin of species and even genera of plants that are similar to each other entails the flow of variability along parallel paths. Genera and species characterized by genetically similar hereditary changes can serve as a way to determine changes in the manifestation of characters for other, related species. As in the chemical table of D.I. Mendeleev, the homological law makes it possible to determine and predict the existence of unknown taxonomic units of plants with selective features that are valuable. This law was formulated through the study of parallelisms manifested in the hereditary variability of generations.

Conclusion

The homologous series of substances, characterized by a common formula structure, but differing in homological differences, has allowed man to increase the chemical potential of substances, to discover and obtain many new compounds used in all spheres of life. Better understand the fundamental phenomenon that physical and chemical quality characteristics can be determined by the molecular structure of a compound.

ALKANE

Saturated hydrocarbons ( alkanes ) are compounds consisting of carbon and hydrogen atoms connected to each other only by σ-bonds and not containing rings. In alkanes, the carbon atoms are in the degree of hybridization sp 3 .

1. The concept of homological series

The simplest compound of this class is methane, a hydrocarbon containing one carbon atom and four hydrogen atoms. Considering the formula of ethane - a saturated hydrocarbon with two carbon atoms, we see that from a formal point of view it is, as it were, formed from methane: one of the equivalent CH bonds is broken and a -CH 2 - group is inserted instead of the break. In the same way, a saturated hydrocarbon with three carbon atoms can be formed from ethane - propane, etc.:

Such a series of compounds similar in structure, possessing similar chemical properties, in which individual members of the series differ from each other only in the number of -CH 2 - groups, is called homologous series . In this case we are talking about a homologous series of alkanes.

For members of any homologous series (for example, a series of alcohols, aldehydes or acids), the vast majority of reactions proceed in the same way (with the exception of sometimes only the first members of the series). Consequently, knowing the chemical reactions of only one member of a homologous series, it can be stated with a high degree of probability that the same type of transformation occurs with the remaining members of this series.

This once again emphasizes that the properties of an organic compound are determined mainly by the functional group, which makes it possible to systematize reactions according to homologous series, or, as is often said, according to classes of organic compounds. A functional group is usually considered to be the part of the molecule of an organic compound that most easily changes in reactions, usually containing atoms and groups other than C and H, or multiple bonds.

For any homologous series, a general formula can be derived that reflects the relationship between the carbon and hydrogen atoms of the members of this series; this formula is called general formula of the homologous series . Having examined the structural formula of any member of the homologous series of saturated hydrocarbons with an unbranched carbon chain, we see that its molecule consists of P groups -CH 2 - and two more hydrogen atoms at the terminal groups. Thus, on P carbon atoms in it are (2p+ 2) hydrogen atoms, therefore, the general formula of the homologous series is C n H 2 n +2.

Table 19 shows the members of the homologous series of saturated hydrocarbons and their physical constants.

2. Isomerism

If two or more individual substances have the same quantitative composition, i.e. the same molecular formula, but differ from each other in some chemical or physical properties, then in general they are called isomers .

One type of isomerism is structural isomerism , when isomers differ from each other in the order of bonds between individual atoms in the molecule.

In methane, ethane and propane, there is only one single order of bonds between the atoms. But already four carbon atoms can be connected in two different ways:

In both cases, hydrocarbons have the same molecular formula C 4 H 10. However, in the first case, all four carbon atoms form an unbranched, or normal, chain, and in the second, a branched at the end, or an isostructured chain. These are different substances: butane and isobutane, which have different physical constants (see Table 19).

For the hydrocarbon C 5 H 12, there are already three isomers

As the number of carbon atoms in a hydrocarbon molecule increases, the number

isomers increases rapidly: for C 6 it is 5; for C 7 - 9; for C 8 - 18; for C 20 - 366 319; for C 40 - 62 491 178 805 831 isomer. This type of isomerism is sometimes called isomerism of the carbon skeleton.

Let's consider a branched hydrocarbon with this structure:

This hydrocarbon has four different types of carbon atoms. Atoms marked with the symbol C a are connected to one carbon atom, they are called primary respectively, the three hydrogen atoms at the primary carbon atom are called primary. A carbon atom, indicated by the symbol C b, is connected to two carbon atoms, it is called secondary, and its two hydrogen atoms are called secondary hydrogen atoms. The C atom is called tertiary, as well as the only hydrogen atom with it; and carbon atom C g - Quaternary.

Alkanes are a class of hydrocarbons with the general formula C n H 2n+2. Related compounds that differ by one methylene group -CH 2 - form a homologous series of alkanes. The simplest substance in the series is methane with one carbon atom (CH 4).

Homologues

Related compounds - homologues - are similar chemically, but have different physical properties. Depending on the number of carbon atoms, gaseous, liquid, and solid alkanes are distinguished. The first four representatives are gases, homologs with 5-15 carbon atoms - flammable liquids. Higher alkanes are waxes and solids with 16-390 carbon atoms.

Rice. 1. Combustion of methane.

The names of alkanes are distinguished by the suffix -ane after the Greek numeral designation:

• un- or gen- - one;
• to- - two;
• three- - three;
• tetra- - four;
• pent- - five;
• hex - six;
• hept- - seven;
• Oct- - eight;
• non- - nine;
• Dec- - ten.

The names of the first four homologues have been fixed historically. Every tenth name “moves on” to the next nine substances, retaining the numeral prefixes and class suffix. The table of the homologous series of alkanes describes the first 20 homologues.

Name	Formula	Physical properties
		Gases. Burn with a blue flame, releasing a large amount of heat
		Flammable oily liquids. Contained in oil. Used to produce liquid fuel - gasoline, kerosene, fuel oil
Tridecan
Tetradecane
Pentadecane
Hexadecane		Waxes and solids. Used to make Vaseline, paraffin
Heptadecane
Octadecan
Nanadekan

The melting and boiling points of alkanes increase with increasing number of carbon atoms and, accordingly, molecular weight. Moreover, all alkanes have a density less than unity. Alkanes float on the surface of water and dissolve only in organic solvents.

Isomers

Alkanes are non-cyclic saturated hydrocarbons. Molecules are long or branched carbon chains. Homologous alkanes can form isomers. The more carbon atoms, the more isomer variants. The first three alkanes (methane, ethane, propane) do not form isomers. Butane, pentane, hexane have only structural isomers. Butane has two: n-butane and isobutane. Pentane forms n-pentane, isopentane, neopentane. Hexane has five isomers: n-hexane, isohexane, 3-methylpentane, diisopropyl, neohexane.

Homologs from heptane and above, in addition to structural isomers, form stereoisomers or spatial isomers, differing in the position of atoms in space. The two molecules are identical in structure and structure, but look like an object and its mirror image.

Rice. 2. Stereoisomers.

Long names of isomers are compiled according to the international IUPAC nomenclature. The verbal designation consists of three parts:

• numbers and prefixes indicating the number of affiliated groups;
• group names;
• names of the main (longest) chain.

For example, the name of the heptane isomer, 2,3-dimethylpentane, indicates that the molecule consists of five carbon atoms (pentane) and two methyl groups attached to the second and third carbon atoms.

Structural formulas are used to display the structure of isomers. The methyl group -CH 3 is written either with a bar up or down from the carbon atom, or in parentheses after the -CH 2 group in the carbon chain. For example, H 3 C-CH 2 -CH(CH 2 CH 3)-CH 2 -CH 3.

Rice. 3. Structural formula.

The number of isomers for each alkane can be calculated mathematically. Therefore, many isomers exist only in theory. It is assumed that hectane (C 100 H 202) can have 592 107 ∙ 10 34 isomers, and this is far from the last alkane in the homologous series.

What have we learned?

Alkanes are formed by the homologous series of methane with the general formula C n H 2n+2. Each subsequent homologue differs from the previous one by one CH 2 group. With an increase in carbon atoms in a homologous series, the physical state of substances changes. Higher alkanes are compounds containing more than 15 carbon atoms. These are solids. Liquids contain 5-15 carbon atoms, gases - 1-4. Starting from the fourth homologue, all alkanes form structural isomers. In addition, alkanes from heptane and above can form stereoisomers.

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