Formulas of homologous series of organic compounds. Homologous series

The general formula of the homologous series of alkanes is CnH2n+2. Homologous series can be constructed for all classes of organic compounds. The composition of the molecules of all members of the homologous series can be expressed by one general formula. For the considered homologous series of saturated hydrocarbons, this formula will be CnH2n+2, where n is the number of carbon atoms. HOMOLOGICAL SERIES - organic compounds related in chemical composition (for example, alcohols).

CH3 - CH3, ethylene CH2 = CH2, acetylene CH ≡ CH. Genetic series are groups of organic ones. CH2 (so-called homologous difference) in the molecule. Homologues arranged in increasing order of their relative molecular weight form a homologous series. The CH2 group is called a homological difference.

The homologous series of alkanes can be easily compiled by adding each time a new carbon atom to the previous chain and supplementing its remaining valencies with up to 4 hydrogen atoms. All organic compounds, depending on the nature of the carbon skeleton, can be divided into acyclic and cyclic. Functional groups are groups of atoms that determine the chemical properties of a given class of compounds.

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The radical resulting from the abstraction of one hydrogen atom from a saturated hydrocarbon molecule is called an alkyl, the general formula of alkyls is CnH2n+1. In both formulas, the C atoms of the ring and the H atoms not participating in the reaction are omitted (for brevity). Using the example of one pair of isomers, indicate the similarities and differences between these compounds.

Thus, the boiling temperatures of neighboring members in the middle of the river. (for compounds with an unbranched chain) differ by approximately 20-25 °C (in the higher members of the G. R. this value gradually decreases). A well-known example is ALKANES of the hydrocarbon group. Thus, homologous compounds belong to the same class of compounds, and the properties of the closest homologues are most similar.

Lesson 4. Covalent bonds of organic compounds

Compounds that are similar in chemical properties, but whose composition differs from each other by the CH2 group, are called homologues. The suffix -an is characteristic of the names of all alkanes. Having the same qualitative composition and the same type of chemical bonds, homologues have similar chemical properties.

Bond lengths and bond angles in CCl4 and C2H6 molecules

Classification of organic compounds. Carbocyclic compounds are divided: 1) into alicyclic (saturated and unsaturated), similar in properties to aliphatic ones; 2) aromatic - contain benzene rings.

The presence of these groups makes it possible to divide the types of organic compounds into classes and facilitate their study. In polyalkenes with conjugated (alternating) double bonds, delocalization of π electrons occurs. Compounds with double bonds polymerize easily. Alkynes (acetylene hydrocarbons) contain a triple bond. The general formula of an alkyne with one triple bond is CnH2n–2.

5 atoms) and get the basis of the name (5 - pentane). Thus, in our example, the alkane would be called 2,3-dimethylpentane. In the above reaction the product will be 2-iodopropane CH3CH(I)CH3. According to Markovnikov's rule, the hydration reaction also occurs, i.e., the reaction of addition of water in the presence of sulfuric acid.

Polymethylbutadiene rubber is a polymer that exists in nature (natural rubber), and polybutadiene rubber is produced artificially (S.V. Lebedev, 1932) and is called synthetic rubber. In alkynes, the carbon atomic orbitals at the triple bond are sp-hybridized (linear structure).

Alkanes (paraffins) are compounds of carbon with hydrogen, in the molecules of which the carbon atoms are connected to each other by a single bond (saturated hydrocarbons). HOMOLOGICAL SERIES - HOMOLOGICAL SERIES, groups of organic compounds with the same chemical. function, but differing from each other in one or more methylene (CH2) groups. CH2-. The -CH2- group is called a homologous difference.

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.

Material from Wikipedia - the free encyclopedia

Homologous series- a number of chemical compounds of the same structural type (for example, alkanes or aliphatic alcohols - fatty alcohols), differing from each other in composition by a certain number of repeating structural units - the so-called homological difference. Homologues- substances included in the same homologous series.

The simplest example of a homologous series is alkanes (general formula C n H 2n + 2): methane CH 4, ethane C 2 H 6, propane C 3 H 8, etc.; the homologous difference of this series is the methylene unit -CH 2 -.

Homology and structure of compounds

The concept of homology in organic chemistry is based on the fundamental position that the chemical and physical properties of a substance are determined by the structure of its molecules: these properties are determined by both the functional groups of the compound (hydroxyl alcohols, carboxyl group of carboxylic acids, aryl group of aromatic compounds, etc.) , and its carbon skeleton.

The complex of chemical properties itself and, accordingly, whether a compound belongs to a certain class is determined precisely by functional groups (for example, the presence of a carboxyl group determines whether the compound exhibits acidic properties and its membership in the class of carboxylic acids), but on the degree of manifestation of chemical properties (for example, reactivity and dissociation constant) or physical properties (boiling and melting points, refractive index, etc.) are also influenced by the carbon skeleton of the molecule (see Fig. 1).

In the case of similarity of carbon skeletons of compounds, that is, the absence of isomerism, the formula of homologous compounds can be written as X-(CH 2) n -Y, compounds with different numbers n methylene units are homologues and belong to the same class of compounds (for example, H-(CH2)n-COOH- aliphatic carboxylic acids). Thus, homologous compounds belong to the same class of compounds, and the properties of the closest homologues are most similar.

In homologous series, there is a regular change in properties from the younger members of the series to the older ones, however, this pattern can be violated, first of all, at the beginning of the series, due to the formation of hydrogen bonds in the presence of functional groups capable of their formation (see Fig. 2, melting temperature).

When studying parallelisms in the phenomena of hereditary variability, N.I. Vavilov, by analogy with the homologous series of organic compounds, introduced the concept Homologous series in hereditary variability.

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An excerpt characterizing the homological series

After Prince Andrei, Boris approached Natasha, inviting her to dance, and the adjutant dancer who started the ball, and more young people, and Natasha, handing over her excess gentlemen to Sonya, happy and flushed, did not stop dancing the whole evening. She did not notice anything and did not see anything that occupied everyone at this ball. She not only did not notice how the sovereign spoke for a long time with the French envoy, how he spoke especially graciously to such and such a lady, how prince such and such did and said this, how Helen was a great success and received special attention from such and such; she did not even see the sovereign and noticed that he left only because after his departure the ball became more lively. One of the merry cotillions, before dinner, Prince Andrei danced with Natasha again. He reminded her of their first date in the Otradnensky alley and how she could not sleep on a moonlit night, and how he involuntarily heard her. Natasha blushed at this reminder and tried to justify herself, as if there was something shameful in the feeling in which Prince Andrei involuntarily overheard her.
Prince Andrei, like all people who grew up in the world, loved to meet in the world that which did not have a common secular imprint on it. And such was Natasha, with her surprise, joy and timidity and even mistakes in the French language. He treated and spoke to her especially tenderly and carefully. Sitting next to her, talking with her about the simplest and most insignificant subjects, Prince Andrei admired the joyful sparkle of her eyes and smile, which related not to the speeches spoken, but to her inner happiness. While Natasha was being chosen and she stood up with a smile and danced around the hall, Prince Andrei especially admired her timid grace. In the middle of the cotillion, Natasha, having completed her figure, still breathing heavily, approached her place. The new gentleman invited her again. She was tired and out of breath, and apparently thought of refusing, but immediately again cheerfully raised her hand on the gentleman’s shoulder and smiled at Prince Andrey.
“I would be glad to rest and sit with you, I’m tired; but you see how they choose me, and I’m glad about it, and I’m happy, and I love everyone, and you and I understand all this,” and that smile said a lot more. When the gentleman left her, Natasha ran across the hall to take two ladies for the figures.
“If she approaches her cousin first, and then another lady, then she will be my wife,” Prince Andrei said to himself quite unexpectedly, looking at her. She approached her cousin first.
“What nonsense sometimes comes to mind! thought Prince Andrey; but the only thing that is true is that this girl is so sweet, so special, that she won’t dance here for a month and get married... This is a rarity here,” he thought when Natasha, straightening the rose that had fallen back from her bodice, sat down next to him.
At the end of the cotillion, the old count approached the dancers in his blue tailcoat. He invited Prince Andrei to his place and asked his daughter if she was having fun? Natasha did not answer and only smiled a smile that reproachfully said: “How could you ask about this?”
- More fun than ever in my life! - she said, and Prince Andrei noticed how quickly her thin arms rose to hug her father and immediately fell. Natasha was as happy as she had never been in her life. She was at that highest level of happiness when a person becomes completely trusting and does not believe in the possibility of evil, misfortune and grief.

At this ball, Pierre for the first time felt insulted by the position that his wife occupied in the highest spheres. He was gloomy and absent-minded. There was a wide crease across his forehead, and he, standing at the window, looked through his glasses, not seeing anyone.
Natasha, heading to dinner, passed him.
Pierre's gloomy, unhappy face struck her. She stopped in front of him. She wanted to help him, to convey to him the excess of her happiness.
“How fun, Count,” she said, “isn’t it?”
Pierre smiled absently, obviously not understanding what was being said to him.
“Yes, I’m very glad,” he said.
“How can they be unhappy with something,” Natasha thought. Especially someone as good as this Bezukhov?” In Natasha’s eyes, everyone at the ball were equally kind, sweet, wonderful people who loved each other: no one could offend each other, and therefore everyone should be happy.

The next day, Prince Andrei remembered yesterday's ball, but did not dwell on it for long. “Yes, it was a very brilliant ball. And also... yes, Rostova is very nice. There is something fresh, special, not St. Petersburg, that distinguishes her.” That's all he thought about yesterday's ball, and after drinking tea, he sat down to work.
But from fatigue or insomnia (the day was not a good one for studying, and Prince Andrei could not do anything), he kept criticizing his own work, as often happened to him, and was glad when he heard that someone had arrived.
The visitor was Bitsky, who served on various commissions, visited all the societies of St. Petersburg, a passionate admirer of new ideas and Speransky and a concerned messenger of St. Petersburg, one of those people who choose a direction like a dress - according to fashion, but who for this reason seem to be the most ardent partisans of directions . He worriedly, barely having time to take off his hat, ran to Prince Andrei and immediately began to speak. He had just learned the details of the meeting of the State Council this morning, opened by the sovereign, and was talking about it with delight. The sovereign's speech was extraordinary. It was one of those speeches that are given only by constitutional monarchs. “The Emperor directly said that the council and the senate are state estates; he said that government should not be based on arbitrariness, but on solid principles. The Emperor said that finances should be transformed and reports should be made public,” said Bitsky, emphasizing well-known words and significantly opening his eyes.

Differing in similar properties and united by a general formula that describes the pattern of structural differences between each subsequent member of the group and the previous one. For example, a homologous series of alkanes, or other groups. is of great importance for research, forecasting or practical application. For organic substances combined into a group, regular changes in chemical and physical properties are observed, and all of them correlate with changes in molecular weight.

Equally important are the rules that describe how the properties of substances change when moving from one group to another. To understand what a homological series is, we should consider specific examples. Any group of compounds is characterized by increasing melting (crystallization), boiling (condensation) and density temperatures with increasing molecular weight and the number of carbon atoms in the molecule.

Called saturated or paraffinic; they are acyclic (no cycles) compounds of normal or branched structure, the atoms in the molecules of which are connected by single bonds. The general formula is CnH2n+2 and describes the homologous series of alkanes. The molecule of each next member increases compared to the previous one by one C atom and two H atoms. These include:

methane;
ethane;
propane and so on.

This also includes cycloparaffins. This is a large group of organic compounds whose molecules are closed in rings. Their homologous series has the formula CnH2n, starting with a chemical substance with three carbon atoms. Examples of cycloparaffins:

cyclopropane;
cyclobutane;
cyclopentane and so on

Unsaturated or unsaturated hydrocarbons are also acyclic. These include substances of normal and isostructure. The homologous series of alkenes has the general formula CnH2n. These compounds are distinguished by the presence of one double bond between two carbon atoms. If the previous series began with a hydrocarbon with one carbon atom (methane), then this one begins with a substance whose molecule contains two carbon atoms. Examples of alkenes:

ethene;
propene;
butene and so on.

Hydrocarbons, in the molecule of which two carbon atoms are connected by a triple bond, are even more unsaturated, otherwise they are called acetylene. They are united by a homologous series of alkynes. It is described by the formula CnH2n-2 and starts with acetylene, which has two C atoms in the formula. Examples of alkynes:

ethin;
propyne;
butine-1 and so on.

Unsaturated acyclic hydrocarbons, the molecule of which has two double bonds, are called diene. They have the general formula CnH2n-2. Their homologous series begins with a hydrocarbon with three carbon atoms in the molecule. Double bonds can be conjugated (separated by one single bond), cumulated (located on adjacent atoms), or isolated (separated by several single bonds). Examples of dienes:

1,2-propadiene;
1,3-butadiene;
isoprene and so on

A special group is formed by a cyclic structure, the molecule of which contains a benzene ring. The homologous series of the simplest aromatic hydrocarbons begins with a compound with six carbon atoms - benzene. series are formed when one or more hydrogen atoms attached to the benzene ring are replaced by radicals. Thus, a number of substances are obtained: benzene, toluene, xylene. If a molecule has two or more substituents, then they speak of the presence of isomers for these substances. Other homologous series of aromatic compounds are formed from naphthalene, anthracene and other substances.

If a hydrocarbon molecule contains a functional group, then such chemical compounds also form a homologous series.

A number of alcohols are distinguished by the presence of a hydroxyl group (—OH) in the molecule. For monohydric alcohols, one hydrogen atom in the acyclic hydrocarbon is replaced by a hydroxyl group; their formula: CnH2n+1OH. There are also rows
A number of phenols are characterized by the presence of a hydroxyl group (—OH) in the molecule, but it replaces hydrogen in the benzene ring.
A number of aldehydes are distinguished by the presence in the molecule of a chemical compound of a carbonyl group (>C=O); general formula of aldehydes: R—CH=O.
A number of ketones are also distinguished by the presence of a carbonyl group (>C=O), but if in aldehydes it is connected to one radical, then in hydrocarbon ketones there are two radicals. Ketone formula: R1-CO-R2.
A number of carboxylic acids are distinguished from other chemical substances by the carboxyl group, which combines carbonyl and hydroxyl groups. Formula - RCOOH.

For each series, be it a homologous series of aldehydes, carboxylic (organic) acids, alcohols or other substances, their properties will largely be determined by the type of functional group and will naturally change as the molecular weight of the substance increases. This classification of a broad class of chemical compounds helps to understand the nature and study their properties.