Homological series definition. homologous series. Homology and structure of compounds

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Rice. 2 - temperature melting and boiling in the homologous series of aliphatic carboxylic acids C 1 ... C 8

Homologous series(from other Greek. ὅμοιος "similar" + λογος "word, law") - a series of chemical compounds of one 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 homologous difference. Homologs- substances that belong to the same homologous series.

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

Homology and structure of compounds

At the heart of the concept of homology in organic chemistry lies the fundamental premise that chemical and physical properties substances are determined by the structure of its molecules: these properties are determined both by 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 itself chemical properties and, accordingly, the belonging of a compound to a certain class is determined precisely by functional groups (for example, the presence of a carboxyl group determines the manifestation by the compound acid properties and its belonging to the class of carboxylic acids), but the degree of manifestation of chemical properties (for example, reactivity and dissociation constant) or physical properties (boiling and melting points, refractive index, etc.) is also affected by the carbon skeleton of the molecule (see Fig. . one).

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

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 homologous difference. Homologs- substances that belong to 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 homological 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 defined as 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, the belonging of a compound to a certain class, is determined precisely by functional groups (for example, the presence of a carboxyl group determines the manifestation of acid properties by the compound and its belonging to 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.) also affects the carbon skeleton of the molecule (see Fig. 1).

In the 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 point).

In the study of parallelisms in the phenomena of hereditary variability, N. I. Vavilov, by analogy with homological series organic compounds, the concept was introduced 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 that adjutant dancer who started the ball, and still young people, and Natasha, passing her excess gentlemen to Sonya, happy and flushed, did not stop dancing the whole evening. She did not notice 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 with such and such a lady, how the prince did such and such and said how Helen had great success and received special attention to 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 supper, Prince Andrei again danced with Natasha. He reminded her of their first meeting in Otradnenskaya Alley and how she could not fall asleep on a moonlit night, and how he could not help hearing 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. And such was Natasha, with her surprise, joy and timidity, and even mistakes in French. He spoke with her especially tenderly and carefully. Sitting beside her, talking to her about the simplest and most insignificant subjects, Prince Andrei admired the joyful gleam in her eyes and smile, which related not to spoken speeches, but to her inner happiness. While Natasha was chosen and she got up with a smile and danced around the hall, Prince Andrei admired in particular her timid grace. In the middle of the cotillion, Natasha, having finished the figure, still breathing heavily, approached her place. The new gentleman again invited her. She was tired and out of breath, and apparently thought of refusing, but immediately again cheerfully raised her hand on the cavalier's shoulder and smiled at Prince Andrei.
“I would be glad to rest and sit with you, I am 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 pieces.
“If she comes first to her cousin, and then to another lady, then she will be my wife,” Prince Andrei said quite unexpectedly to himself, looking at her. She went first to her cousin.
“What nonsense sometimes comes to mind! thought Prince Andrei; but it’s only true 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 corsage, sat down beside him.
At the end of the cotillion, the old count in his blue tailcoat approached the dancers. He invited Prince Andrei to his place and asked his daughter if she was having fun? Natasha did not answer and only smiled with such a smile that said reproachfully: "How could you ask about this?"
- So much fun, like never before in my life! - she said, and Prince Andrei noticed how quickly her thin hands rose to hug her father and immediately fell. Natasha was as happy as ever in her life. She was on that higher level happiness, when a person becomes completely trusting and does not believe in the possibility of evil, misfortune and grief.

Pierre at this ball for the first time felt insulted by the position that his wife occupied in higher spheres. He was sullen and distracted. There was a wide crease across his forehead, and he, standing at the window, looked through his glasses, seeing no one.
Natasha, on her way to dinner, walked past him.
The gloomy, unhappy face of Pierre struck her. She stopped in front of him. She wanted to help him, to convey to him the surplus 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 am very glad,” he said.
“How can they be dissatisfied with something,” thought Natasha. Especially one as good as this Bezukhov?” In Natasha's eyes, all those who were at the ball were equally kind, sweet, beautiful people loving each other: no one could offend each other, and therefore everyone should have been happy.

The next day, Prince Andrei remembered yesterday's ball, but did not dwell on it for a long time. “Yes, the ball was very brilliant. And yet ... yes, Rostova is very nice. There is something fresh, special, not Petersburg, which 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 good for classes, and Prince Andrei could not do anything), he criticized his own work, as often happened to him, and was glad when he heard that someone had arrived.
The visitor was Bitsky, who served in various commissions, visited all the societies of St. Petersburg, a passionate admirer of new ideas and Speransky, and an anxious news reporter of St. Petersburg, one of those people who choose a trend like a dress - according to fashion, but who for this reason seem to be the most ardent partisans of trends . He anxiously, barely having time to take off his hat, ran to Prince Andrei and immediately began to speak. He just learned the details of the meeting state council this morning, opened by the sovereign, and enthusiastically talked about it. The emperor's speech was extraordinary. It was one of those speeches only given by constitutional monarchs. “The sovereign directly said that the council and the senate are state estates; he said that government should not be based on arbitrariness, but on firm principles. The sovereign said that the finances should be transformed and the reports should be made public,” Bitsky said, hitting on well-known words and opening his eyes significantly.

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

What is a homologous series

A homologous series is a chemical compound that has a similar structural type, but differs in the number of repetitions of the elementary units of the substance. Difference structural components, namely identical units, and is called the homological difference. Homologues are substances that are in the same homologous series.

Examples of homologues are alcohols, alkanes, alkynes, ketones. If we consider the homologous series using the example of alkanes - the simplest representatives (characteristic formula: C n H 2 n + 2), then we see similarities in the structure of a number of representatives of this kind of substances: 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 the homology of substances in organic chemistry is based on the understanding that both physical and chemical quality characteristics 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, the belonging of a homologue to a particular class by its functional group. As an example, you can pay attention to the carboxyl group, which is responsible for the manifestation of acidic properties and the belonging of the substance to carboxylic acids. However, it is possible to determine the level of manifestation of chemical or physical qualities 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 the class of compounds of the same species. Similar types of homologues are the closest.

The homologous series of substances has some general patterns of change in properties from younger representatives to older ones. A similar phenomenon can be violated, which is associated with the formation of hydrogen bonds in the presence of a group that can form them.

Homology of aldehydes

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 elemental 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 have occurred as a result of the replacement of the paraffinic hydrocarbon atom H with an aldehyde group. For such chemical compounds, isomerism and homology are similar to other derivatives of saturated monosubstituted hydrocarbons.

The name of aldehydes is compiled depending 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 - isobutyric aldehyde, etc.

Alkyne homology

Alkynes are hydrocarbon chemical compounds that carry triple bonds between C atoms. They form a number 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: ethyne (C2H2), propyne (C3H4), butyne (C4H6), pentyne (C5H8), hexine (C6H10), heptine (C7H12), octine (C8H14), nonine (C9H16), decine (C10H18).

The physical properties of alkynes are defined in a similar way to alkenes. For example, the boiling and melting point gradually increases with increasing length of the dominant carbon chain and molecular weight. The chemical properties include the reactions of halogenation, hydrohalogenation, hydration, polymerization. Alkynes are also characterized by substitution reactions.

Homology in biology

The homologous series is used in biology, but has a slightly different character. 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 traits for other, related species. As in chemical table 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 of a valuable nature. This law was formulated through the study of parallelisms, manifested in the hereditary variability of generations.

Conclusion

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

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 defined as functional groups of the compound (hydroxyl of alcohols, carboxyl group of carboxylic acids, aryl group of aromatic compounds, etc.) , and its carbon skeleton.

homologous series

Rice. 2 - Melting points (blue) and boiling points (violet) in the homologous series of aliphatic carboxylic acids C1…C8.

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 "homologous difference". Most often these are methylene units: ... -CH 2 - ... The simplest example of a homologous series is the lower homologues of alkanes (general formula C n H 2n + 2): methane CH 4, ethane C 2 H 6, propane C 3 H 8, etc. .

Homology and structure of compounds

In the study of 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.