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Hydrogen communications(N-Communication) is a special type of interaction between reaction-capable groups, while one of the groups contains a hydrogen atom prone to such interaction. Hydrogen bond is a global phenomenon covering all chemistry. In contrast to conventional chemical bonds, the N-connection appears not as a result of targeted synthesis, but arises in suitable conditions itself and manifests itself in the form of intermolecular or intramolecular interactions.

Features of hydrogen bonds.

A distinctive feature of the hydrogen bond is relatively low strength, its energy is 5-10 times lower than the energy of the chemical bond. By energy, it occupies an intermediate position between chemical bonds and van der Waals interactions, those that hold molecules in a solid or liquid phase.

In the formation of the N-bond, the electronegability of atoms involved in the connection is plays a defining role - the ability to delay the electrons of the chemical bond from an atom partner participating in this regard. As a result, a partial negative charge D- occurs on atom a with increased electronegitricness, and a positive D + atom, the chemical bond be poorerized: and D- -d D +.

The resulting partial positive charge on the hydrogen atom allows it to attract another molecule, also containing an electroneary element, thus, electrostatic interactions are made into the formation of n-communication.

Three atoms are involved in the formation of n-bonds, two electronegative (A and B) and the hydrogen atom between them, the structure of such a relationship can be presented as follows: b ··· H D + -A D- (hydrogen bonds are usually denoted by a point line ). Atom A, chemically associated with N, is called the proton donor (lat. Donare - give, sacrificing), and b - its acceptor (lat. Acceptor - acceptor). Most often, the true "donation" is not, and H remains chemically associated with A.

Atoms - donors A, supplying H for the formation of n-links, not many, almost only three: n, o and f, at the same time a set of acceptor atoms B is very wide.

The very concept and term "hydrogen bond" introduced V.Lothimer and R. Uerbush in 1920, in order to explain the high boiling points of water, alcohols, liquid HF and some other connections. Comparing the boiling point of related compounds H 2 O, H 2 S, H 2 SE, and H 2 TE, they noticed that the first member of this series - water - boils much higher than it followed from the patterns that the rest were formed Members of a series. From this pattern, water should be boiled to 200 ° C lower than the observed true value.

Exactly the same deviation is observed for ammonia in a row of related compounds: NN 3, H 3 P, H 3 AS, H 3 SB. Its true boiling point (-33 ° C) is 80 ° C above the expected value.

When the liquid is boiling, only van der Waals interaction, those that hold molecules in the liquid phase are destroyed. If the boiling point is unexpectedly high, then, therefore, molecules are associated additionally some other forces. In this case, this is hydrogen bonds.

Similarly, the increased boiling point of alcohols (in comparison with the compounds that do not contain group -one) is the result of the formation of hydrogen bonds.

Currently, a reliable way to detect n communications give spectral methods (most often infrared spectroscopy). The spectral characteristics of human-related groups associated with hydrogen bonds are noticeably different from those cases when this connection is absent. In addition, if structural studies show that the distance between the b-n atoms is less than the amount of van der Waals radius, it is believed that the presence of the N-connection is established.

In addition to the elevated boiling point, hydrogen bonds also appear in the formation of a crystal structure of a substance, increasing its melting point. In the crystal structure of ice, the N-bonds form a bulk mesh, while water molecules are arranged so that hydrogen atoms of one molecule are directed to the oxygen atoms of adjacent molecules:

Boric acid B (OH) 3 has a layered crystalline structure, each molecule is bound by hydrogen bonds with three other molecules. Packaging of molecules in the layer forms a parquet pattern collected from hexagons:

Most organic substances are not soluble in water, when such a rule is broken, then, most often, this is the result of the intervention of hydrogen bonds.

Oxygen and nitrogen are the main donors of protons, they take on the function of the atom A in the previously considered triade b ··· H D + -A D-. They, most often, act as acceptors (atom b). Due to this, some organic substances containing O and N in the role of an atom b can be dissolved in water (the role of an atom A performs water oxygen). Hydrogen bonds between organic matter and water help to "remove" organic matter molecules, translating it into an aqueous solution.

There is an empirical rule: if the organic substance contains no more than three carbon atoms by one oxygen atom, it is easily dissolved in water:

Benzol is very slightly soluble in water, but if you replace one group of CH on N, we obtain pyridine C 5 H 5 n, which is mixed with water in any relations.

Hydrogen bonds can show themselves in non-aqueous solutions when a partial positive charge occurs on hydrogen, and there is a molecule containing a "good" acceptor, as a rule oxygen. For example, HCCl 3 chloroform dissolves fatty acids, and acetylene HCєCH soluble in acetone:

This fact found an important technical application, acetylene under pressure is very sensitive to easy concussions and easily explodes, and its solution in acetone under pressure is safe in circulation.

Hydrogen bonds in polymers and biopolymers play an important role. In cellulose, the main component of wood - hydroxyl groups are located in the form of lateral groups of the polymer chain collected from cyclic fragments. Despite the relatively weak energy of each individual n-bond, their interaction throughout the polymer molecule leads to such a powerful intermolecular interaction that the dissolution of cellulose becomes possible only when using an exotic high-polar solvent - the reagent of the Swisser (ammonia complex of copper hydroxide).

In polyamides (Capron, nylon) n-bonds arise between carbonyl and amino groups\u003e C \u003d O ··· H-N

This leads to the formation of crystalline regions in the structure of the polymer and an increase in its mechanical strength.

The same thing happens in polyurethanes having a structure close to polyamides:

NH-C (O) O- (CH 2) 4 -OC (O) -NH- (CH 2) N -NH-C (O) O-

The formation of crystalline regions and subsequent strengthening of the polymer occurs due to the formation of n-bonds between carbonyl and amino groups\u003e C \u003d O ··· H-N<.>

Similarly, there is a union of parallel polymeric chains in proteins, but n-bonds provide protein molecules as a different packaging method - in the form of a spiral, while the spirals are fixed by all the same hydrogen bonds arising between the carbonyl and amino group:

In the DNA molecule, all information on the particular living organism in the form of alternating cyclic fragments containing carbonyl and amino groups are recorded. Such fragments are four types: adenine, thymine, cytosine and guanine. They are located in the form of side suspensions along the entire DNA polymer molecule. The procedure for the alternation of these fragments determines the individuality of each living being., With pair, the interaction of carbonyl C \u003d O and amino groups NH, as well as the amino groups NH and nitrogen atoms that do not contain hydrogen arise, n-bonds occur, it is they who holds two DNA molecules in the form of widely known double Spiral:

The complexes of some transition metals are prone to the formation of n-communications (in the role of protons acceptors); Most are located to participate in n-bonds Metal complexes of VI-VIII groups. In order for such a link to arise in a number of cases, the participation of a powerful proton donor, for example, trifluoroacetic acid is necessary. In the first stage (see Figure below), a n-bond occurs with the participation of the Iridiya metal atom (complex I), which plays the role of the acceptor B.

Next, with a decrease in temperature (from room to -50 ° C), the proton moves to the metal and the usual connection of Mn. All transformations are reversible, depending on the temperature, the proton can move either to the metal or to its donor - an acid anion.

In the second stage, the metal (complex II) adopts a proton, and with it a positive charge and becomes a cation. An ordinary ionic connection is formed (as NaCl). However, by turning to the metal, the proton retains its constant thrust to various acceptors, in this case to an anion of acid. As a result, it appears n-bond (marked with asterisks), an additional tightening ion pair:

The hydrogen atom can participate in the role of an atom b, that is, a proton acceptor in the case when a negative charge is concentrated on it, it is realized in metals hydrides: M D + -N D-, connections containing metal - hydrogen. If the metal hydride interacts with the proton donor of the middle force (for example, fluorinated tert-Bathannol), it arises an unusual amber-free bridge, where hydrogen itself organizes the N-bond: M D + -H D- ··· H D + -A D-:

In the shown complex with wedge-shaped lines with solid fill or transverse hatching, chemical bonds are indicated to the heights of the octahedron.

Mikhail Levitsky

Hydrogen Communication -the relationship between a positively charged hydrogen atom of one molecule and a negatively charged atom of another molecule.

The mechanism of the occurrence of hydrogen bonds is partly electrostatic and partially donor-acceptor.

A characteristic feature of the hydrogen bond is the distance between the hydrogen atom and the other atom, it formes. It should be less than the sum of the radii of these atoms.

Hydrogen Communication is divided:

1. Intermolecular hydrogen bond It is formed between molecules of substances, which includes hydrogen and highly electronegative element - fluorine, oxygen, nitrogen, chlorine, sulfur. Highly displaced general electron steam from hydrogen to the atom of a negatively charged element, while the positive charge of hydrogen is concentrated in a small volume, the interaction of the proton with an interimalized electronic pair of another atom or ion, to generalize it.

Hydrogen communication is denoted by points, indicating that it is much weaker than a covalent bond (approximately 15-20 times).

2. Intramolecular hydrogen bond Present in polyhydric alcohols, carbohydrates, proteins and other organic substances.

Substances with hydrogen bonds have molecular crystalline lattices, in the nodes of which are molecules.

Examples: Water in the form of ice, iodine, chlorine, bromine, "dry ice" (solid carbon dioxide), solid ammonia, as well as solid organic substances (methane, benzene, phenol, naphthalene, proteins, etc.).

The physical properties of substances with hydrogen bond.

Hydrogen bond provides low-molecular weight substances under normal conditions in a liquid aggregate state (ethanol, methanol, water) or liquefying gases (ammonia, fluorine hydrogen).

Higher boiling point of water (100 ° C) compared with hydrogen compounds of the elements of the subgroup of oxygen ( H 2 S., H 2 S., H 2 Th.) Since additional energy is spent on the destruction of hydrogen bonds.

Also during water melting, its density increases. This is explained by the fact that in the structure of ice, each oxygen atom is associated through hydrogen atoms with four other oxygen atoms of other water molecules. As a result, a loose "openwork" structure is formed.

In molecules of HF, H 2 O, NH 3 compounds there are hydrogen bonds with a strongly electronegative element (H-F, H-O, H-N). Between molecules of such compounds can be formed intermolecular hydrogen bonds. In some organic molecules containing the bonds of H-O, H-N, may arise intramolecular hydrogen bonds.

The mechanism for the formation of hydrogen bonds has partially electrostatic, partially donor - acceptor character. At the same time, the donor of the electron pair is the atom of a strong electronegative element (F, O, N), and the calculator - hydrogen atoms connected to these atoms. As for covalent communications, for hydrogen bonds are characteristic food in the space I. saturability.

Hydrogen communication is made to mark points: n ··· F. Hydrogen bond is stronger than the higher the electronegability of the partner atom and the smaller its size. It is characteristic primarily for fluorine compounds, as well as oxygen, to a lesser degree of nitrogen, to an even lesser extent for chlorine and sulfur. Accordingly, the energy of a hydrogen bond is also changing (Table 1).

Table 1. The average values \u200b\u200bof the energy of hydrogen bonds

Intermolecular and intramolecular hydrogen bond

Thanks to hydrogen bonds, the molecule is combined into dimers and more complex associates.Water molecules form associates (H 2 O) 2, (H 2 O) 3, (H 2 O) 4; alcohol (C 2 H 5) 4. This explains the increase in the boiling point of alcohols compared with hydrocarbons, there is a good dissolution of methanol and ethanol in water.Hydrogen bond, resulting from molecules, is called intermolecular.

For example, the formation of parachidroxibenzaldehyde dimer can be represented by the following scheme (Fig. 1).

Fig. 1. The formation of intermolecular hydrogen ties inparagidroxybenzaldehyde.

Hydrogen bonds may occur both between different molecules (intermolecular hydrogen bonds) and inside the molecule (intramolecular hydrogen bond).Intramolecular hydrogen tiesand are available in polyatomic alcohols, carbohydrates, proteins and other organic substances.

The effect of hydrogen bonds on the properties of substances

The most convenient indicator of the existence of an intermolecular hydrogen bond is the boiling point of the substance. The higher boiling point of water (100 o C compared to hydrogen compounds of the elements of the oxygen subgroup (H 2 S, H 2 SE, H 2 TE) is explained by the presence of hydrogen bonds: the destruction of intermolecular hydrogen bonds in water must be expected to cost additional energy.

Hydrogen bond can significantly affect the structure and properties of substances. The existence of an intermolecular hydrogen bond increases the melting and boiling point of substances. The presence of intramolecular hydrogen bond leads to the fact that the deoxyribonucleic acid molecule (DNA) is turned out to be rolled in water double spirals.

The hydrogen bond also plays an important role in the dissolution processes, since solubility depends on the ability of the compound to give hydrogen bonds with the solvent. As a result, containing on-groups such substances such as sugar, glucose, alcohols, carboxylic acids are usually soluble in water.

Examples of compounds: monohydric (methanol, ethanol) and polyhydric alcohols (glycerin, ethylene glycol), carboxylic acids, amines, amino acids, proteins, water, ammonia, fluoride fluoron, oxygen-containing carboxylic acids.

Structure hydrogen bond We will analyze with you interaction Water molecules among themselves.

Water molecule is dipolem. This is explained by the fact that atom hydrogenassociated with more electric negative Element oxygenhaving, experiencing disadvantage electrons And therefore is able to interact With an oxygen atom, another water molecule.

As a result interaction arises hydrogen communications (Fig. 2.1):

2.1. The mechanism of the formation of hydrogen bond between water molecules

This is explained by atom hydrogenassociated with more electric negative Element having various electronic couple (nitrogen, oxygen, fluorine, etc.) is experiencing disadvantage electrons and therefore can interact with virtual pair of electrons another electronegative atom this same or other molecules.

As a result, it also arises hydrogen Communicationwhich is graphically designated three dots (Fig.):

Fig. 2.2. The mechanism for the formation of hydrogen bond between the proton ( . δ + ) and more electronegative sulfur atoms (:S. δ - ), oxygen (:O. δ - ) and nitrogen (:N. δ - )

This connection is significant weaker other chemical connections ( energy Her Education 10-40 kJ / mol), and, mainly, is determined by electrostatic and donor-acceptor interactions.

Hydrogen bond can be like intramolecular, so I. intermolecular.

2.1.4. Hydrophobic interactions

Before considering nature hydrophobic interaction, it is necessary to introduce the concept " hydrophilic " and " hydrophobic " Functional group.

Groups that can form hydrogen bonds with water molecules are called hydrophilic.

These groups include polar Groups: amino group (-NH 2 ) , carboxyl(- Coool), carbonyl group(- Cho.) I. sulfgidrile Group ( - Sh).

Usually, hydrophilic Connections Good soluble in water. !!! This is due to the fact that the polar groups are able to form hydrogen bonds with water molecules .

Appearance Such connections are accompanied energy release, therefore, there is a tendency to maximum increase in contact surface charged groups and water ( Fig. 2.3.):

Fig. 2.3. The mechanism of formation of hydrophobic and hydrophilic interactions

Molecules or parts of molecules, unable to form hydrogen bonds with water are called hydrophobic groups.

These groups include alkyl and aromatic radicals that notolar and do not bear Electric charge.

Hydrophobic groups – badly or not at all soluble in water.

This is explained by atoms and group of Atomsincluded hydrophobic Groups are electrophetraland therefore) can not form hydrogen ties with water.

!!! Hydrophobic interactions arise as a result of contact between non-polar radicals, unable to break the hydrogen bonds between water molecules.

As a result water molecules Ostive on surface hydrophilic molecules ( Fig. 2.3.).

2.1.5. Van der Waals interaction.

In molecules there are also quite weak and short-range attraction forces between electrically neutral atoms and functional groups.

These are the so-called van der Waals interaction.

They are due electrostatic interaction between negatively charged electrons one Atom and a positively charged core other Atom.

As the nuclei of atoms shielded surrounding their own electricians from nuclei of adjacent atoms, then arising between different atoms van der Waalsy interaction quite nearby.

All these types of interactions Take part in formation, maintaining and stabilization spatial structure ( conformations) protein molecules ( Fig. 2.4.):

Fig. 2.4. The mechanism of formation of covalent bonds and weak non-rigal interactions:1 - Electric static interactions;2 - hydrogen bonds;3 - hydrophobic interactions,4 - disulfide connections

Forces that contribute the formation of the spatial structure of proteins and holding it in a stable stateare very weak forces. The energy of these forces on 2-3 The order is less than the energy of covalent bonds. They act between individual atoms and groups of atoms.

However, a huge number of atoms in biopolymers (proteins) molecules leads to the fact that the total energy of these weak interactions becomes comparable to covalent bonds.

Hydrogen is the simplest chemical element in the universe. Its atom consists of only one proton in the kernel and one electron. Despite its physical and chemical simplicity, hydrogen is the main element of the universe, the stars are lit and lightly, our planet is covered with water, and the most complicated organic compounds began the beginning of the most amazing phenomenon in the Universe.

In contact with

Features of matter

In nature, to meet hydrogen as part of other elements of the Mendeleev table can be everywhere. The most striking example of such a compound is such a substance as.

Hydrogen has three isotopes:

• (the very first element of the Mendeleev table, habitual to we all hydrogen);
• deuterium (the so-called heavy hydrogen containing in the nucleus not only proton, but also neutron);
• trithium is a radioactive hydrogen isotope, whose core consists of a proton and two neutrons.

Hydrogen bond is characteristic and present in most organic compounds. Connecting with chlorine, hydrogen forms chloroic acid, with oxygen - water, with nitrogen - ammonia. These phenomena found at the end of the 19th century were opened by Russian chemists M. Ilyinsky and N. Beketov.

Scientists have established that the hydrogen containing a group of atoms most often forms stable associations with a charged atom, which can be part of a molecule (it is not excluded that even in the same one). This additional "coupling" is called hydrogen bond.

Nature phenomena

We give the definition of hydrogen bond (VS). This is the interaction between negatively charged particles of molecules implemented by a hydrogen atom.

If the feature indicate the connection of a covalent type, and three points - hydrogen, then it is symbolically reflected by V.S. between molecules A and in this way :.

The nature of this interatomic phenomenon is quite simple. Atom n is the positive charges δ +, if it meets on its path charged negatively and possessing the charge Δ-, then it comes to electrostatic contact with it.

Important! Most often by V.S. Noticeably weaker than covalent. However, they are much stronger than the standard molecular attraction of particles, characteristic of solid and liquid bodies.

Covalency

Despite the fact that V.S. It may occur within two particles a pair of completely different molecules, hydrogen chemical is not a molecular connection. The property of direction and saturation is one of the qualities of V.S., which makes it very similar to covalent. Note that in many theories, V.S. It is considered a view and it does not at all affect the results, so this opinion can be considered correct. Moreover, Nature itself, V.S. Very close to covalent.

This can be easily demonstrated using traditional chemical methods that calculate orbital inside molecules. In this calculation, it will be three center-centrogen bonds. Once again, this proves that the attribution of the sun to a variety of covalent does not bear anything anti-scientific.

Education process

What is the method of education. Hydrogen bonds are formed between electronegative atoms, one of which has a free electronic pair.

The most convincing sign of V.S. is the distance between the H atom and the second atom. The thing is that the distance between atoms is less than the sum of two atomic radius. Despite the frequently found asymmetry (when B, the distance exceeds the distance) is still the sum of the radii of atoms more than the distance between them.

Yes, asymmetry in V.S. It often occurs, however, there are symmetric structures, such as HF. The angle between the first and second atom in the system is close to 180 degrees. Remembering the HF fluorine breeders, it should be noted that the connection with fluorine is one of the strongest. HF is a symmetric type ion . In it, the energy of hydrogen compounds is about 150 kilodzhoules in one mole. The covalent bond of fluoride is approximately the same. In water N 2 O V.S. Significantly less - about 20 kilodzhoules per mole.

The compound of particles through hydrogen is found in a large number of different connections. Chemical bond often occurs between fluorine, nitrogen and, since the latter are the most electronegable elements. It is rarely discovered between chlorine, gray and other non-metals.

Important! Nitrogen and oxygen - the basis of life, these elements are contained in a particularly high concentration in carbohydrates, proteins and nucleic acid. If there were no durable contact through atom n, life on Earth would be impossible.

Intermolecular hydrogen bond is a type of formation of a strong structure that binds through an atom H is one molecule on the other. A bright example is formic acid. It is a molecule consisting of two or more simple molecules (dimer).

Intramolecular Sun is a type at which atom H is a link in the framework of one molecule.

The same applies to the fluoride sodium, which is in a gaseous state. It contains such polymeric structures that may consist of four simple HF molecules combined with each other through hydrogen.

Examples of the hydrogen intermolecular design does not have to search: the solubility of glucose, fructose, sucrose in the aqueous solution is due precisely using hydrogen and its connecting properties. Molecular structures of living organisms (molecule, for example) contain millions of complex hydrogen-related structures.

Function connections

How important is the social role of these connections. Consider several substances that exist due to the hydrogen joint. We will compare these water molecules. So that our reflections are honest, we will choose for comparison exclusively non-metals. These substances are called chalcogencogelodes.

For example, Tellur. H 2 TE hydrogen compound boils at a temperature -2 degree. As for selenium, H 2 SE boils at -42 degrees, and sulfur chalcogenic hydrogen H 2 S boils at -60 degrees. It is amazing that water boils with +100 degrees.

Attention! If there were no VS, and oxygen did not have so "chain" qualities, with an existing climate on Earth, there would be no water in a liquid state. Such a high boiling point is the direct consequence of hydrogen bonds.

The "clutch" of oxygen atoms with hydrogen is shown in the following image.

But on this, the amazing properties of water do not end. It should also be remembered about its melting. And again a hydrogen bond - precisely because of it when melting, the density begins to grow. When melting ice, each tenth hydrogen compound is destroyed, which is why the water molecules are approaching each other.

Types and properties of hydrogen bonds.

Hydrogen bond. Self-preparation to the exam and CT in chemistry

Output

The formation of hydrogen bonds affect the acidity of substances. For example, the hydric acid NF is rather weak. In this case, other halogen-hydrogen acids are quite strong. The reason for this is that H is connected immediately with two atoms F, and this does not allow them to be pulled out. It is precisely due to this, the NF is the unique acid forming the sour salt of NaHF 2.