Hydrogen communications - this is the interaction between the two electronegative atoms of one or different molecules By means of a hydrogen atom: A-H ... in (a covalent bond is indicated, three points - hydrogen bond).

One of the signs of the hydrogen bond can be 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.

They occur, as a rule, between fluorine, nitrogen and oxygen atoms (the most electronegative elements), less often - with the participation of chlorine, sulfur atoms and other non-metals. Strong hydrogen bonds are formed in such liquid substances such as water, fluoride hydrogen, oxygen-containing inorganic acids, carboxylic acids, phenols, alcohols, ammonia, amines. When crystallization, hydrogen bonds in these substances are usually preserved.

Addiction physical properties Substances with molecular structure on the nature of intermolecular interaction. The effect of hydrogen bond on the properties of substances.

Intermolecular hydrogen bonds determine the association of molecules, which leads to an increase in boiling and melting temperature. For example, ethyl alcohol C2H5OH, capable of association, boils at + 78.3 ° C, and the dimethyl ether of CH3OSN3, which does not form hydrogen bonds, only at -24 ° C ( molecular formula Both substances C2N6O).

The formation of n-bonds with solvent molecules contributes to the improvement of solubility. So, methyl and ethyl alcohols (CH3OH, C2N5Y), forming n-bonds with water molecules, dissolved in it indefinitely.

The intramolecular hydrogen bond is formed with a favorable spatial location in the molecule of the corresponding groups of atoms and specifically affects the properties. For example, n-bond inside salicylic acid molecules increases its acidity. Hydrogen bonds play an extremely important role in the formation of the spatial structure of biopolymers (proteins, polysaccharides, nucleic acids), which largely determines their biological functions.

Forces of intermolecular interaction (van der Waals strength). Orientational, induction and dispersion interaction.

Intermolecular interaction- Interaction between electrically neutral molecules or atoms.

TO van der Waals Forces include interactions between dipoles (permanent and induced). The name is due to the fact that these forces are the cause of the internal pressure amendment in the equation of the state of the real gas of Van der Waals. These interactions are mainly determined by the forces responsible for the formation of the spatial structure of biological macromolecules.

Orientational: Polar molecules in which the centers of gravity of positive and negative charges do not coincide, for example HCl, H2O, NH3, are focused in such a way that ends with opposite charges are nearby. There is an attraction between them. (Keesoma energy) is expressed by the ratio:

E K \u003d -2 μ 1 μ 2 / 4π ε 0 R 3,

where μ1 and μ2 are dipole moments of interacting dipoles, R is the distance between them. The attraction dipole-dipole can be carried out only when the energy of attraction exceeds the thermal energy of molecules; This usually takes place in solid and liquid substances. The dipole-dipole interaction is manifested in polar fluids (water, fluoride hydrogen).

Induction: Under the action of the charged ends of the polar molecule, electronic clouds of non-polar molecules are shifted towards a positive charge and away from the negative. The non-polar molecule becomes polar, and the molecules begin to attract each other, only much weaker than two polar molecules.

(Debye energy) is determined by the expression:

E d \u003d -2 μ mA 2 γ / R 6,

where μ kin is the moment of the induced dipole.

The attraction of constant and induced dipoles is usually very weak, since the polarizability of the molecules of most substances is small. It acts only at very low distances between dipoles. This type of interaction is manifested mainly in the solutions of polar compounds in non-polar solvents.

Dispersion: There may also be attracted between non-polar molecules. The electrons that are in constant motion may turn out to be focused on the one side of the molecule, that is, a non-polar particle will become polar. This causes redistribution of charges in neighboring molecules, and short-term relationships are established between them.

(London Energy) is given by the relation:

E L \u003d -2 μ MGN 2 γ 2 / R 6,

where μ MGN is the moment of an instant dipole. London forces of attraction between non-polar particles (atoms, molecules) are very short-range. The energy values \u200b\u200bof such attraction depend on the size of particles and the number of electrons in the induced dipoles. These relationships are very weak - the weakest of all intermolecular interactions. However, they are the most versatile, as they arise between any molecules.

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 and with increased electronegitility, and a positive D + atom-partner is positive. chemical communications At the same time polarizes: 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 Communication" introduced V.Lothimer and R. Uerbush in 1920, in order to explain high temperatures Boiling 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 increased temperature Boiling hydrogen bonds also appear in the formation of a crystalline 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 organic 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).

Hydrogen Communication (N-Communication)- This is a bond formed by the protonated hydrogen atom with a strongly electron atom of the same or other molecule. Under normal conditions, the hydrogen valence is equal to 1, and it is able to generalize with other atoms one electron pair, forming a covalent bond: a hydrogen atom can attach an electron, forming a hydride ion H +.

The hydrogen atom has a feature that distinguishes it from all other atoms: giving up its electron, it remains in the form of a core without electrons, i.e. In the form of a particle, a diameter, which is thousands of times less than the diameter of the remaining atoms. In the absence of electrons, ion H + is not repelled by electronic shells of other atoms or ions, but, on the contrary, attracts; It can closely approach other atoms, interact with their electrons and even embed into their electronic shells. In H +-yions liquids, it is mostly not preserved in the form of an independent particle, and bind to molecules of two substances: in water with water molecules, forming ion H 3 o + -Ion hydroxonium; With ammonia molecule - NH 4 + -yone ammonium.

Being associated with the atom of one of the most electronegative elements: with a fluorine atom, oxygen, chlorine and nitrogen, a hydrogen atom acquires a relatively high positive charge that does not exceed one. Since this charge is concentrated on an extremely small atomic cooker, it is very close to another atom carrying a negative charge. This causes the formation of a rather strong dipole-dipole connection with the energy of 20-30 kJ / mol and more. Hydrogen bond occurs as a result of interdapole interaction of two strongly polar bonds belonging to various molecules or one and the same molecule. It is weaker than ordinary covalent CommunicationThe energy of which is approximately 125-420 kJ / mol and can be enhanced due to the mutual polarization of bonds due to these features of the hydrogen atom. Hydrogen bond (N-Communication) is denoted by x-n × × × x.

A hydrogen atom involved in hydrogen bond can be located precisely in the middle between two strongly negative atoms - a symmetrical location or shifting closer to that, which has greater electronegativity - asymmetrical location.

The energy of the hydrogen bond is sufficient so that with conventional and reduced temperatures, cause a noticeable dissociation of molecules. Fluoride hydrogen even near the boiling point has an average composition (HF) 4. The association leads to abnormally high temperatures of melting and boiling fluoride hydrogen. The existence of dimer H 2 F 2 explains the formation of acid salts of type KHF 2 × Nahf 2. The fact that hydrofluoric acid is unlike chloride hydrochloric, bromide hydrochloric and orodoyhedral, represents weak acid (K d \u003d 7 × 10 -4) is also a consequence of the association of HF molecules due to hydrogen bonds.

In the presence of an asymmetric hydrogen bond, which occurs in oxygen and nitrogen compounds, hydrogen is slightly closer to one of the two adjacent atoms, here intermolecular N-Communication. Each H 2 o molecule is involved in the formation of two n-bonds, so that an oxygen atom turns out to be associated with four hydrogen atoms. Associated water molecules form an openwork spatial structure, where each oxygen atom is located in the center of the tetrahedron, and hydrogen atoms are located in the corners.

Openwork Spatial Water Structure

The openwork ice structure explains its smaller density than water. When melting, part of the n-links is torn and the density of water increases, because Molecules are arranging more tight. X-ray examination showed that for most of the molecules in liquid water, a tetrahedral environment is also persisted: the location of the neighboring molecules is almost the same as in the ice crystal, and the subsequent layer is repeated

some deviation from the specified orderliness; The deviation increases as the molecule is removed. For water, the presence of "near-order" is characterized as for other liquids, and to a lesser extent, compared with other liquids, the presence of a "long-range". This explains the presence of a crystal structure in water.

Water properties such as large values \u200b\u200bof heat capacity and heat evaporation, abnormally high melting and boiling temperatures, high dielectric constant - due to the boundanness of water molecules with hydrogen bonds. Without n-links T pl.V. \u003d -100 O C, T KIP. Water \u003d -80 o C.

Hydrogen bonds are present in liquid ammonia. The hydrogen atom associated with carbon can acquire the ability to form a hydrogen bond if the remaining carbon valence is saturated with high electronegative atoms or corresponding atomic groups, for example, chloroform (SNSL 3), pentachloroethane (CCL 3 -CHCL 2), i.e. The neighborhood of electronegative atoms can activate the formation of hydrogen bonds at the CH-Group atoms, although the electronegability of atoms C and H is almost the same. This explains the occurrence of n-links between molecules in liquid HCN, CHF 3, etc.

Hydrogen bond is peculiar to any aggregate states of the substance. It is formed between the same and between different molecules, between different parts of the same molecule - intramolecular hydrogen bond. The most common is the N-connection between molecules containing hydroxyl groups it -.

Easy ethers with a greater molar mass of more volatile than alcohols, since all the hydrogen atoms are associated with carbon atoms and are not capable of forming H-bonds.

The role of H-bonds in biochemical systems is great. Properties of proteins and nucleic acids are largely due to the presence of hydrogen bonds. N-bond plays a big role in dissolution processes. Hydrogen bonds in protein molecules, nucleic acids and other biologically important compounds are especially common, so these bonds play an important role in the chemistry of life processes.