If you look at the chronology of the study in chemical science of the ability of atoms of various elements to interact with each other, we can single out the middle of the 19th century. At that time, scientists drew attention to the fact that hydrogen compounds of oxygen, fluorine, nitrogen are characterized by a group of properties that can be called anomalous.
These are, first of all, very high melting and boiling points, for example, for water or hydrogen fluoride, which are higher than for other similar compounds. At present, it is already known that these features of these substances are determined by the property of hydrogen atoms to form an unusual type of bond with the atoms of elements that have a high electronegativity index. They called it hydrogen. The properties of the bond, the specifics of its formation and examples of compounds containing it are the main points that we will focus on in our article.
Reason for connection
The action of the forces of electrostatic attraction isthe physical basis for the appearance of most types of chemical bonds. The types of chemical bonds that have arisen due to the interaction of oppositely charged atomic nuclei of one element and electrons of another are well known. These are covalent non-polar and polar bonds, characteristic of simple and complex compounds of non-metallic elements.
For example, between the fluorine atom, which has the highest electronegativity, and the electroneutral particle of hydrogen, the one-electron cloud of which initially belonged only to the H atom, there is a shift in the negatively charged density. Now the hydrogen atom itself can rightly be called a proton. What happens next?
Electrostatic interaction
The electron cloud of the hydrogen atom almost completely passes towards the fluorine particle, and it acquires an excess negative charge. Between the naked, that is, devoid of negative density, hydrogen atom - a proton, and the F- ion of the neighboring hydrogen fluoride molecule, the force of electrostatic attraction is manifested. It leads to the appearance of intermolecular hydrogen bonds. Due to its occurrence, several HF molecules can form stable associates at once.
The main condition for the formation of a hydrogen bond is the presence of an atom of a chemical element with a high electronegativity and a hydrogen proton interacting with it. This type of interaction is most pronounced in oxygen and fluorine compounds (water, hydrogen fluoride), less in nitrogen-containing substances, such as ammonia, and even less in sulfur and chlorine compounds. Examples of hydrogen bonds formed between molecules can also be found in organic substances.
Thus, in alcohols between the oxygen and hydrogen atoms of the functional hydroxyl groups, electrostatic attraction forces also arise. Therefore, already the first representatives of the homologous series - methanol and ethyl alcohol - are liquids, not gases, like other substances of this composition and molecular weight.
Energy characteristic of communication
Let's compare the energy intensity of covalent (40–100 kcal/mol) and hydrogen bonds. The examples below confirm the following statement: the hydrogen type contains only 2 kcal/mol (between ammonia dimers) to 10 kcal/mol energy in fluorine compounds. But it turns out to be enough for the particles of some substances to be able to bind into associates: dimers, tetra - and polymers - groups consisting of many molecules.
They are not only in the liquid phase of the compound, but can be preserved without decomposing, when passing into a gas state. Therefore, hydrogen bonds, which hold molecules in groups, cause abnormally high boiling and melting points of ammonia, water, or hydrogen fluoride.
How water molecules associate
Both inorganic and organic substances have several types of chemical bonds. The chemical bond that arises in the process of association of polar particles with each other, and is called intermolecular hydrogen, can radically change the physicochemicalconnection characteristics. Let us prove this statement by considering the properties of water. Molecules H2O have the form of dipoles - particles whose poles carry opposite charges.
Neighboring molecules are attracted to each other by the positively charged hydrogen protons and the negative charges of the oxygen atom. As a result of this process, molecular complexes are formed - associates, leading to the appearance of abnormally high boiling and melting points, high heat capacity and thermal conductivity of the compound.
The unique properties of water
The presence of hydrogen bonds between H2O particles is responsible for many of its vital properties. Water provides the most important metabolic reactions - the hydrolysis of carbohydrates, proteins and fats occurring in the cell - and is a solvent. Such water, which is part of the cytoplasm or intercellular fluid, is called free. Thanks to hydrogen bonds between molecules, it forms hydration shells around proteins and glycoproteins, which prevent sticking between polymer macromolecules.
In this case, the water is called structured. The examples we have given of the hydrogen bond that occurs between the particles of H2O prove its leading role in the formation of the basic physical and chemical properties of organic substances - proteins and polysaccharides, in the processes of assimilation and dissimilation occurring in living organisms. systems, as well as in ensuring their thermal balance.
Intramolecular hydrogen bond
Salicylic acid is one of the well-known and long-used medicines with anti-inflammatory, wound healing and antimicrobial effects. The acid itself, bromo derivatives of phenol, organic complex compounds are capable of forming an intramolecular hydrogen bond. The examples below show the mechanism of its formation. So, in the spatial configuration of the salicylic acid molecule, the approach of the oxygen atom of the carbonyl group and the hydrogen proton of the hydroxyl radical is possible.
Due to the greater electronegativity of the oxygen atom, the electron of the hydrogen particle almost completely falls under the influence of the oxygen nucleus. A hydrogen bond occurs inside the salicylic acid molecule, which increases the acidity of the solution due to an increase in the concentration of hydrogen ions in it.
Summing up, we can say that this type of interaction between atoms manifests itself if the group of the donor (particle that donates an electron) and the acceptor atom that accepts it are part of the same molecule.
