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 distinguish the middle of the XIX century. At this time, scientists drew attention to the fact that the hydrogen compounds of oxygen, fluorine, nitrogen are characterized by a group of properties that can be called abnormal.
First of all, these are very high melting and boiling points, for example, of water or hydrogen fluoride, which are higher than 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 atoms of elements having a high electronegativity index. It was called hydrogen. Communication properties, the specifics of its formation and examples of compounds containing it are the main points that we will focus on in our article.
Causes of Communication
The action of electrostatic attraction forces is the physical basis for the appearance of most types of chemical bonds. The types of chemical bonds arising from the interaction of oppositely charged atomic nuclei of one element and the electrons of another are well known. This is a covalent non-polar and polar bond, characteristic of simple and complex compounds of non-metallic elements.
For example, between a fluorine atom, which has the highest electronegativity, and an electroneutral hydrogen particle, whose one-electron cloud initially belonged only to the H atom, a negatively charged density is shifted. Now the hydrogen atom itself can rightfully be called a proton. What happens next?
Electrostatic interaction
The electron cloud of the hydrogen atom almost completely goes over to the side of the fluorine particle, and it acquires an excess negative charge. Between a bare, that is, negative density, hydrogen atom — a proton, and an ion F — an adjacent 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 having 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.
So, between alcohols between oxygen and hydrogen atoms of functional hydroxyl groups, electrostatic attraction forces also arise. Therefore, the very 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 us compare the energy intensity of covalent (40–100 kcal / mol) and hydrogen bonds. The examples given below confirm the following statement: the hydrogen type contains only from 2 kcal / mol (between ammonia dimers) to 10 kcal / mol of energy in fluorine compounds. But it turns out to be enough so that particles of certain substances can 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 upon transition to a state of gas. Therefore, hydrogen bonds ensuring the retention of molecules in groups cause abnormally high boiling and melting temperatures of ammonia, water or hydrogen fluoride.
How is the association of water molecules
Both inorganic and organic substances have several types of chemical bonds. The chemical bond that arises in the process of the association of polar particles with each other, and is called intermolecular hydrogen, can fundamentally change the physicochemical characteristic of the compound. Let us prove this statement by considering the properties of water. H 2 O molecules have the form of dipoles — particles whose poles carry opposite charges.
Neighboring molecules are attracted to each other by positively charged hydrogen protons and 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 temperatures, high heat capacity and thermal conductivity of the compound.
Unique properties of water
The presence of hydrogen bonds between H 2 O particles is the cause of many of its vital properties. Water provides the most important metabolic reactions - the hydrolysis of carbohydrates, proteins, and fats that occurs in the cell - and is a solvent. Such water, which is part of the cytoplasm or intercellular fluid, is called free. Due to the hydrogen bonds between the molecules, it forms hydration shells around proteins and glycoproteins, which prevent adhesion between the macromolecules of the polymers.
In this case, the water is called structured. Our examples of the hydrogen bond arising between H 2 O particles 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 in living systems, and also in ensuring their thermal balance.
Intramolecular hydrogen bond
Salicylic acid is one of the well-known and long-used in medicine therapeutic agents with anti-inflammatory, wound healing and antimicrobial effects. The acid itself, phenol bromine derivatives, and organic complex compounds are capable of forming an intramolecular hydrogen bond. The examples below show the mechanism of its formation. Thus, in the spatial configuration of the salicylic acid molecule, the oxygen atom of the carbonyl group and the hydrogen proton of the hydroxyl radical can come closer together.
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 appears 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 (the particle giving the electron away) and the atom of the acceptor accepting it are part of the same molecule.