What is a hydrogen bond? A common example of this relationship is ordinary water (H2O). Due to the fact that the oxygen atom (O) is more electronegative than two hydrogen atoms (H), it seems to pull away binding electrons from the hydrogen atoms. As a result of the creation of such a covalent polar bond , a dipole is formed. An oxygen atom acquires a not very large negative charge, and hydrogen atoms - a small positive charge, which is attracted to the electrons (their lone pair) on the oxygen atom of a neighboring H2O molecule (i.e. water). Thus, we can say that the hydrogen bond is the generated attractive force between the hydrogen atom and the electronegative atom. An important feature of a hydrogen atom is that when its bonding electrons are attracted, its core is exposed (that is, a proton that is not shielded by other electrons). Although the hydrogen bond is weaker than the covalent bond, it is it that determines a number of anomalous properties of H2O (water).
Most often, this bond is formed with the participation of atoms of the following elements: oxygen (O), nitrogen (N) and fluorine (F). This is due to the fact that the atoms of these elements are small and are characterized by high electronegativity. With larger atoms (sulfur S or chlorine Cl), the hydrogen bond formed is weaker, despite the fact that by their electronegativity these elements are comparable to N (i.e., nitrogen).
There are two types of hydrogen bonding:
1. Hydrogen intermolecular bond - appears between two molecules, for example: methanol, ammonia, hydrogen fluoride.
2. Intramolecular hydrogen bond - appears inside one molecule, for example: 2-nitrophenol.
Also, at present, there is an opinion that the hydrogen chemical bond is weak and strong. They differ from each other in energy and bond length (distance between atoms):
1. Hydrogen bonds are weak. Energy - 10-30 kJ / mol, bond length - 30. All substances listed above are examples of normal or weak hydrogen bonds.
2. Hydrogen bonds are strong. Energy - 400 kJ / mol, length - 23-24. Data obtained experimentally indicate that strong bonds are formed in the following ions: ion-hydrogen difluoride [FHF] -, ion-hydrated hydroxide [HO-H-OH] -, oxonium ion hydrated [H2O-H-OH2] + , as well as in various other organic and inorganic compounds.
The effect of hydrogen intermolecular bonds
The abnormal values of boiling and melting temperatures, enthalpies of evaporation, and surface tension of some compounds can be explained by the presence of hydrogen bonds. Water has anomalous values of all these properties, and hydrogen fluoride and ammonia have boiling and melting points. Solid and liquid water and hydrogen fluoride are considered polymerized due to the presence of hydrogen intermolecular bonds in them. This relationship explains not only the too high melting point of these substances, but also their low density. Moreover, during melting, the hydrogen bond is partially destroyed, due to which water molecules (H2O) are packed more densely.
The dimerization of certain substances (carboxylic acids, for example, benzoic and acetic) can also be explained by the presence of a hydrogen bond in them. A dimer is two molecules that are linked together. For this reason, the boiling point of carboxylic acids is higher than that of compounds having approximately the same molecular weight. For example, in acetic acid (CH3COOH), the boiling point is 391 K, while in acetone (CH3COCH3) it is 329 K.
The effect of hydrogen intramolecular bonds
This bond also affects the structure and properties of various compounds, such as: 2- and 4-nitrophenol. But the most famous and important example of hydrogen bonding is deoxyribonucleic acid (abbr .: DNA). The molecules of this acid are folded in the form of a double helix, the two strands of which are interconnected by a hydrogen bond.