What is a hydrogen bond?A well-known example of this connection is ordinary water (H2O). Due to the fact that the oxygen atom (O) is more electronegative than two hydrogen atoms (H), it seems to detach binding electrons from hydrogen atoms. As a result of the creation of such a covalent polar bond, a dipole is formed. The oxygen atom acquires a negatively negative charge, and the hydrogen atoms - a small positive charge, which is attracted to the electrons (their unshared pair) on the oxygen atom of the neighboring H2O molecule (that is, water). Thus, it can be said that the hydrogen bond is the forming attractive force between the hydrogen atom and the electronegative atom. An important feature of the hydrogen atom is that when its binding electrons attract, its core (that is, the proton, other electrons not shielded) is exposed. And although the hydrogen bond is weaker than the covalent bond, it causes a number of abnormal properties of H2O (water).
Most often this bond is formed with the participation of atomsthe following elements: oxygen (O), nitrogen (N) and fluorine (F). This is due to the fact that the atoms of these elements have small dimensions and are characterized by high electronegativity. With larger atoms (sulfur S or chlorine Cl), the resulting hydrogen bond is weaker, despite the fact that in terms of their electronegativity these elements are comparable to N (i.e., nitrogen).
There are two types of hydrogen bond:
1. Hydrogen intermolecular bonding - appears between two molecules, for example: methanol, ammonia, hydrogen fluoride.
2. Hydrogen bond intramolecular - 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 in the length of the bond (the distance between the atoms):
1. Hydrogen bonds are weak. The energy is 10-30 kJ / mol, the bond length is 30. All the substances listed above are examples of a normal or weak hydrogen bond.
2. Hydrogen bonds are strong.The energy is 400 kJ / mol, the length is 23-24. The 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.
Influence of hydrogen intermolecular bonds
Anomalous boiling points andmelting, enthalpy of evaporation and surface tension of some compounds can be explained by the presence of hydrogen bonds. Water has abnormal values for all of these properties, and hydrogen fluoride and ammonia have boiling and melting points. Water and hydrogen fluoride in the solid and liquid states are considered polymerized because of 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. And during melting the hydrogen bond is partially destroyed, because of which the water molecules (H2O) are packed more densely.
Dimerization of some substances (carboxylicacids, for example, benzoic and acetic) can also be explained by the presence of a hydrogen bond in them. The 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.
Influence of hydrogen intramolecular bonds
This relationship also affects the structure and propertiesvarious compounds, such as: 2- and 4-nitrophenol. But the most famous and important example of a hydrogen bond is deoxyribonucleic acid (abridged: DNA). The molecules of this acid are folded in the form of a double helix, two strands of which are connected together by a hydrogen bond.
