What is the hydrogen bond?The famous example of all this communication is ordinary water (H2O).Due to the fact that the oxygen atom (O) is more electronegative than two hydrogen atoms (H) as if he pulls hydrogen atoms bonding electrons.As a result of the creation of such a polar covalent bond is formed dipole.The oxygen atom gets is not very large negative charge and the hydrogen atoms - a small positive charge, which is attracted to the electrons (their lone pair) on the oxygen atom of the neighboring molecule H2O (ie water).Thus, we can say that the hydrogen bond - a way of the force of attraction between the hydrogen atom and an electronegative atom.An important feature of the hydrogen atom is the fact that the attraction of its electronic connection laid bare its nucleus (ie, protons, electrons, other unshielded).Although the hydrogen bond is weaker than covalent, that it causes a whole range of the anomalous properties of H2O (water).
Most often, this bond is formed with the participation of the atoms of the following elements: oxygen (O), nitrogen (N) and fluorine (F).This occurs because the atoms of these elements are small and have a high electronegativity.With the size of the larger atoms (sulfur S or chlorine Cl) forms a hydrogen bond is weaker, despite the fact that by their electronegativity these items are comparable to N (i.e. nitrogen).
There are two types of hydrogen bonds:
1. intermolecular hydrogen bond - appears between two molecules, such as methanol, ammonia, hydrogen fluoride.
2. intramolecular hydrogen bond - appears in one molecule, such as 2-nitrophenol.
Also now it is believed that hydrogen chemical bond is weak and the strong.They differ from each other in energy and bond length (the distance between the atoms):
1. Hydrogen bonds are weak.Energy - 10-30 kJ / mol, the bond length - 30. All the materials listed above are examples of normal and weak hydrogen bond.
2. Hydrogen bonds are strong.Energy - 400 kJ / mol, length - 23-24.Data obtained by experiment indicate that strong bonds are formed in the following ions: an ion-vodoroddiftorid [FHF] -, ion-hydrated hydroxide, [HO-H-OH] -, ion oxonium hydrated [H2O-H-OH2] +as well as various other organic and inorganic compounds.
Influence of hydrogen-linking
Abnormal temperatures boiling and melting enthalpy of vaporization and surface tension of some of the compounds can be explained by the presence of hydrogen bonds.Water has anomalous values of all these properties, hydrogen fluoride and ammonia - boiling point and melting point.Water and hydrogen fluoride in solid and liquid states because of the presence in them of hydrogen intermolecular bonds are considered to be polymerized.This relationship explains not only too high melting point of these substances, but also of their low density.Moreover, hydrogen bonding by melting partially collapses, due to which the water molecules (H2O) are packed more densely.
dimerization of certain substances (carboxylic acid, e.g., benzoic and acetic) can also be explained by the presence of a hydrogen connection.Diemer - two molecules that are linked together.For this reason, the boiling point of the carboxylic acids is higher than that of compounds having approximately the same molecular weight.For example, acetic acid (CH3COOH) is equal to the boiling point of 391 K, while acetone (SN3SOSN3) it is equal to 329 K.
Effect of hydrogen intramolecular bonds
This relationship 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 - a deoxyribonucleic acid (abbr .: DNA).This acid molecule folded into a double helix, the two strands of which are interconnected by hydrogen bonds.
