Hydrogen bonds – nature, types and importance.
The hydrogen atom is involved in two bonds: one is a covalent bond, and the second is known as a hydrogen bond.
hydrogen is known to form the proton, H+and the hydride anion, H:-
The large difference in electronegativity between hydrogen (2.1) and the second element (4.1 for fluorine, 3.5 for oxygen, 3.0 for nitrogen) leads to a highly polar covalent bond in which the hydrogen bears fractional positive charge.
The intermolecular attraction that results when a hydrogen atom comes between two small, highly electronegative atoms are called a hydrogen bond.
The hydrogen bond is not a true bond but a particular strong form of dipole - dipole interaction between the hydrogen atom in a polar bond such as O —H, N—H, or F—H, and electronegative atoms F, O, or N.
This interaction is written A—H. B or A—H..A.
A and B represent F, O, or N; A-H is one molecule and B is a part of another molecule
A-H; the dashed line represents the hydrogen bond. The three atoms usually lie along a straight line, but the angle AHB (or AHA) can deviate as much as 30o from linearity.
Some examples of this interaction are:
HF .... HF, H2O ... HOH, H3N ... HNH2, H3N .... HOH, H2O ... HO –CH3
Hydrogen bonds energies of about 4 - 40 kJ/mol and are therefore much weaker than covalent bonds such as C—H or O—H, which have bond energies of about 400 kJ/mol.