Explanation of Hydrogen Bonding

When a hydrogen atom combines with a very high electrically negative element to form a covalent compound, the electrons participating in the bond are more attracted to a very high electronegative element.

As a result, polarity is created between them. When such polar molecules come close to each other, the positive hydrogen end is particularly attracted to the negative end of the other molecule and forms a bond through a weak attraction.

This weak attraction is called hydrogen bonding. The hydrogen bond is expressed by the dot (. . .) sign. A hydrogen-bonding strength is about 0.01.

For example, Hydrogen bonding is observed in molecules like hydrogen fluoride(HF), water(H₂O), ammonia(NH₃), ethanoic acid(CH₃COOH), and phenol(C₆H₅OH), etc.

Properties of hydrogen bond

1. Hydrogen bonds are weak bonds. Even more so than covalent bonds. Hydrogen bonding strength 8–42 kJ/mol. On the other hand, the covalent bond strength is 200–450kj/mol.
2. The strength of the hydrogen bond depends on the value of the electronegativity of the hydrogen atom to the other atom. The higher the value of electronegativity of the connected atom, the higher the hydrogen bonding force. The values of electronegativity of elements F, O, and N are 4.0, 3.5, and 3.0. A sequence of hydrogen bonding energy H — F> H — O> H — N.
3. Hydrogen bonds have specific bonding orientations.
4. The position of the hydrogen bond depends on the direction of the unpaired electron present in the atom of the electronegative element associated with the hydrogen atom.
5. A large number of molecules are joined together by hydrogen bonding. As a result, the molecules remain in a cohesive state.
6. The physical position of the molecule changes in hydrogen bonding.
7. The size of a hydrogen atom is smaller than the size of an atom of another electronegative element bound by a hydrogen bond. As a result, a strong repulsive force acts between the last orbital electrons of the two atoms.
8. To minimize the value of repulsion, the positions of hydrogen and electronegativity are linear.
9. The effect of hydrogen bonding is to change the melting point, boiling point, solubility, density, viscosity, and surface texture of the compound.

Prerequisites for Hydrogen Bonding

1. The corresponding molecules must have hydrogen atoms.
2. The atom attached to the hydrogen atom in the corresponding molecule must be an extremely electronegative element. E.g., O, F, N.
3. Molecules must have unpaired electrons.
4. The influence of unpaired electrons plays a major role in the formation of hydrogen bonds.
5. The bond between the electronegative element and the hydrogen atom must be more polarized.
6. The size of the electronegative atom attached to the hydrogen atom must be small. The smaller the size of the atom attached to the hydrogen atom, the more effective the polarization between the positive edge of the hydrogen atom and the negative edge of the electronegative element becomes more effective. The efficiency of hydrogen bonding is also increased. For this reason, Cl, Br, S, and P form compounds with negative elements of hydrogen but do not form hydrogen bonds.
7. Must have static electron attraction. This attraction results in the formation of hydrogen bonds.

Types of Hydrogen Bonding

There are two types of hydrogen bonds.

i. Intermolecular hydrogen bonding.
ii. Intramolecular hydrogen bonding.

Intermolecular hydrogen bonding

Hydrogen bonds formed between different molecules of the same or different compounds are called intermolecular hydrogen bonds. Hydrogen bonds are formed between individual molecules of the same or different compounds. E.g. HF, H₂O, CH₃COOH.

Intramolecular hydrogen bonding

Hydrogen bonding between different parts of the same molecule of the same compound is called intramolecular hydrogen bonding. The formation of hydrogen bonds is called chilation.

E.g., C₆H₄(OH)(NO₂), hydroxy-benzaldehyde C₆H₄(OH)CHO, salicylic acid C₆H₄(OH)COOH. Intramolecular hydrogen bonds exist between these molecules.