4.9 Hydrogen Bonding

Nitrogen, oxygen and fluorine are the highly electronegative elements. When they are attached to a hydrogen atom to form covalent bond, the electrons of the covalent bond are shifted towards the more electronegative atom. This partially positively charged hydrogen atom forms a bond with the other more electronegative atom. This bond is known as hydrogen bond and is weaker than the covalent bond. For example, in HF molecule, the hydrogen bond exists between hydrogen atom of one molecule and fluorine atom of another molecule as depicted below :
\(
-~-~-\mathrm{H}^{\delta+}-\mathrm{F}^{\delta-}-~-~-\mathrm{H}^{\delta+}-\mathrm{F}^{\delta-}-~-~-\mathrm{H}^{\delta+}-\mathrm{F}^{\delta-}
\)
Here, hydrogen bond acts as a bridge between two atoms which holds one atom by covalent bond and the other by hydrogen bond. Hydrogen bond is represented by a dotted line \((-~-~-)\) while a solid line represents the covalent bond. Thus, hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electronegative atom ( \(\mathrm{F}, \mathrm{O}\) or \(\mathrm{N})\) of another molecule.

Cause of Formation of Hydrogen Bond

When hydrogen is bonded to strongly electronegative element ‘ \(X\) ‘, the electron pair shared between the two atoms moves far away from hydrogen atom. As a result, the hydrogen atom becomes highly electropositive with respect to the other atom ‘ \(X\) ‘. Since there is displacement of electrons towards \(X\), the hydrogen acquires fractional positive charge \(\left(\delta^{+}\right)\)while ‘ \(\mathrm{X}\) ‘ attain fractional negative charge \(\left(\delta^{-}\right)\). This results in the formation of a polar molecule having electrostatic force of attraction which can be represented as :
\(
\mathrm{H}^{\delta+}-\mathrm{X}^{\delta-}-~-~-\mathrm{H}^{\delta+}-\mathrm{X}^{\delta-}-~-~-\mathrm{H}^{\delta+}-\mathrm{X}^{\delta-}
\)
The magnitude of H-bonding depends on the physical state of the compound. It is maximum in the solid state and minimum in the gaseous state. Thus, the hydrogen bonds have strong influence on the structure and properties of the compounds.

Types of H-Bonds

There are two types of H-bonds

• Intermolecular hydrogen bond: It is formed between two different molecules of the same or different compounds. For example, H-bond in case of HF molecule, alcohol or water molecules, etc.
• Intramolecular hydrogen bond: It is formed when hydrogen atom is in between the two highly electronegative \((\mathrm{F}, \mathrm{O}, \mathrm{N})\) atoms present within the same molecule. For example, in o-nitrophenol the hydrogen is in between the two oxygen atoms.