Hydrogen bond

Hydrogen bond is a weak electrostatic bond formed between a partially positively charged hydrogen atom and highly electronegative atom of the same molecule or another molecule. It is formed when the Hydrogen is bonded to small, highly electronegative atoms like F, O and N. A partial positive charge will be on hydrogen atom and partial negative charge on the electronegative atom.

The bond dissociation energy of H bond is 40 kJ/mole. H bond is represented with dotted lines (- – – -).  It stronger than van der Waals’ forces and weaker than covalent bond.

H bond is two types.

  1. Intermolecular hydrogen bonding                2.  Intramolecular hydrogen bonding

Intermolecular hydrogen bonding :

If the H bond is formed between two polar molecules it is called intermolecular hydrogen bonding. i.e., the bond is formed between the hydrogen atom of one molecule and highly electronegative atom of another molecule is known as intermolecular hydrogen bond.

Ex : Water (\text H_{2} \text {O}) ; HF : \text {NH}_{3} ; p – nitrophenol, \text {CH}_{3} \text {COOH}, ethyl alcohol etc.

Water :

hydrogen bond in water
Water molecule forms an associated molecule through intermolecular hydrogen bondin. Due to molecular association water possess high boiling point.

HF:

hydrogen bond
\text {NH}_{3} :

p – nitrophenol :

Intramolecular hydrogen bonding :

If the H bond is formed within the molecule it is known as intramolecular hydrogen bonding.

Ex: O – nitrophenol ; O – hydroxy benzaldehyde

Intramolecular hydrogen bond

Abnormal behaviour due to Hydrogen bond :

The physical state of the substance may alter. They have high melting and boiling points.

Ammonia has a higher boiling point than HCl even though nitrogen and chlorine have the same electronegativity values (3.0). Ammonia forms an associated molecule through intermolecular hydrogen bonding.

P – hydroxy benzaldehyde have a higher boiling point than o – hydroxy benzaldehyde. This is due to intermolecular hydrogen bonding in para isomer.

Ethyl alcohol is highly soluble in water due to association and co – association through intermolecular hydrogen bonding.

\text H_{2} \text {O} is liquid but \text H_{2}\text S is a gas, since H – bond is present in \text H_{2} \text {O}.