Hydrogen Bonding

Overview

The following topics are covered on this page :

• What is a hydrogen bond
• Important points regarding hydrogen bond
• Types of hydrogen bond
• Effects of hydrogen bond

Let's get started.

What is a Hydrogen Bond

Nitrogen, oxygen and fluorine are highly electronegative elements. When they form a covalent bond with hydrogen atom, due to difference in electronegativity, dipoles are created where hydrogen atom acquires a partial positive charge and the electronegative atom acquires a partial negative charge. This partially positively charged hydrogen atom forms a weak bond with another highly electronegative atom nearby as depicted below :

This weak bond, usually represented by dotted lines, is known as hydrogen bond.

Important Points Regarding H-Bond

• Hydrogen bond involves dipole-dipole attractive interaction.
• H-bond is formed between a hydrogen atom and a highly electronegative atom.
• The size of the electronegative atom should be small in order to form H-bond. Thus, only nitrogen, oxygen, and fluorine form hydrogen bonds.
  Chlorine having the same electronegativity as that of nitrogen atom does not form hydrogen bond due to its large size.
• Hydrogen bond is a weaker bond than covalent and ionic bond, but stronger than van der Waal's forces.

Types of Hydrogen Bond

There are two types of hydrogen bonds.

1. Intermolecular hydrogen bond
2. Intramolecular hydrogen bond

Intermolecular hydrogen bond

Intermolecular hydrogen bond is formed between two different molecules of the same or different compounds.

Examples of intermolecular hydrogen bonding (different molecules of the same compound) : HF, R-OH (R is alkyl group), H₂O etc.

Intermolecular hydrogen bond in hydrogen fluoride (HF) ↓

Intermolecular hydrogen bond in alcohol molecule↓

Examples of intermolecular hydrogen bonding in different compounds : 'Water and alcohol' , 'Ammonia and water' etc.

Intermolecular hydrogen bond between ethanol and water

Intramolecular hydrogen bond

Intramolecular hydrogen bond is formed when hydrogen atom is in between the two highly electronegative atoms present within the same molecule. Example : In salicylic acid, hydrogen is attached to one oxygen with a covalent bond and to other with intramolecular hydrogen bond.

Salicylic acid

Effects of Hydrogen Bond

Compounds that are capable of forming hydrogen bond tend to show abnormality in their physical properties. Some effects of hydrogen bond are given below :

• On melting and boiling point
  The melting and boiling points of compounds having hydrogen bond are abnormally high. Example :
  1. NH₃ has higher boiling point than PH₃ even though PH₃ is a larger molecule with greater dispersion forces than ammonia (NH₃).
  2. Because of the presence of hydrogen bond in ethanol, the boiling point of ethanol is higher than that of ethoxy ethane.
• On solubility
  Compounds that form hydrogen bond are polar; hence, they are soluble in polar solvents such as water. Example :
  1. Lower alcohols are soluble in water.
  2. Ammonia is soluble in water.
• On H₂O
  Although oxygen, sulfur, selenium, tellurium all belong to group 16, only H₂O exists as liquid whereas H₂S, H₂Se and H₂Te are all gases at normal temperature. The reason being the presence of hydrogen bond in H₂O molecule.

  Intermolecular hydrogen bonding in water

  
• On carboxylic acid
  

  Many carboxylic acids exist as dimer due to intermolcular hydrogen bonding.

  Intermolecular hydrogen bond in carboxylic acid↓

  
• On viscosity and surface tension
  Substances that are capable of forming hydrogen bond tend to exist as associated molecules. As a result, they have higher viscosity and surface tension.