A hydrogen bond is an intermolecular force of attraction between two molecules. It is weaker than covalent and ionic bonds. However, stronger than van der Waals’s interactions. It is of great importance in chemistry because it hugely affects the properties of compounds. For instance, boiling point, acidity, and solubility. Many compounds have hydrogen bonds, such as water is an excellent example of hydrogen bonding.
What is a hydrogen bond?
The electrostatic force of attraction between a highly electronegative atom and a positively charged hydrogen atom is known as a hydrogen bond.
The general form of the hydrogen bond is as follows:
- ‘Z’ is a small and strong electronegative atom. Such as oxygen, nitrogen, and fluorine.
- ‘Y’ are those elements that are potential hydrogen acceptors.
Types of Hydrogen bond
There are two types of hydrogen bonds.
- Intermolecular hydrogen bond
- Intramolecular hydrogen bond
Intermolecular hydrogen bond
The type of hydrogen bonds between two or more similar or different molecules is known as intermolecular hydrogen bonds.
In this type of bond, two molecules are associated together. For example, ammonia, alcohol, water, etc contains this type of hydrogen bond.
Intramolecular hydrogen bonds
When a hydrogen bond is present within a single molecule is known as an intramolecular hydrogen bond.
This type is a kind of chelation. For example, ortho-nitrophenol, and ortho-benzoic acid contains this bonding.
Strength of hydrogen bond
The hydrogen bonds are intermediate in strength between van der Waals forces and covalent bonds. The strength of the hydrogen bonds is between 8 – 42 kJ/mol. Whereas the strength of van der Waal forces and covalent bond has 4 kJ/mol and 464 kJ/mol bond energies respectively. The strength of the hydrogen bond increases with increasing acidity. It may also increase by increasing the basicity of ‘Y’.
Effect of hydrogen bonding on properties of compounds
On Boiling point
The boiling point of compounds is greatly affected by hydrogen bonding. The compounds containing intermolecular hydrogen bonds have high boiling points. This is the reason why, water (H2O), hydrogen fluoride (HF), and ammonia (NH3) show anomalous behavior and have high boiling points than other compounds
The boiling points of some compounds are given below:
It is important to note that the boiling point of Hydrogen fluoride is expected to be higher than water but, it is lower. The reason is that fluorine can make one hydrogen bond on the other hand, in water, there are two hydrogen bonds per molecule. However, intramolecular hydrogen bonds cause a lowering in the boiling points of the substance.
Hydrogen bonds affect the solubility of compounds. Many compounds dissolve in water due to the formation of hydrogen bonds. For example, para-nitro phenol dissolves in water due to hydrogen bonding.
However, the solubility of o-nitrophenol in water is lower as compared with p-nitro phenol. This is because of the presence of intramolecular hydrogen bonding in o-nitro phenol.
The hydrogen bonds greatly affect the acidity of carboxylic acid. For example, o-hydroxybenzoic acid is more acidic than para-hydroxybenzoic acid. This is because of the greater stability of the conjugate base of ortho-hydroxybenzoic acid which is stabilized by intramolecular hydrogen bonding.
Examples of Hydrogen bonds
Hydroen bond in hydrogen floride
The HF has low acidic strength as compared to the HCl, HBr, and HI. This exceptional case of HF is due to the very small size of the fluoride ion which does not allow the proton to get apart.
Hydrogen bonds in water
Water is the best example of hydrogen bonding. The oxygen atom in water is highly electronegative and contains two loan pairs. These lone pairs attract the two naked hydrogens in the vicinity to form hydrogen bonds as shown below
Hydrogen bonds in Ammonia
In ammonia, highly electronegative nitrogen is bonded to hydrogen. Therefore, it can form a hydrogen bond.
Applications of compounds containing hydrogen bonds
Preservation of polar molecules
Water and alcohol both are polar molecules as well as can form hydrogen bonds, that’s why alcohol can be dissolved in water and vice versa. Similarly, carboxylic acids are also soluble in water. This property of polar compounds makes them important in the preservation of food such as pickles, jams, and fermented products.
The cleansing action of soap and detergents
The cleansing action of soap and detergents is because of hydrogen bonding. The non-polar part interacts with nonpolar oils and dirt present in clothes or hands whereas, the polar part of these molecules forms hydrogen bonds with water.
Hydrogen bonds in biological molecules
The hydrogen bonds also exist in living organisms. For example, proteins are present in the hair, silk, and muscles. These consist of long chains of amino acids. In amino acid groups, ›NH and ›C=O are joined together by hydrogen bonds.
For example, DNA (Deoxyribonucleic acid) has two spiral strands connected with each other through hydrogen bonds as shown below:
Paints, polymers, and the textile industry
The adhesive property of paints is due to hydrogen bonding.
The rigidity and tensile strength of thread-making materials are also because of hydrogen bonding. Additionally, Kevlar® a polyamide that is 300 times stronger than steel, containing hydrogen bonds.
Why do hydrogen bonds occur?
The electron pair is not equally shared between the highly electronegative atom and a hydrogen atom. which makes the hydrogen naked to be attracted by any other electronegative atom.
What is the strongest hydrogen bond?
Florine has a greater electronegativity than all other atoms. Therefore it forms the strongest hydrogen bond.
What is the importance of hydrogen bonds?
The water molecules are attracted to each other by hydrogen bonds. That is the reason for its cohesive and adhesive properties.
How does electronegativity affect hydrogen bonding?
The highly electronegative atom attracts the electron pair more easily. Therefore, it makes the molecules polar. Hence it makes a strong hydrogen bond.
What is the difference between hydrogen bonds and polar bonds?
A polar bond is a type of covalent bond. In which electrons are shared between two atoms. Whereas a hydrogen bond is an intermolecular force between two atoms.
How to tell if a molecule can form a hydrogen bond?
A molecule in which a hydrogen atom is attached to the oxygen, nitrogen, and fluorine atoms is capable of forming hydrogen bonds.
- The Weak Hydrogen Bond in Structural Chemistry and Biology By Gautam R. Desiraju (University of Hyderabad) and Thomas Steiner (Freie University Berlin)
- A textbook of Organic Chemistry by M. Younas (University of Punjab)
- Hydrogen bonding (byjus.com)