Hydrogen Bond: Types, Strength, Effects & Examples

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 forces. As it greatly affects the properties of compounds. Such as boiling point, acidity, and solubility.  Many compounds have hydrogen bonds. Although among them 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 hydrogen bond.”

The general form of the hydrogen bond is the following:

  • 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 bonds

“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 bonds.

Intramolecular hydrogen bonds

“When a hydrogen bond is present within a single molecule is known as intramolecular hydrogen bond.”

This type is a kind of chelation. For example, ortho-nitrophenol, ortho-benzoic acid, etc 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 is 4 kJ/mol. The bond dissociation energy of the covalent bond in water is 464 kJ/mol.  The strength of the hydrogen bonds 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 hydrogen bonds have high boiling points. The boiling points of some compounds are given below:

Hence H2O, HF, and NH3 have high boiling points than other compounds. This is because of the presence of hydrogen bonding in these compounds. We expect that the boiling point of fluorine should be higher than water. However, it is lower. The reason is that fluorine can make one hydrogen bond. Whereas the water formed two hydrogen bonds per molecule.

On solubility

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. This is because of the presence of intramolecular hydrogen bonding in o-nitro phenol.

On acidity

The hydrogen bonds greatly affects 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. This is stabilized by intramolecular hydrogen bonding.

Examples of Hydrogen bonds

hYDROGEN BOND IN HYDROGEN FLORIDE

The HF has low acidic strength as compared to the HCl, HBr, and HI. This exceptional case is due to the hydrogen bonding in HF.

Hydrogen bond in water

The water molecule is the best example of hydrogen bonding. The oxygen atom in water is more electronegative, as compared to hydrogen. The electrostatic force of attraction between these atoms is responsible for the hydrogen bond.

Hydrogen bond in Ammonia

In ammonia, highly electronegative nitrogen is bonded to hydrogen. Therefore, it can form a hydrogen bond.

 

Applications of compounds containing hydrogen bonds

Solubility of hydrogen-bonded molecules

Water and alcohol both can form hydrogen bonds. Therefore, alcohol can dissolve in water. Carboxylic acids are also dissolved in water. This is because both can form hydrogen bonds with each other. Hydrocarbons cannot dissolve in water. This is because they are non-polar. There is no possibility of the formation of the hydrogen bond.

The cleansing action of soap and detergents

The cleansing action of soap and detergents is because of hydrogen bonding. The polar part of their molecules forms hydrogen bonds with water.  Whether the non-polar part is insoluble in 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.

fig

DNA (Deoxyribonucleic acid)  has two spiral chains. These chains are joined together by hydrogen bonds between their subunits.

fig

Hydrogen bonds in paints dyes and textile materials

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.

Concepts Berg

Why do hydrogen bonds occur?

The electron pair is not equally shared between the highly electronegative atom and a hydrogen atom. Therefore, the hydrogen bond is formed.

What is the strongest hydrogen bond?

Florine has a greater electronegativity than all other atoms. Therefore it forms a strong hydrogen bond.

What is the importance of a hydrogen bonds?

The water molecules are attracted to each other by hydrogen bonds. That is the reason it is very cohesive. We can see the cohesiveness of water in our daily life. For example, in drops of water.

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 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 atom is capable of forming hydrogen bonds.

 

Reference Books

  • 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 the Punjab)

Reference links

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