Hydrogen Bonding - Types - Examples

Hydrogen Bond

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as N, O, or F atom and another very electronegative atom. Hydrogen bond strengths range from 4 kJ to 50 kJ per mole of hydrogen bonds.

Hydrogen bond between ethanol and water molecule

A hydrogen bond is a kind of bonding that is present between an atom of hydrogen and a pair of other atoms having a high electronegativity. Hydrogen-bonding used to be competitively weaker than ionic bonding or covalent bonding, but it is stronger than van der Waals forces. Hydrogen bonding can exist in two ways. One is that it can occur between atoms of different molecules or in the atoms of the same molecule.

And another one in which atom of the pair, which is also known as donor as it donated electrons  ( mostly fluorine (F), nitrogen (N), or oxygen (O) atom), is covalently bonded to a hydrogen atom (-FH, -NH, -OH). Its high electron affinity makes the hydrogen atom take on a slight positive charge. The other pairs of atoms, i.e. F, N, or O, contain an electron pair that is not shared, which provides it with a slight negative charge. To form a bond the donor atom effectively shares its hydrogen with the acceptor atom mainly through electrostatic attraction.

Effects of Hydrogen Bonding on Elements

Association

The molecules of carboxylic acids exist as dimer because of the hydrogen bonding. The molecular masses of such compounds are found to be double than those calculated from their simple formula.

Dissociation

In an aqueous solution, HF dissociates and gives the difluoride ion instead of the fluoride ion. This is due to hydrogen bonding in HF. The molecules of HCl, HBr and HI do not form a hydrogen bond. This explains the non-existence of compounds like KHCl2, KHBr2 and KHI2.

Conditions for Hydrogen Bonding:

In a molecule, when a hydrogen atom is linked to a highly electronegative atom, it attracts the shared pair of electrons more, and so this end of the molecule becomes slightly negative while the other end becomes slightly positive. The negative end of one molecule attracts the positive end of the other, and as a result, a weak bond is formed between them. This bond is called the hydrogen bond.

As a result of hydrogen bonding, a hydrogen atom links the two electronegative atoms simultaneously, one by a covalent bond and the other by a hydrogen bond. The conditions for hydrogen bonding are as follows:

·        The molecule must contain a highly electronegative atom linked to the hydrogen atom. The higher the electronegativity, the more the polarization of the molecule.

·        The size of the electronegative atom should be small. The smaller the size, the greater the electrostatic attraction.

Types of Hydrogen Bonding:

There are two types of hydrogen bondings which are given below.

1. Intramolecular Hydrogen Bonding
2. Intermolecular Hydrogen Bonding

 

Intramolecular Hydrogen Bonding:

Within a single molecule, an intramolecular hydrogen bond is formed. It is formed when two functional groups of a molecule form hydrogen bonds with each other. In order to form intramolecular hydrogen bonding, the hydrogen donor and hydrogen acceptor must be present within one molecule. They must be present within each other’s proximity.

Examples Intramolecular Hydrogen Bonding:

Ethylene glycol (C₂H₄(OH)₂) has two hydroxyls groups. Here hydrogen bonding is present between the two groups due to molecular geometry.

One more example of a molecule that shows this type of hydrogen bonding is salicylaldehyde(C₇H₆O₂).

hydrogen bonding in salicylaldehyde

Intermolecular Hydrogen Bond:

An intermolecular hydrogen bond is formed between two or more separate molecules in a substance. The donor atoms and acceptor atoms that took part in hydrogen are present in suitable positions where they can interact. 

Examples of Intermolecular Hydrogen Bond:

Some Examples of Intermolecular Bonds of Hydrogen Bonds are given below.

·        Water (H₂O)

·        Ethanol (C₂H₅OH)

·        Ammonia (NH₃)

·        Hydrogen fluoride (HF)

Properties of Hydrogen Bonding:

Some of the properties of hydrogen bonding are given below.

·        It is a weak force of attraction between molecules.

·        Any compound or molecule having hydrogen bonding shows different physical properties. For example, if hydrogen bonding is present, it increases the melting and boiling point of a substance. As a result of this, it makes it less volatile.

·        It is stronger than the weak Van der Waals bonds.

·        Hydrogen bonds are comparatively weaker than covalent and ionic bonds.

Hydrogen Bonding Examples:

Some examples of hydrogen bonding are as follows.

1. Water

A water molecule is composed of a highly electronegative oxygen atom linked to the hydrogen atom. The electrons of the shared pair are attracted more by the oxygen atom, and this end of the molecule becomes negative, and the hydrogen atoms become positive.

Chemical structure of water molecule

2. Ammonia

The hydrogen bonds in ammonia (NH3) are formed between nitrogen and hydrogen atoms. Nitrogen is a highly electronegative atom that is linked to hydrogen atoms in order to make hydrogen bonds.

chemical structure of Ammonia

3. Hydrogen Fluoride

Fluorine is an element that has the highest value of electronegativity, and it forms the strongest hydrogen bond.

Hydrogen Fluoride chemical structure

4. Alcohols

Alcohols are organic compounds. It contains at least one -OH group. When any molecule containing the hydrogen atom is connected to either oxygen or nitrogen directly, it usually has the tendency to form hydrogen bonding.

chemical structure of primary, secondary and tertiary alcohols

5. Carboxylic Acid

Hydrogen bonding can occur in a pure carboxylic acid in between two molecules of acid in order to produce a dimer. The hydrogen bonding in carboxylic acid doubles the size of the molecule.

chemical structure of Carboxylic Acid

Applications of Hydrogen Bonding:

In different types of chemical and biological processes, hydrogen bonding is necessary. Some of the applications and effects of hydrogen bonds are given below. 

Hydrogen Bonds in Plants:

Water has the property to stick to itself (cohesion) and also with other molecules (adhesion). When water droplets fall on a leaf, the hydrogen bonds present between the molecules of water are more substantial than the intermolecular forces of adhesion between the water molecules and the leaf. The high surface tension of water is explained by this property.

Hydrogen Bonds in Proteins:

Intramolecular hydrogen bonding is responsible for different types of proteins such as secondary proteins, tertiary proteins, and quaternary proteins and as well as for the structure of the nucleic acids.

Hydrogen Bonds in DNA (Deoxyribonucleic Acid)

The double-helix model of DNA consists of two intertwined strands held together by a base pair. The hydrogen bonding present between the bases on adjacent strands is responsible for this. Because of different structure bases, adenine (A) always forms hydrogen bonds with thymine (T). Guanine (G) always forms hydrogen bonds with cytosine (C) in contrast.

Conclusion 

A hydrogen bond is a type of chemical bonding that is formed because of the electrostatic attraction between a hydrogen atom and an atom containing a lone pair of electrons in a substance. We get important information related to hydrogen bonding, such as hydrogen bonding examples, properties and effects of hydrogen bonds etc.

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