Hydrogen Bond

A hydrogen bond also expressed as an H-bond can be defined as the electrostatic bond of a hydrogen atom with a covalent bond of another electronegative group of elements or atoms. 

Electrostatic force can be explained as the force generated between two objects (in the case of a hydrogen bond- between two atomic particles) due to electric charges, which makes the two bodies attracted towards each other.

A covalent bond can be explained as a chemical bond between two atomic particles, which is created due to sharing of electron pairs between the two respective atomic particles. A covalent atomic bond is also referred to as bonding pairs or shared pairs. 

What is a Hydrogen bond?

A hydrogen bond can be explained as a weak bond created between a hydrogen atom and an electronegative element that has a lone pair to share with the hydrogen atom to create an electrostatic attraction between portions of the molecule of the two atoms.

The nature of a hydrogen bond is either dipole-dipole type, ion-dipole type, or dipole-induced dipole type.

The hydrogen atom forms a bond with mainly the chemical elements of fluorine, nitrogen, and oxygen. In the context of organic chemistry, it forms a bond with the elements of carbon and chlorine.

There are certain prerequisites for forming a hydrogen bond, such as the element forming a bond with the hydrogen atom must be electronegative. Also, the atom’s electronegativity must be equal to or greater than 3. And finally, the atom must have a lone pair to share with the hydrogen atom, which must be smaller in size.

The elements which form a bond with hydrogen have the following electronegative charge:

• Fluorine -4.0
• Oxygen -3.5
• Nitrogen -3
• Chlorine -3 (the study of the formation of a hydrogen bond between the hydrogen atom and the atom of chlorine is known as organic chemistry)

Types of Hydrogen Bonds

There are three types of hydrogen bonds. They are:

1. Intramolecular: This type of bond is formed when the hydrogen donor (the electronegative element that hydrogen is covalently bonded with) and the hydrogen acceptor (the electronegative element or group with which the hydrogen atom forms the hydrogen bond) are present close to each other in the molecule. For example, the hydroxyl ions in ethylene glycol form hydrogen bonds. The two hydroxyl ions are present close together. So the hydrogen atom of one ion forms a hydrogen bond with the oxygen atom of the other ion.
2. Intermolecular: These hydrogen bonds can be formed between any molecules as long as the hydrogen donor and the hydrogen acceptor interact. Such intermolecular hydrogen bonds can form between ammonia molecules and water molecules alone or between the molecules of ammonia and water. The hydrogen bonds between water molecules are responsible for the great range in temperature that water can remain a liquid in.
3. Symmetric: This type of hydrogen bond is stronger than other hydrogen bonds because the proton is situated exactly halfway between the donor and the acceptor. So the bonds on both sides of the proton are equal in strength. Its strength can be compared to covalent bonds. It is an example of a three-center four-electron bond. This kind of bond is found in ice and anhydrous acids at high pressure.

Dihydrogen Bond

This type of bond is worth mentioning because it is related to hydrogen bonds but is not a hydrogen bond. In hydrogen bonds, there is generally an acceptor, a non-metallic atom (usually N or the chalcogen group). But in dihydrogen bonds, the acceptor is usually a metal hydride. Thus the interaction is between two hydrogen atoms forming a hydrogen-hydrogen bond. They are similar to hydrogen bonds in molecular geometry.

Hydrogen bonding as it occurs in nature:

1. Water: A lot of the unique properties of water are because it has hydrogen bonding. The following properties of water are affected by hydrogen bonding:
   1. High surface tension
   2. High boiling point
   3. Low vapour pressure
2. Plants: Transport of water in the xylem of the plants takes place because not only do the water molecules form hydrogen bonds with each other but with the cellulose of the plant vessel. It is how capillary action takes place, allowing water to rise in the transport vessels of the plants. That is why tall trees can pull water up and transport it to the highest parts even against the force of gravity.
3. Proteins: The secondary structure of proteins exhibits hydrogen bonding frequently. The N and O atoms are relatively electronegative, and, therefore, the hydrogen atom can form a sufficiently polar covalent bond with nitrogen and hydrogen bonds with the oxygen atoms. These bonds may be weak individually, but they are repeated enough in a pattern in proteins to give the proteins enough strength.

Conclusion

Hydrogen bonding is the key to understanding several properties and interactions of elements. It is also helpful in designing drugs. Most orally active drugs have five to ten hydrogen bonds. Since proteins too exhibit hydrogen bonding, it plays an important part in illuminating several aspects of how certain proteins behave. It is because of hydrogen bonding that several elements display anomalous behavior. But with a knowledge of how hydrogen bonding affects their molecular interactions, these anomalies can be explained.