Hydrogen Bonding

Introduction

A hydrogen bond is formed between a hydrogen atom, which is already covalently bonded to another electronegative atom or group, and another atom that has a lone pair of electrons. The electronegative atom forming the hydrogen bond is called the acceptor. Such a bond is generally denoted as Dn-H…Ac. The solid line is the polar covalent bond, and the dotted line is the hydrogen bond.

General Characteristics and Formation of the Hydrogen Bond

Hydrogen bonds are weaker than covalent or ionic bonds, but they are stronger than van der Waal’s forces. Hydrogen bonding can be intermolecular, i.e. between different molecules or form in the same molecule. What usually happens is that one atom which is relatively more electronegative than hydrogen is bonded covalently to a hydrogen atom. It causes the hydrogen atom to take on a slight positive charge. When another more electronegative element – usually F, N, or O, with a pair of electrons that are free from being shared – comes in contact with slightly positively charged hydrogen atoms, they form a bond known as a hydrogen bond. There is a dipole-dipole attraction between the slightly positive hydrogen atom and the electronegative element it is bound to.

 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 can interact with each other. 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 the molecules of water are responsible for the great range in temperature that water can remain a liquid in.
3. Symmetric hydrogen bond: This type of hydrogen bond is stronger than other hydrogen bonds because, in it, 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-centre 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.

 Effects of Hydrogen Bonding

1. Boiling point: When we consider the boiling points of H2O, HF, NH3, H2S, HCl and PH3, we find that the first three have higher boiling points than the rest. It is because all three have chemical bonding, meaning that the intermolecular forces are stronger in these elements. So hydrogen bonding makes the boiling point higher.
2. Viscosity: The presence of hydrogen bonds means that the viscosity of the substance will be higher. In fact, the higher the number of hydrogen bonds that the molecules of a substance can form, the higher the viscosity.

 What prevents Hydrogen Bonds?

1. The low difference in electronegativity: For a hydrogen bond to form successfully, there should be a considerable amount of difference between hydrogen and the potential acceptor. It is the reason why PH3 cannot form hydrogen bonds. Phosphorus and hydrogen are electronegativity similar. So they cannot form hydrogen bonds.
2. The large size of the atom: When the atoms of the potential acceptor are large, the nuclei cannot interact enough to result in a hydrogen bond.

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 quite 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.