Chemical Bonding and Molecular Structure Part - 16 | Class 11th

  ü  Hydrogen Bonding: The Bond which is formed between hydrogen and electronegative species such as N, O and F. It is weaker than a covalent bond but stronger than van der Waal’s force. It is represented by dotted line (……...).

Reason for Hydrogen Bonding: When H is bonded to a strongly electronegative atom X, the shared electron pair is shifted more towards X. So, H atom gets a slight positive charge (δ⁺) and the electronegative atom gets a slight negative charge (δ^-). This results in the formation of a polar molecule. The electrostatic force of attraction between these polar molecules is termed as H-bonding.

Thus, the hydrogen bond between the hydrogen atom bonded to the electronegative atom X may be represented as follows:

– – – X – H – – – X – H – – – X – H – – –

In such situation the hydrogen atom is attached simultaneously to two electronegative atoms. Hence it acts as a bridge between the two and is, therefore, called the hydrogen bridge.

Types of Hydrogen Bonding: There are two types of H bonds- inter molecular H-bonding and intra molecular H-bonding.

Inter-molecular H bonding: It is the H bond formed by H atom of one molecule and the electronegative atom of another molecule.

Inter molecular H bonding influences the physical properties of the compounds. For example, water (H₂O) is a liquid with high boiling point but hydrogen sulphide (H₂S) is a gas. This is because in water inter molecular H bonding is possible which is not possible in H₂S.

Ø  Intra-molecular Hydrogen Bonding: It is the H bond formed between H atom and the electronegative atom of the same molecule.

An important example of intra–molecular hydrogen bonding is that of o–nitrophenol. Boiling point of o–nitrophenol is 214 ⁰C as compared to 279 ⁰C for p–nitrophenol. Further o–nitrophenol is more volatile in steam and less soluble in water than the other two isomers. All the above facts can be explained on the assumption that o–nitrophenol contains hydrogen bond represented as

              

Due to the intra–molecular hydrogen bonding, the solubility of o–nitrophenol in water is also reduced. In m– and p– isomers, intra–molecular hydrogen bonding is not possible so inter–molecular hydrogen bonding takes place. This explains higher boiling points of m– and p–isomers.

Ø  Conditions for Hydrogen Bonding: In case of inter molecular hydrogen bonding, the most important condition is that the molecules must contain one hydrogen atom linked to one highly electronegative atom. In case of intra–molecular hydrogen bonding, the following conditions are favorable for hydrogen bonding:

(i)  The molecule should contain two groups such that one group contains H–atom linked to a highly electro–negative atom and the other group should also contain a highly electronegative atom linked to a lesser electronegative atom.

(ii)    The molecule should be planar.

Importance of Hydrogen Bond:

(i) Hydrogen Bonds are important in fixing properties such as solubilities, melting points and boiling points and in determining the form and stability of crystalline structures.

(ii) Hydrogen bonds are important in biological systems. Proteins, for example, contain both CO and NH group and hydrogen bonds can be formed to bridge the space between H and O. The structure and hence the properties of proteins depend upon the existence of hydrogen bonds.

(iii)  Hydrogen bonding within a single molecule is one of the chief factors in determining the structure of important biological substances.