Hydrogen Bonding
What is Hydrogen Bonding?
Let us consider a molecule, H-A, in which the H atom is linked with a strongly electronegative atom A (A maybe F, O, N, etc.) by a normal covalent bond. The electron pair is shared between the H atom and the A atom.
Due to the high electronegativity of atom A, the electron pair will evidently lie far away from the H atom, and thus partial positive and negative charges will be developed on the H and A atoms, respectively. Consequently, the H-A molecule will have a dipole represented as Hδ+-Aδ-, If another molecule H –B which also forms a dipole Hδ+– Bδ- (B is strongly electronegative) is brought near H- A dipole, these two dipoles will be linked together by a special type of bond which is called “Hydrogen bond” and is represented by dotted lines. Aδ- – Hδ+……. Bδ- – Hδ+.
H-atom acts as a bridge between the electronegative atoms A and B. Thus hydrogen bonds can be defined as follows:
The attractive force that binds a hydrogen atom, which is already covalently attached with a strongly electronegative atom of a molecule with another electronegative atom of some other molecule, is known as a hydrogen bond.
Types of Hydrogen Bonding
Hydrogen bonds have been classified into two major classes:
- Intermolecular H – Bonding (Association)
- Intramolecular H – Bonding (Chelation).
1. Intermolecular Hydrogen Bonding
Hydrogen bonds occurring between two or more similar or different molecules are called intermolecular Hydrogen bonding.
For example, molecules like HF, NH3, and H2O form intermolecular hydrogen bonds.
2. Intramolecular Hydrogen Bonding
H-bonding occurring within a single molecule is called intra-molecular H-bonding.
An important example of a molecule having intramolecular H-bonding is furnished by O-nitro phenol, which boils at 214°C while the m– and P– isomers boil at 290°C and 270°C, respectively. Another well-known example of intramolecular hydrogen bonding is the hydrogen bonding of Salicylic acid.
The energy of Hydrogen Bonding
The bond energy of the H – bond is in the range of 3 – 10 Kcal/mole, while that of a normal covalent bond is in the range of 50 – 100 Kcal/mole. Thus H – bond is much weaker than a covalent bond. The formation of the H – bond does not involve any sharing of electrons pair. It is, therefore, quite different from a covalent bond. Since hydrogen bonding occurs only with electronegative atoms, it is likely that it involves mainly electrostatic interaction. This simple electrostatic model is in fair agreement with experimental data. The energy of the hydrogen bond decreases as the electronegativities of the bonded atom decrease.
