Chemical Bonding and Molecular Structure
7.0 Hydrogen Bonding
7.0 Hydrogen Bonding
In $1920$, Latimer and Rodebush introduced the idea of “hydrogen bond” to explain the nature of association in liquid state of substance like water, hydrogen fluoride, ammonia, formic acid etc.
In a hydrogen compound, when hydrogen is bonded to highly electronegative atom (such as $F$, $O$, $N$) by a covalent bond, the electron pair is attracted towards electronegative atom so strongly that a dipole results i.e., one end carries a positive charge ($H-$end) and other end carries a negative charge ($X-$end).
If a number of such molecules are brought nearer to each other, the positive end of one molecule and negative end of the other molecule will attract each other and weak electrostatic force will develop. Thus, these molecules will associate together to form a cluster of molecules.
The attractive force that binds hydrogen atom of one molecule with electronegative atom of the other molecule of the same or different substance is known as hydrogen bond.
Hydrogen bonding is of two types:
$1.$ Intermolecular hydrogen bonding:
This type of bonding results between the positive and negative ends of different molecules of the same or different substances.
Example:
$(a)$ Ammonia -
$(b)$ Water -
$(c)$ Acetic acid -
This type of hydrogen bonding increases the boiling point of the compound and also its solubility in water. The increase in boiling point is due to association of several molecules of the compound.
$2.$ Intramolecular hydrogen bonding:
This type of bonding results between hydrogen and an electro-negative element both present in the same molecule. This type of bonding is generally present in organic compounds. Examples are $o-$nitro-phenol, $o-$hydroxy benzoic acid, etc.
This type of bonding decreases the boiling point of the compound. The solubility of the compound also decreases. Hence compound becomes more volatile.
