An atom of hydrogen linked covalently to a strongly electronegative atom can establish an extra weak attachment to another electronegative atom in the same or different molecules. This attachment is called a hydrogen bond.
To distinguish from a normal covalent bond, a hydrogen bond is represented by a broken line eg X – H…Y where X & Y are two electronagative atoms.
The strength of hydrogen bond is quite low about 2-10 kcal mol-1 or 8.4-42 kJ mol-1 as compared to a covalent bond strength 50-100 kcal mol-1 or 209 – 419 kJ mol-1
Conditions for Hydrogen Bonding:
∎ Hydrogen should be linked to a highly electronegative element.
∎ The size of the electronegative element must be small.
These two criteria are fulfilled by F, O, and N in the periodic table. Greater the electronegativity and smaller the size, the stronger is the hydrogen bond which is evident from the relative order of energies of hydrogen bonds.
TYPES OF HYDROGEN BONDING
Intermolecular hydrogen bonding:
This type of bonding takes place between two molecules of the same or different types.
Inter molecular hydrogen bonding leads to molecular association in liquids like water etc. Thus in water only a few percent of the water molecules appear not to be hydrogen bonded even at 90°C.
Breaking of those hydrogen bonds throughout the entire liquid requires appreciable heat energy. This is indicated in the relatively higher boiling points of hydrogen bonded liquids.
Crystalline hydrogen fluoride consists of the polymer (HF)n. This has a zig-zag chain structure involving H-bond.
Intramolecular hydrogen bonding:
This type of bonding occurs between atoms of the same molecule present on different sites. Intramolecular hydrogen bonding gives rise to a closed ring structure for which the term chelation is sometimes used.
Examples are: o-nitrophenol , salicylaldehyde
Effect of Hydrogen bonding
Hydrogen bonding has got a very pronounced effect on certain properties of the molecules. As State of the substance:
H2O exists in liquid state whereas H2S in gaseous state because hydrogen bonding exist in water and no H-bonding exists in H2S.
The organic compounds like alkane, alkenes, alkynes are insoluble in water due to absence of H-bonding whereas alcohols, organic acids, amines are soluble in water due to H-bonding.
High boiling and melting points of and HI in comparison to hydrides of other elements of V, VI and VII groups to which N, O and F belong respectively are due to hydrogen bonding.
Acidity of different isomers:
Strength of certain acids and bases can be explained on the basis of hydrogen bonding eg. when we compare the acidic strength of o – m – and p – hydroxyl benzoic acid.
The abnormally high dissociation constant of o – hydroxybenzoic acid is due to the fact that the conjugate base is stabilised by hydrogen bonding.
Illustration : H2O is a liquid at ordinary temperature while H2S is a gas although both O and S belong to the same group of the periodic table.
Solution: H2O is capable of forming intermolecular hydrogen bonds. This is possible due to high electronegativity and small size of oxygen.
Due to intermolecular H-bonding, molecular association takes place. As a result the effective molecular weight increases and hence the boiling point increases. So H2O is a liquid.
But in H2S no hydrogen bonding is possible due to large size and less electronegativity of S. So it’s boiling point is that of an isolated H2S molecule and therefore it is a gas.
IMPORTANCE OF HYDROGEN BONDING IN BIOLOGICAL SYSTEMS :
Hydrogen bonding plays a vital role in physiological systems. Proteins contain chains of amino acids. The amino acid units are arranged in a spiral form somewhat like a stretched coil spring (forming a helix). The N-H group of each amino acid unit and the fourth C=O group following it along the chain, establishes the N-H—O hydrogen bonds.
These bonds are partly responsible for the stability of the spiral structure. Double helix structure of DNA also consists of two strands forming a double helix and are joined to each other through hydrogen bond
Also Read :
Maximum Covalency & Resonance
Deviation from ideal behaviour & FAJAN’S RULE
Role of φ ( ionic Potential )