The electrostatic interaction between a strongly electronegative atom and a partially positively charged hydrogen atom is called hydrogen bonding.
Two strong electronegative elements are mostly N, O, F, and rarely CI.
Two molecules involved in H – bonding may be the same or different.
Example and Explanation
Hydrogen bonding in H₂O
Consider H₂O. In H₂O, strong electronegative O attracts a shared pair of electrons more towards itself. Thus O gets large & charge and H gets large 8 charges. Thus, dipole-dipole interaction may be developed among water molecules. However, forces of attraction among water molecules are stronger than simple dipole-dipole interaction.
Oxygen has two lone pairs of electrons, Moreover, ‘ H ‘ creates a strong electric field due to its small size the oxygen atom of one H₂O molecule links to the H – atom of another H₂O molecule through lone pair by of coordinate covalent bond. This bond formed is called a hydrogen bond. This bonding acts as a bridge between electronegative oxygen atoms.
Hydrogen bonding in HF and NH3
The hydrogen bond between HF and NH3, molecules are represented as
Hydrogen bonding in acetone and chloroform
Some elements (other than N, Q, and F) are also involved in H – bonding e.g. chloroform, the three Cl atoms make the carbon highly δ+ which in turn makes the H-atom also highly 8. This H – atom can now form an H- bond with a strongly electronegative atom of another molecule e.g. with the oxygen atom of acetone as shown in the figure
Properties of Hydrogen Bond
• Hydrogen bond is longer than a normal covalent bond
• Hydrogen bond is stronger than dipole dipole-interaction but weaker than a normal covalent bond. It is generally 20 times weaker than a covalent bond.
• Hydrogen bond is a directional bond.
• Hydrogen bond results in the formation of long chains and networks of molecules
Properties and Application of Compounds Containing Hydrogen – Bonding
1. Strength of Acids
• HF Is a Weaker Acid than HCL, HBr, and HI
In HF, molecules are H – bonded in a zigzag manner. Thus, His entrapped between two F atoms as shown in the figure
Hence, HF cannot easily donate its H ‘ ions, so it is a weaker acid
Thermodynamic Properties of Covalent Hydrides
In covalent hydrides of Group IV A to VII A H – bonding effects are very clear Consider the graph for boiling points of covalent hydrides plotted against their period numbers
Boiling points of hydrides of Group IV A are the lowest among all covalent hydrides.
It is because elements of groups IVA are least electronegative, therefore, they have the weakest intermolecular forces among all covalent hydrides. e.g. CH has the lowest boiling point because it is a very small molecule and it has the least polarizability.
The boiling points of NH, and HF. And H, O are Highest in Their Respective Series
It is because these hydrides have strong electronegativity elements N, F, and O, which form hydrogen bonding among their molecules. Thus, their boiling points are high.
H₂O has a High boiling point than HF, Although F is More electronegative than O.
It is because, the F atom can make only one H – bond per molecule due to the presence of one hydrogen, while H₂O can form two H – bonds per molecule because it has two hydrogen atoms and two lone pairs of electrons. Hence, due to the presence of strong hydrogen bonding in H₂O, its boiling point is greater than HF
The boiling point of NH is lower than HF and H₂O.
NH can form one H – bond per molecule. It is because N has only one lone pair of electrons. Also, its electronegativity is lower than O and F. Hence, its boiling point is lower than HF and H₂O.
H₂O is a liquid but H₂S and H₂Se are gases
In H₂O, strong H – bonding is present which makes it a liquid. In HS and H₂Se weak intermolecular forces are present. Thus, H₂S and H₂Se are gases at room temperature.
The boiling point of HBr Is Higher Than HCI
It is due to the bigger size of Br than Cl. Due to the bigger size of BrFso/, HBr has greater polarizability and strong London forces among its molecules than HCL. Hence, the boiling point of HBr is greater than HCI.
The hydrides of the fourth period e.g. GeH4, AsH3, H₂Se, and HBr show greater boiling points than those of the period due to larger size and greater polarizabilities.
Both H₂O and ethyl alcohol can form hydrogen bonding with Each Other
→ Substances, which can form hydrogen – bond with each other, are highly soluble in each other. Since both H₂O and ethyl alcohol can form hydrogen bonding with each other. Therefore, they are miscible with each other in all proportion
→ However, larger alcohols are not soluble in water due to non – the polar nature of the bigger hydrocarbon chains in them.
→ Similarly, small carboxylic acids (RCOOH) are also soluble in H₂O.
• Hydrocarbons are Insoluble in water
It is because they are non-polar and cannot develop H – bonding or other attractions with H₂O. Hence, hydrocarbons are insoluble in water.
3. Cleansing Action
Soaps and detergents are made up of a long non-polar hydrocarbon tail (generally alkyl or benzyl) and a polar anion head. In water, the head is stabilized by making an H – bond with H₂O, while non – polar tail remains outside because it is not soluble in water. Thus, hydrogen bonding helps in cleansing action.
4. H – Bonding in Biological Compounds and Food Materials
H – bonding is very important in living organisms.
• Large Protein molecules in living organisms are stabilized due to H – bonding.
Many fibrous proteins e.g. horns, nails, skin, feathers, hair, etc. are composed of long chains of amino acids. These chains are coiled around each other and form a spiral. This spiral is called a helix. Such helix may either be right-handed or left-handed. In right-handed helix groups like NH and C – Q are vertically adjacent to one another and they form H – bonds. These hydrogen bonds link one spiral to the other.
X-ray analysis has shown that on average there are 3.6 amino acids for each turn of the helix.
• DNA (deoxyribonucleic acid) occurs in cells
It consists of two spiral chains which are coiled about each other on a common axis bonding between their subunits and forming a double helix. This is 18-20 A in diameter. They are linked together by hydrogen bonding between their subunits.
5. H-Bonding in Paints and Dyes
The adhesive nature of certain paints and dyes is also due to H – bonding with the surfaces. Similarly, the sticky action of Glue and Honey is also due to H-bonding.
6. Food Materials such as carbohydrates e.g. glucose, fructose, and sucrose are also stabilized due to H – bonding. All these contain the OH groups which produce H – bonding.
Both natural and artificial fibers have rigidity and tensile strength due to H – bonding.
8. Structure of Ice
Hydrogen bonds H₂O have a tetrahedral electronic structure.
Ice floats on water:
In liquid H₂O, molecules form temporary H – bonds with each other. It is because due to the movement of molecules, bonds are broken and reformed. Hence, there is less regularity and less free space.
However, when the temperature of H₂O is lowered below 4 ° C, its molecules become regular and form a permanent H – bond. So, empty spaces are developed between the molecules, and their volume increase. Ice occupies 9 % more space than liquid water. Thus, the density of ice becomes less than water. Hence, it floats over water.
Similarity between the structure of Ice and diamond:
The structure of ice is just that of a diamond because each atom of carbon in the diamond is at the cenletrahedron just like the oxygen of water molecule in ice. It has a hexagonal structure with large empty spaces.
Application of low-density of Ice in cold climates:
The density of ice is less, Therefore, in a cold climate, when the temperature falls below 4 ° C, cold water lighter comes to the surface and freezes to ice. Thus an insulating layer of formed above warm water. This layer of ice prevents further heat loss from underneath the water. Thus, it protects aquatic life from cold.