Define Hydrogen Bonding in Chemistry
Hydrogen bond or intermolecular and intramolecular hydrogen bonding is the unstable types of bonds due to very unstable attractive forces responsible for the formation of H-bond in chemistry or chemical science. The electronic configuration of hydrogen is 1s1. Hence hydrogen nucleus surrounded by only one electron in 1s orbital with the maximum capacity of two-electrons. Thus hydrogen has the capability for forming the single covalent chemical bond. When hydrogen covalently bonded to the strongly electronegative element the bonds become strongly dipolar. Due to polarization, the positive hydrogen end of the dipole can weakly link some other negative dipole end present in the same molecule like o-nitrophenol or another molecule like water, ice, ammonia, etc. This weak secondary link between two electronegative atoms through hydrogen atom is called the hydrogen bond.
Therefore, the hydrogen bond is an unstable link uses to explain the unexpected properties like density, melting point, boiling point, the solid, liquid, gaseous state of the inorganic, and organic molecule. For example, the crystalline form of phenol or carboxylic acid supported by hydrogen bonds.
Properties of Hydrogen Bond
- The bond energy of the H-bond in the range of 3 to 10 kcal mol-1 but in normal covalent bond in the range of 50 to 100 kcal mol-1. Therefore, the hydrogen bond is much weaker than the normal covalent bond for learning chemistry.
- The formation of H-bonding does not involve any sharing of the electron particles, hence H-bond is quite different from the normal covalent bond.
- The strength of hydrogen bonds directly related to the electronegativity of polar bonding periodic table elements. With the increasing electronegativity or affinity, the strength of hydrogen bonds also increases. Hence H-bonging strength of ammonia < water < hydrogen fluoride.
- The hydrogen bond detected by electromagnetic absorption spectroscopy, infrared spectrum, and x-ray method.
Intermolecular Hydrogen Bonding
The hydrogen linking occurring between two or more similar or different molecules is called intermolecular hydrogen bonding. For example, water, ammonia, and hydrogen fluoride. In Hydrogen fluoride, the positive end of the one dipole attracts the negative end of another similar dipole, and these molecules associated together to form the cluster, (HF)n. Molecular association of water form the polymerize molecule like (H2O)n in which hydrogen acts as a bridge between two highly electronegative oxygen atoms. But in hydrogen sulfide, no such cluster formation through hydrogen link. Therefore, water exist in liquid form but hydrogen sulfide exists as a gas molecule.
Hydrogen Bonding in Water and Ice Molecule
Ice is a crystalline solid. In the ice crystal lattice, the oxygen-atom tetrahedrally surrounded by four hydrogen atom. Two hydrogen atoms linked to the oxygen atom by covalent bonding and remaining two hydrogen atom linked to the oxygen atom of other water molecules by hydrogen bonding. Therefore, in ice crystal dimensions every water molecule associated with other four water molecules by hydrogen bonding in a tetrahedral fashion with a large amount of space. When ice heat to melt from o to 4°C, the hydrogen bonds are broken down and the space between water molecules decreases. Therefore, the density of water maximum at o to 4°C. Above 4°C, the kinetic energy of the molecules sufficient to disperse from each other, and the concentration steadily decrease to form water.
Intramolecular H-Bonding in Molecules
The hydrogen linking occurring within the single molecules is called intra-molecular hydrogen bonding. Hence intramolecular hydrogen bonding gives rise to ring formation or chelation. For an example of aromatic alcohol like o-nitrophenol have have an intramolecular hydrogen bond. Therefore, o-nitrophenol boils at 214°C, while m- and p-isomer boils at 290°C and 270°C respectively. Here o-nitrophenol has hydrogen bonding limited within the same molecule but p- and m-nitrophenol extends to the neighboring molecules. Therefore, the p- and meta nitrophenol gathering a large number of the molecule through the H-bond.
o-hydroxy benzaldehyde also restricted H-bond within the molecule. Thus this leads to weakening acid properties of the compound. Due to H-bonding, the scope for the formation of hydrogen ion in the solution limited which increases the pH of the solution.
Biological Importance of Hydrogen Bonds
In chemical and biological science, the importance of hydrogen bonding is very important for the existence in our environment. Due to the absence of hydrogen bonding, the liquid water or ice would be in the gas phase at ordinary temperatures. Hence without liquid water, we can not imagine the existence of an animal or vegetable life of our environment.