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Hydrogen Bonding

Hydrogen Bonding Definition in Chemistry

Hydrogen bonding or intermolecular and intramolecular hydrogen bond is the weak type of bonds due to very unstable attractive forces responsible for the formation of H-bond in learning chemistry or chemical science. The electronic configuration of hydrogen is 1s1 and the nucleus surrounded by only one electron in 1s orbital with the maximum capacity of two-electrons. Therefore, hydrogen has the capability for forming the single covalent chemical bond but if the covalently bonded another atom is a strongly electronegative chemical element, the bonds become polar. From the definition of electric polarization, the positive hydrogen end of the dipole can weakly link or bonding with another 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 defined as the hydrogen bond or bonding.

The unstable link or hydrogen bond uses to explain the unexpected physical properties like density, melting point, the boiling point of the solid, liquid, gaseous inorganic, and organic molecule. For example, the crystalline form of phenol or carboxylic acid supported due to the formation of the hydrogen bond.

Hydrogen bonding and intermolecular and intramolecular hydrogen bond in water, ice, ortho-nitrophenol molecule

Properties of Hydrogen Bond

  • The bond energy of the hydrogen bonding 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, a normal covalent bond is a strong bond than weaker hydrogen bonds present in organic or inorganic compounds.
  • The formation of Hydrogen 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 and polarity of bonds between the periodic table elements. The increasing electronegativity or electron affinity, the strength of hydrogen bonds also increases. For example, the H-bonding strength of ammonia < water < hydrogen fluoride.
  • The hydrogen bond detected by electromagnetic spectrum in 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 bond and water, ammonia, and hydrogen fluoride are examples of such type of bonding. 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 exists 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 a covalent bond. The remaining two hydrogen atoms linked to the oxygen atom of other water molecules by hydrogen bond. Therefore, in ice crystal dimensions every water molecule associated with the 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 due to breaking Hydrogen-bond.

Intramolecular Hydrogen Bond in Molecule

The hydrogen linking occurring within the single molecules is called intra-molecular hydrogen bonding. Hence intramolecular H-bond gives rise to ring formation or chelation. For an example of aromatic alcohol like o-nitrophenol define 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. Hence the p- and meta nitrophenol gathering a large number of the molecule through the hydrogen-bond.

o-hydroxy benzaldehyde also restricted hydrogen-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 Bonding

In the chemical and biological science, hydrogen bonding is very important for the existence in our environment or our life. Due to the absence of hydrogen bonds, the liquid water or ice would be in the gas phase at ordinary temperature, and without liquid water or absence of hydrogen bonding, we can not imagine the existence of an animal or vegetable life of our environment.