NCERT Solutions for Class 12 Chemistry Hydrogen Bonding Notes
A Hydrogen Bond is an electromagnetic attraction between polar molecules in which hydrogen atom (H) is bound to a larger atom, such as oxygen (O), nitrogen (N) or fluorine (F). This is an attraction between positive and negative poles of the charged atoms.
The electrons carry with them a negative charge, so wherever the electrons move they give negative charge, this results in unequal sharing of electrons. In a molecule, when the hydrogen atom, which is covalently linked to a highly electronegative atom like oxygen, nitrogen or fluorine, experiences the electrostatic field of another highly electronegative atom of the nearby molecule, then hydrogen bonds are formed. One atom of the pair (the donor), generally a fluorine, nitrogen or oxygen atom, is covalently bonded to a hydrogen atom (-FH, -NH or –OH), whose electrons it shares unequally. Its high electron affinity gives hydrogen a slight positive charge. The other atom of the pair, typically F, N, or O has an unshared pair of the electron; hence it has a slight negative charge. Mainly through electrostatic attractions, the donor atom shares its hydrogen with the acceptor atom hence forming a hydrogen bond.
The small sizes of nitrogen, oxygen, and fluorine are essential to H bonding because it makes those atoms electro-negative that their covalently bonded H is highly positive. Another reason is that it allows the lone pair on the other oxygen, nitrogen, or fluorine to come close to the hydrogen.
The Hydrogen bond is stronger than Van der Waals forces, but they are weaker than covalent bonds and ionic bonds. Hydrogen bonds can occur between the molecules (intermolecular) or within different parts of a single molecule (intramolecular).
The vitality of Hydrogen bonding in the living system
- Due to the extensive hydrogen bonding in water molecules, water is a liquid, and it is considered to be a universal solvent for most of the substances, as it readily forms a hydrogen bond with many solutes. All the biochemical reactions occurring inside the living system takes place in water.
- Hydrogen bonding between amino acids (building blocks of proteins) in a primary/linear structure of protein molecules helps to determine the tertiary or the functional configuration of the protein Hence, the hydrogen bonding plays an important role in stabilizing the three-dimensional structure of protein molecules.
The Hydrogen bonds between the nitrogenous bases (the building blocks of nucleotides, which are the building blocks of nucleic acids) on the two strands of DNA (deoxyribonucleic acid) gives rise to a double-helical structure of DNA which is vital for the transmission of genetic information from one generation to another generation in the living organisms.
Properties of Hydrogen Bonds
- Hydrogen bonding affects the boiling point of the molecules. The boiling point usually increases with the increase of the molecular mass. However, molecules that are involved in intermolecular H-bonding have much higher boiling points in comparison with the molecules of the same molecular mass that are not involved in H-bonding. This is because the unusually strong H-bonding forces allow for stronger interaction. For example in water molecules the strong hydrogen bond results in a higher boiling point of water. In addition, H-bonding is also responsible for many unusual proprieties of water, such as its high melting point, the heat of vaporization, high dielectric constant, surface tension, capillary action, etc.
- As we have discussed hydrogen bonding can occur between hydrogen and four other elements. Oxygen (the most common), Fluorine, Nitrogen, and Carbon. Carbon is the special case in that it only really interacts in hydrogen bonding when it is bound to very electronegative elements.
- Hydrogen bonding is an important component of the three major macromolecules/biomolecules in biochemistry such as proteins, nucleic acids, and carbohydrates. These are all hydrocarbons containing carbon as an important constituent. The H-bonding is responsible for the structure and properties of all the macromolecules. Hydrogen bonding is applicable in these biomolecules because of the presence of functional groups present, like carboxylic acid (-COOH), alcohol (-OH) or even amine groups (NH2).
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