Hydrogen Bonding: Overview, Questions, Preparation

A hydrogen bond is not the result of the combination of atoms and the formation of a molecular orbital, unlike covalent bonds. In simple terms, it is an intermolecular electrostatic attraction force. In general, hydrogen bonds are weaker than covalent or ionic bonds, but stronger than other intermolecular forces like van der Waals forces.

When a hydrogen atom that shares a covalent bond with a highly electronegative atom like fluorine (F), oxygen (O), or nitrogen (N) experiences an electrostatic attraction to another electronegative atom in a nearby molecule or within the same molecule, it is said to be Hydrogen bonding.

When the hydrogen bonding happens within the same molecule, it is called Intramolecular hydrogen bonding. When it happens with the nearby atom of another molecule, it is called Intermolecular hydrogen bonding.

Hydrogen bonding is a type of intermolecular force that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like fluorine (F), oxygen (O), or nitrogen (N), experiences an attraction to another electronegative atom in a nearby molecule or within the same molecule. Hydrogen bonding occurs due to the dipole-dipole interaction between a hydrogen atom that is bonded to a highly electronegative atom and another highly electronegative atom in either the same molecule or in another nearby molecule. 

As per the NCERT Definition," A hydrogen bond can be defined as the attractive force which binds a hydrogen atom of one molecule with the electronegative atom (F, O or N) of another molecule." 

Hydrogen bonding is highly influenced by the physical state of the chemical compound; the magnitude of the hydrogen bonding is highest for the solid state and lowest for the gaseous state. The presence of a hydrogen bond in the chemical compound has a huge impact on properties such as melting point, boiling point, density, and others.

Hydrogen bonds are mid-range bonds in terms of strength. The hydrogen bonds are weaker than covalent bonds but stronger than van der Waals forces. This special nature of hydrogen bonds causes changes in the chemical and physical properties of compounds.

NCERT Textbooks describe the formation of the hydrogen bond in a simple manner. As you know, a hydrogen bond is nothing but a simple electrostatic attraction between two opposite charges. Hence, if you understand how these two charges are formed, it is simple to understand how they exert force on each other. Here are a few simple stages that explain the formation of hydrogen bonding.

• STAGE 1:  A highly electronegative atom (A) shares one electron with hydrogen to complete an electron pair and form a covalent bond.
• STAGE 2: Electrons experience a greater attraction force towards the nucleus of the highly electronegative atom (A).
• STAGE 3: Due to the displacement of electrons, hydrogen near the highly electronegative atom (A) develops a slight electropositive charge(δ+), and atom (A) gets an equal electronegative charge(δ-).
• STAGE 4: This condition leads to the formation of a polar molecule having an electrostatic force of attraction between a hydrogen (+ve) and the Electronegative atom (-ve). 
• If the hydrogen is within the same molecule, Intramolecular Hydrogen Bond.
• If the hydrogen is attracted to another molecule: Intermolecular Hydrogen Bond.

There are a few essential conditions for the formation of hydrogen bonds. These conditions are often asked in one way or another in CBSE and State board exams. Check the important conditions below.

• A Hydrogen Atom: A molecule must have a hydrogen atom.
• A Highly Electronegative Atom: The molecule must have a highly electronegative atom (such as F, O, or N).
• Covalent Bond: Hydrogen and the electronegative atom must share a pair of electrons to form a covalent bond between them.
• Availability of Lone Pairs: The electronegative atom must have a lone pair of electrons that allows the displacement of electrons.
• Size of Molecule: The Smaller the distance between the hydrogen (+ve) and the Electronegative atom (-ve), the greater the attraction.
• Polar Molecule: The hydrogen bonding has its root in electrostatic attraction, which means polar-polar interaction facilitates the hydrogen bonding. 

There are two main types of hydrogen bonding based on the interaction with another atom. Check below.

Intermolecular Hydrogen Bonding

When a hydrogen atom and a highly electronegative atom of two different molecules interact to form a hydrogen bond is called the intermolecular hydrogen bond.

For Example, water is the prime example of the intermolecular hydrogen bond. The hydrogen atom of one water molecule interacts with the oxygen's lone pair of another water molecule. This is the reason water has a higher melting point than expected.

Intramolecular Hydrogen Bonding

When a hydrogen atom and a highly electronegative atom of the same molecule interact to form a hydrogen bond is called the intramolecular hydrogen bond.

For Example, the hydrogen bonding in o-nitrophenol is an intramolecular hydrogen bond. In o-nitrophenol, the hydrogen of the hydroxyl group forms a hydrogen bond with the oxygen of the nitro group of the ring.

 

Special Cases: Symmetrical and Weak Hydrogen Bonds

• Symmetrical Hydrogen Bonds: When the hydrogen atom is located precisely midway between two same electronegative atoms, it forms a symmetrical hydrogen bond.
• Weak Hydrogen Bonds: When the interaction forms a weak hydrogen bond, it is called a weak hydrogen bond. It includes weaker interactions of hydrogen atom with other atoms like sulfur (S), chlorine (Cl), and even carbon (C).

Hydrogen bond in chemical compounds induces several important characteristics in the compounds:

• Higher Boiling and Melting Points: Compounds having hydrogen bonding exhibit higher boiling and melting points due to the increase in the net intermolecular attraction force. For ex: water and ammonia ( ${\mathrm{N}\mathrm{H}}_3,{\mathrm{H}}_2\mathrm{O}$ ), have higher boiling points due to the presence of intermolecular hydrogen bonds.
• Increased Solubility: Hydrogen bonding increases solubility in polar solvents. Ethanol ( ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$ ) is soluble in water because of the hydrogen bonding possible between water and alcohol molecules. 
• Increased Viscosity and Surface Tension: The basis of viscosity and surface tension is the intermolecular attraction. The Hydrogen bonding increases the net intermolecular attraction. Therefore viscosity and surface tension increase due to greater intermolecular cohesion. For example, Glycerol has high viscosity due to extensive hydrogen bonding.
• DNA Structure and Protein Folding: The hydrogen bonding between base pairs (A-T, G-C) is responsible for the double helix structure. The hydrogen bonds also maintain the secondary and tertiary structures of proteins.
• Water Density Anomaly: Water has the highest density at 277K instead of logically 273 K. This also happens due to the presence of hydrogen bonds in the water molecule. The intermolecular hydrogen bonding provides a tetrahedral structure of four water molecules combined. During the melting of ice, this tetrahedral structure dismantles, leading to closer water molecules that increase density. That is why ice floats on water. 

 

Several types of questions are asked based on the hydrogen bonding. Here are a few important points for JEE Mains exam preparation.

• Based on comparison of physical properties: For example, comparing boiling points (e.g., ${\mathrm{H}}_2\mathrm{O}$ vs. ${\mathrm{H}}_2\mathrm{S}$ ).
• Identifying molecular structure: intramolecular and intermolecular hydrogen bonding.
• Anomalies: Explaining water's density or high specific heat.