Hydrogen Bonding is a type of intermolecular force in which a hydrogen atom is shared between two molecules. This type of bonding is stronger than van der Waals forces, but weaker than covalent bonds.

Hydrogen bonding is a special type of intermolecular force that is created when a hydrogen atom, which is covalently bonded to a highly electronegative atom, interacts with another highly electronegative atom that is nearby. As an example, in water molecules (H2O), hydrogen is covalently bonded to the more electronegative oxygen atom, and so hydrogen bonding occurs due to the dipole-dipole interactions between the hydrogen atom of one water molecule and the oxygen atom of another H2O molecule.

The O-H bond has its bond pair of electrons close to the oxygen nucleus, due to the large difference in electronegativities of oxygen and hydrogen. This results in the oxygen atom developing a partial negative charge (-δ) and the hydrogen atom developing a partial positive charge (+δ). This then allows hydrogen bonding to occur, which is an electrostatic attraction between the hydrogen atom of one water molecule (with +δ charge) and the oxygen atom of another water molecule (with -δ charge). This is a special class of intermolecular attractive forces, as it only occurs when hydrogen is bonded to a highly electronegative atom. Hydrogen bonds are usually strong compared to normal dipole-dipole and dispersion forces, but weak compared to true covalent or ionic bonds.

Table of Contents

Conditions for Hydrogen Bonding

Effects of Hydrogen Bonding

Examples of Hydrogen Bonding

Strength of the Hydrogen Bond

Properties of Hydrogen Bond

Types of Hydrogen Bonding

Electron Sea Model

What are the Conditions for Hydrogen Bonding?

A hydrogen bond is formed when a hydrogen atom, linked to a highly electronegative atom, attracts the shared pair of electrons more, making one end of the molecule slightly negative and the other slightly positive. This negative end then attracts the positive end of another molecule, forming a weak bond between them.

A hydrogen bond is formed when a hydrogen atom links two electronegative atoms simultaneously, one by a covalent bond and the other by a hydrogen bond. The conditions for hydrogen bonding are:

1. The molecule must contain a highly electronegative atom linked to the hydrogen atom, resulting in increased polarization of the molecule as the electronegativity increases.

2. The size of the electronegative atom should be small, as the smaller the size, the greater is the electrostatic attraction.

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Chemical Bonding

Covalent Bond

Effects of Hydrogen Bonding on Elements

Association

The molecules of carboxylic acids exist as dimer due to hydrogen bonding. This leads to their molecular masses being double the value calculated from their simple formula.

Dissociation

HF has the ability to form hydrogen bonds in aqueous solution, resulting in the dissociation of the molecule and the formation of the difluoride ion instead of the fluoride ion. This is in contrast to molecules such as HCl, HBr, and HI, which do not form hydrogen bonds and therefore do not form compounds such as KHCl2, KHBr2, and KHI2.

How does hydrogen bonding affect the melting and boiling points of compounds?

Compounds containing hydrogen bonds exhibit abnormally high melting and boiling points. This is because additional energy is required to break these bonds.

The high boiling point of hydrogen fluoride among the halogen acids is attributed to the presence of hydrogen bonding.

H2O is a liquid, while H2S, H2Se, and H2Te are all gases at ordinary temperature. Hydrogen bonding between water molecules results in a higher boiling point for water than for the other compounds.

Ammonia (NH3) has a higher boiling point than PH3 because hydrogen bonding is present in NH3, whereas it is not present in PH3.

Ethanol has a higher boiling point than diethyl ether due to the presence of hydrogen bonding in the ethanol molecule.

Examples of Hydrogen Bonding

1. Water molecules
2. DNA molecules
3. Ammonia molecules

Hydrogen Bonding in Hydrogen Fluoride

Fluorine, with the highest electronegativity, forms the strongest hydrogen bond.

Hydrogen Bonding in Water

A water molecule contains a highly electronegative oxygen atom linked to the hydrogen atom. The oxygen atom attracts the shared pair of electrons more, making this end of the molecule negative, while the hydrogen atoms become positive.

Hydrogen Bonding in Ammonia

It contains a highly electronegative atom, nitrogen, linked to hydrogen atoms.

Hydrogen Bonding in Alcohols and Carboxylic Acids

Alcohol is a type of organic molecule that contains an -OH group. When a hydrogen atom is connected directly to either oxygen or nitrogen, hydrogen bonding is typically formed.

Hydrogen Bonding in Alcohols

Hydrogen Bonding in Carboxylic Acid

Hydrogen Bonding in Polymers

Hydrogen bonding is a key component in determining the 3D structures and properties of synthetic and natural proteins. Additionally, hydrogen bonds are essential in defining the structure of cellulose and its derived polymers, such as cotton and flax.

Strength of the Hydrogen Bond

The hydrogen bond is a weak bond. The strength of hydrogen bond lies somewhere in-between the weak van der Waals forces and the strong covalent bonds.

The dissociation energy of the hydrogen bond depends upon the attraction of the shared pair of electrons and hence on the electronegativity of the atom.

Properties of Hydrogen Bonding

Solubility: Lower alcohols are soluble in water due to the hydrogen bonding that occurs between the water and alcohol molecules.

Volatility: Compounds involving hydrogen bonding between different molecules tend to have a higher boiling point, resulting in lower volatility.

Viscosity and Surface Tension: Substances which contain hydrogen bonding exist as associated molecules, making them more difficult to flow. This leads to higher viscosity and higher surface tension.

The Lower Density of Ice Than Water: Ice has a lower density than water at 273 K due to its cage-like structure of hydrogen-bonded water molecules. The molecules are not as closely packed as they are in a liquid state, thus for the same mass of water, the volume decreases and density increases when ice melts, causing the cage-like structure to collapse and the molecules to come closer together.

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Fajan’s Rule

Hybridization

Molecular Orbital Theory

Types of Hydrogen Bonding

There are two types of H bonds, classified as follows:

Intermolecular Hydrogen Bonding

Intramolecular Hydrogen Bonding

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding occurs when hydrogen bonding takes place between different molecules of the same or different compounds.

For example - hydrogen bonding in water, alcohol, ammonia etc.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding is the hydrogen bonding that takes place within a molecule itself.

It takes place in compounds containing two groups, wherein one group has a hydrogen atom linked to an electronegative atom and the other group has a highly electronegative atom linked to a less electronegative atom.

The bond is formed between the hydrogen atoms of one group and the more electronegative atom of the other group.

Question: Which of the following molecules could form Hydrogen Bonds with other like molecules?

Symmetric Hydrogen Bond

A symmetric hydrogen bond is a special type of hydrogen bond where a proton is placed between two identical atoms, with each atom having equal bond strength. This type of three-centre four-electron bond is much stronger than a “normal” hydrogen bond and is almost as strong as a covalent bond.

Metallic Bonding is a type of chemical bonding that results from the electrostatic attractive force between the positively charged metal ions and the delocalized, or “free” electrons that are in the metal’s electron cloud.

Metals are known for their bright lustre, high electrical and thermal conductivity, malleability, ductility, and high tensile strength. A metallic crystal is composed of a vast number of atoms organized in a regular pattern.

Two of the most important models proposed to explain the nature of metallic bonding are:

1. The free electron model
2. The band theory model

Electron Sea Model

In this model, a metal is assumed to consist of a lattice of positive ions (or kernels) immersed in a sea of mobile valence electrons, which move freely within the boundaries of a crystal. A positive kernel consists of the nucleus of the atom together with its core electrons. The charge on a kernel is, therefore, equal in magnitude to the total valence electronic charge per atom.

The free electrons act as a barrier, protecting the positively charged ion cores from the electrostatic repulsive forces they would otherwise experience. In this sense, the free electrons act as a sort of “glue,” binding the ion cores together.

The forces that hold the atoms together in a metal due to the attraction between positive ions and the freely mobile electrons are referred to as metallic bonds.

The electrical and thermal conductivity of metals can be explained by the presence of mobile electrons in metals, even though the electron sea model predates quantum mechanics.

When an electron field is applied, the mobile electrons in the metal can conduct electricity from one end to the other. Additionally, when one part of the metal is heated, the mobile electrons in that area gain a great deal of kinetic energy. As they are free and mobile, these electrons move quickly throughout the metal, transferring heat to the other part of the metal.

Hydrogen Bonding

Frequently Asked Questions (FAQs)

How many hydrogen bonds can a water molecule form?

Water can form four hydrogen bonds. The two lone pairs of oxygen atoms and the two hydrogen atoms of water are involved in intermolecular hydrogen bonding.How does hydrogen bonding affect acidity?

Hydrogen bonding affects acidity by increasing the strength of the acid. This is because hydrogen bonds are strong and can stabilize the acid’s protons, allowing them to remain in solution for longer periods of time.

Hydrogen bonding increases the solubility of polar compounds because hydrogen atoms form strong bonds with other polar molecules. This allows the molecules to be dispersed in the solvent, making it easier for them to dissolve.

The consequences of hydrogen bonding are increased solubility of polar compounds in water, increased boiling and melting points of substances, and the formation of hydrogen bonds between molecules.

The density of ice is less in water due to stronger hydrogen bonds in solid ice than in liquid water. This is an important consequence of hydrogen bonding. How many hydrogen bonds are present between the base pairs Adenine(A) - Thymine (T) and Guanine (G) - Cytosine (C)?

There are two hydrogen bonds present between each of the base pairs Adenine(A) - Thymine (T) and Guanine (G) - Cytosine (C).

There are two hydrogen bonds between A-T and three hydrogen bonds between G-C.