Hydrogen Bonding

Hydrogen bonding is a special type of attractive force that exists between molecules or between different parts of the same molecule. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, and is attracted to another electronegative atom nearby.

Examples of hydrogen bonds: Water, Ammonia, and Hydrogen Fluoride.

A hydrogen bond is formed when a slightly positive hydrogen ( δ⁺) attached to one atom is attracted to a slightly negative atom ( δ⁻) with a lone pair on another molecule.

Hydrogen bonding is a special type of intermolecular force that arises when a hydrogen atom, covalently bonded to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F), is attracted to another electronegative atom in the same or a different molecule

Example: Water (H₂O)

In Water, hydrogen is bonded to oxygen. Since oxygen is highly electronegative, hydrogen becomes partially positive (δ⁺). The δ⁺ hydrogen of one water molecule is attracted to the δ⁻ oxygen of another water molecule. This forms intermolecular hydrogen bonding.

Properties of Hydrogen Bonding

• Volatility - The boiling point of compounds incorporating hydrogen bonding between distinct molecules is greater, hence they are less volatile.
• Solubility - Because of the hydrogen bonding that can occur between water and the alcohol molecule, lower alcohols are soluble in water.
• Lower density of ice than water - In the case of solid ice, hydrogen bonding causes water molecules to form a cage-like structure. In fact, each water molecule is tetrahedrally connected to four other water molecules. This cage-like structure collapses as ice melts, bringing the molecules closer together. As a result, at 273 K, ice has a lower density than water.
• Viscosity and Surface Tension - Hydrogen bonding is found in compounds that have an associated molecule. As a result, their flow becomes more complicated. They have high surface tension and higher viscosity.

Conditions for Hydrogen Bonding

For hydrogen bonding to occur, certain conditions must be satisfied:

1. Hydrogen must be bonded to a highly electronegative atom: Hydrogen should be covalently bonded to a small and highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F). These atoms strongly attract the shared electrons, creating partial charges.

2. High Electronegativity Difference: There must be a large difference in electronegativity between hydrogen and the atom to which it is bonded. This makes hydrogen partially positive (δ⁺), which is necessary for hydrogen bond formation.

3. Presence of Lone Pair of Electrons: The electronegative atom (N, O, or F) must have at least one lone pair of electrons. The hydrogen bond is formed between the δ⁺ hydrogen and the lone pair of another electronegative atom.

4. Small Size of Electronegative Atom: The atom bonded to hydrogen should be small in size. Smaller atoms (like N, O, F) allow closer approach of molecules, resulting in stronger hydrogen bonding.

5. Proper Orientation: The molecules must be arranged in such a way that the hydrogen atom can approach the lone pair of the electronegative atom effectively.

Types of Hydrogen Bonding

Hydrogen bonding can occur in different ways depending on where the attraction takes place. Based on this difference in location, hydrogen bonding is classified into two main types:

1. Intermolecular Hydrogen Bonding

• Intermolecular hydrogen bonding occurs between two different molecules.
• In this type, the hydrogen atom of one molecule forms a hydrogen bond with the electronegative atom of another molecule.
• This type of hydrogen bonding generally increases boiling point, viscosity, and solubility because molecules become strongly associated with each other.

Example: Hydrogen Fluoride (HF)

In Hydrogen fluoride, hydrogen is bonded to fluorine. Fluorine is highly electronegative, so strong intermolecular hydrogen bonding occurs between HF molecules.

2. Intramolecular Hydrogen Bonding

• Intramolecular hydrogen bonding occurs within the same molecule.
• In this type, the hydrogen atom forms a hydrogen bond with an electronegative atom present in the same molecule.
• This usually happens when hydrogen and the electronegative atom are positioned close to each other in the molecule.
• Intramolecular hydrogen bonding often reduces boiling point because molecules do not associate with each other.
  

Example: o-Nitrophenol

In o-Nitrophenol, the –OH group forms a hydrogen bond with the oxygen atom of the –NO₂ group within the same molecule.

This internal hydrogen bonding makes the molecule less associated with other molecules, so it has a lower boiling point compared to p-nitrophenol.

Examples of Hydrogen Bonding

The following examples illustrate how hydrogen bonding occurs in different substances and how it affects their properties.

1. Hydrogen Bond in Water (H₂O)

An excellent example of Hydrogen Bonding is Water. A highly electronegative oxygen atom is connected to a hydrogen atom in the water molecule. The shared pair of electrons are attracted to the oxygen atoms more, and this end of the molecule becomes negative, while the hydrogen atoms become positive.

2. Hydrogen Bond in Hydrogen Fluoride (HF)

A stronger-than-average hydrogen bond is created by hydrofluoric acid and is known as a symmetric hydrogen bond. Formic acid can also make this type of bond.

3. Hydrogen Bond in Ammonia (NH₃)

Between the hydrogen in one molecule and the nitrogen in another, hydrogen bonds are formed. Since each nitrogen has a single electron pair, the bond that develops in the case of ammonia is relatively weak. Methylamine also has this form of hydrogen bonding with nitrogen.

4. Hydrogen Bond in Alcohol and Carboxylic Acid 

A type of chemical molecule with a -OH group is alcohol. In most cases, hydrogen bonding is easily generated if any molecule containing the hydrogen atom is immediately coupled to either oxygen or nitrogen.

Hydrogen Bond in Alcohol

Hydrogen Bond in Carboxylic Acid 

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• Bond Parameters
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