What is� Hydrogen Bond?
Types and Examples
Chemistry Education
What is Hydrogen Bond?
Definition
A special type of dipole-dipole electrostatic attraction
Formation Mechanism
1
Electronegative atom X pulls electrons → becomes δ⁻
2
Hydrogen loses electron density → becomes δ⁺
3
δ⁺ H attracts lone pair of atom Y
General Notation
X–H · · · Y
where X, Y = N, O, or F
Bond Strength
5-30
kJ/mol
Stronger than van der Waals forces
Weaker than covalent bonds
Types of Hydrogen Bonds
Intermolecular
Between Molecules
Location
Between two or more different molecules
Association
Causes molecular association
Effect on Properties
Increases boiling & melting points
Raises viscosity & surface tension
Examples
H₂O, HF, NH₃, Acetic Acid
Intramolecular
Within Molecule
Location
Within a single molecule
Proximity
Requires close geometric proximity
Ring Formation
Forms stable ring (chelation)
Effect on B.P.
No significant increase
Examples
o-Nitrophenol, Salicylic Acid
Intermolecular Hydrogen Bonds
Definition
Formed between two or more different molecules
Effects
Higher B.P.
Higher M.P.
Increased Viscosity
Higher Surface Tension
Molecular association occurs, causing properties to increase significantly compared to similar compounds without H-bonds
Example: Water (H₂O)
Each water molecule donates 2 H-bonds and accepts 2 H-bonds
Forms extensive 3D tetrahedral network
Water is liquid at room temperature (unusually high B.P.)
Ice is less dense than liquid water
More Intermolecular Examples
HF
Hydrogen Fluoride
Zig-zag chain arrangement in solid/liquid states
Exists as (HF)ₙ polymeric clusters
NH₃
Ammonia
H-bonds form between δ⁺ H and lone pair of N
Boiling Point:
-33°C
vs. PH₃: -87°C
Strong H-bonding leads to molecular association
CH₃COOH
Acetic Acid Dimer
Cyclic Structure
Two simultaneous H-bonds form stable dimer
Closed-ring cyclic structure
Stable in vapour phase & non-polar solvents
Lower volatility due to dimerization
Intramolecular Hydrogen Bonds
Definition
Formed within a single molecule
Proximity Required
H-donor and H-acceptor groups must be in close geometric proximity
Ring Formation
Forms stable ring structure (chelation)
Boiling Point
No significant increase (no molecular association)
Example: o-Nitrophenol
H of -OH group forms H-bond with O of -NO₂ group
Creates six-membered ring within molecule
Lower boiling point than p-nitrophenol (intermolecular H-bonding)
More Intramolecular Examples
C₇H₆O₃
Salicylic Acid
2-Hydroxybenzoic acid
H-bond between -OH and -COOH
Six-membered chelate ring
Intramolecular H-Bond Formation
Hydrogen atom of -OH (hydroxyl) group forms H-bond
With oxygen atom of -COOH (carboxyl) group
Within the same molecule
Chelate Ring Structure
Forms a stable six-membered chelate ring, locking the H-bond within the molecule
Solubility
Reduced due to intramolecular locking
Association
No intermolecular association
Comparison: Intermolecular vs Intramolecular H-Bonds
Key Takeaways
1
Intermolecular H-bonds cause molecular association
2
Intramolecular H-bonds form chelate rings
3
Location determines physical properties
4
Both involve N, O, or F atoms