Why does water have an unusually high boiling point for its molecular mass?

Because of strong intermolecular hydrogen bonding

Due to London dispersion forces

In o-nitrophenol, hydrogen bonding is INTRAMOLECULAR. What does this mean?

H-bond forms within the same molecule, creating a ring

H-bond forms between two o-nitrophenol molecules

H-bond forms with water molecules only

No H-bonding is possible in o-nitrophenol

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Hydrogen Bonding: Types, Examples & Diagrams PDF

X–H ···Y (X, Y = N, O, F): δ⁺H attracts lone pair
Intermolecular → raises B.P., M.P. & viscosity
Intramolecular → chelate ring; no B.P. rise
H₂O: 2 donor + 2 acceptor → extensive 3D network
~5–30 kJ/mol: stronger than vdW, weaker than covalent

What is a Hydrogen Bond (H-Bond)?

A Hydrogen Bond is a special type of dipole–dipole electrostatic attraction.

It forms when a Hydrogen atom that is covalently bonded to a highly electronegative atom (Fluorine, Oxygen, or Nitrogen) is attracted to the lone pair of electrons of another electronegative atom nearby.

How an H-Bond Forms

1.The electronegative atom (X) pulls the shared electron pair toward itself, becoming partially negative (δ⁻).

2.The Hydrogen atom, stripped of electron density, becomes partially positive (δ⁺).

3.This δ⁺ Hydrogen atom is then electrostatically attracted to the lone pair of a neighbouring electronegative atom (Y).

General notation: X–H · · · Y (where X and Y = N, O, or F)

Bond strength: Stronger than van der Waals forces, but weaker than a covalent bond (~5–30 kJ/mol).

1. Intermolecular Hydrogen Bond

An Intermolecular H-bond is formed between two or more different molecules (of the same or different compounds).

It results in the association of molecules, which increases boiling point, melting point, viscosity, and surface tension.

Example 1 — Water (H₂O)

Water (H₂O) Hydrogen Bond Network — Intermolecular

Each water molecule can donate two H-bonds and accept two H-bonds, forming an extensive 3D tetrahedral network. This explains why:

Water is a liquid at room temperature (unusually high boiling point for its molecular mass).

Ice is less dense than liquid water (the open H-bond lattice expands volume).

Example 2 — Hydrogen Fluoride (HF)

Hydrogen Fluoride (HF) — Zig-Zag Chain via H-bonds

Due to the exceptionally high electronegativity of Fluorine, HF molecules associate strongly. In the solid and liquid states, HF adopts a zig-zag chain arrangement due to the tetrahedral geometry around each F atom. It exists as (HF)ₙ polymeric clusters.

Example 3 — Ammonia (NH₃)

Ammonia (NH₃) — Intermolecular Hydrogen Bonding

The H–N bond in ammonia is polar. H-bonds form between the δ⁺ Hydrogen of one NH₃ molecule and the lone pair of Nitrogen on an adjacent NH₃ molecule. This association contributes to ammonia's relatively high boiling point (−33°C) compared to phosphine (PH₃, −87°C).

Example 4 — Acetic Acid Dimer

Acetic Acid Dimer — Cyclic Intermolecular H-Bond

Two molecules of acetic acid join together via two simultaneous H-bonds to form a stable, closed-ring dimer. This cyclic dimer structure is so stable that acetic acid exists largely as dimers in the vapour phase and in non-polar solvents.

2. Intramolecular Hydrogen Bond

An Intramolecular H-bond is formed within a single molecule — between a H-donor and a H-acceptor group present in the same molecule.

This requires the two groups to be in close geometric proximity. It often leads to the formation of a stable ring structure (chelation) and has no effect on boiling point (in contrast to intermolecular H-bonds).

Example 1 — o-Nitrophenol

o-Nitrophenol — Intramolecular H-Bond forming a 6-membered ring

In o-Nitrophenol, the H atom of the –OH group forms a six-membered ring H-bond with one of the Oxygen atoms of the –NO₂ group attached to the adjacent position of the benzene ring. Because the H-bond is locked intramolecularly, o-nitrophenol has a much lower boiling point than its p-nitrophenol isomer (which uses intermolecular H-bonds instead).

Example 2 — Salicylic Acid

Salicylic Acid — Intramolecular H-Bond between –OH and –COOH

In Salicylic Acid, an intramolecular H-bond forms between the –OH (hydroxyl) group and the –COOH (carboxyl) group within the same molecule. This produces a stable six-membered chelate ring, reducing its ability to associate with other molecules and lowering its solubility compared to similar compounds.

Comparison Table: Intermolecular vs. Intramolecular H-Bond

Feature	Intermolecular H-Bond	Intramolecular H-Bond
Location	Between two or more molecules	Within a single molecule
Effect on B.P./M.P.	Increases (molecular association)	No significant increase
Molecular mass	Appears higher (associates)	No association effect
Solubility	Higher in polar solvents	Generally lower
Ring formation	No	Yes (chelation)
Example	H₂O, HF, NH₃	o-Nitrophenol, Salicylic Acid

Read next →Periodic Trends VSEPR Theory

X–H ···Y (X, Y = N, O, F): δ⁺H attracts lone pair
Intermolecular → raises B.P., M.P. & viscosity
Intramolecular → chelate ring; no B.P. rise
H₂O: 2 donor + 2 acceptor → extensive 3D network
~5–30 kJ/mol: stronger than vdW, weaker than covalent

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