What is Hydrogen Bonding?
Hydrogen bonding is a strong type of intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms (oxygen, nitrogen or fluorine) and interacts with a lone pair on another molecule. It is responsible for many of water's unique properties and the structure of DNA.
Hydrogen bonding is an intermolecular force between a hydrogen bonded to a highly electronegative atom (O, N, F) and a lone pair on another molecule. It is much stronger than regular dipole-dipole forces.
- 1↓Electronegativity differenceH bonded to O/N/F creates a large @delta+ on H and @delta@minus on the heteroatom
- 2↓Dipole formsThe O-H/N-H/F-H bond is highly polar
- 3↓Lone pair attractionThe @delta+ H attracts the @delta@minus lone pair on a second molecule
- 4H-bond formsResult: a hydrogen bond (denoted H···O, H···N or H···F)
Step-by-step worked examples
Explain why water has an unusually high boiling point (100@deg C).
Water molecules contain O-H bonds (hydrogen bonding donors)
Each O also has 2 lone pairs (hydrogen bonding acceptors)
Each water molecule can form up to 4 hydrogen bonds: 2 as donor, 2 as acceptor
This extensive hydrogen bonding network requires a lot of energy to break
Result: water boils at 100@deg C, much higher than similar-mass molecules like CH@sub{4} (−164@deg C)Why is ammonia (NH@sub{3}) a gas at room temperature while water (H@sub{2}O) is liquid?
Both have hydrogen bonding capability Water: O has 2 lone pairs; each H can donate @rightarrow up to 4 H-bonds per molecule Ammonia: N has 1 lone pair; each H can donate, but fewer H-bonds form Water's hydrogen bonding network is more extensive than ammonia's Result: water remains liquid; ammonia is a gas (boiling point −33@deg C)
DNA's double helix is stabilised by hydrogen bonds between base pairs. Why are G-C pairs more stable than A-T pairs?
Adenine (A)–Thymine (T): 2 hydrogen bonds Guanine (G)–Cytosine (C): 3 hydrogen bonds More hydrogen bonds = stronger interaction Result: G-C pairs are more stable and require more energy to denature
Flashcards
Quick quiz
Q1.Hydrogen bonding occurs between…
Q2.How many hydrogen bonds can one water molecule form?
Q3.Why is HF a liquid (bp 19.5@deg C) but HCl is a gas (bp −85@deg C)?
Q4.Which molecule exhibits the strongest hydrogen bonding?
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Common mistakes
Thinking hydrogen bonding involves a direct H-H bond. — Correct: It is an intermolecular force between H (@delta+) and a lone pair (@delta@minus).
Believing all H atoms can form hydrogen bonds. — Correct: Only H bonded to O, N or F (highly electronegative).
Ignoring hydrogen bonding in biological molecules. — Correct: H-bonding is critical in DNA, proteins and other biomolecules.
Confusing hydrogen bonding strength with covalent bond strength. — Correct: H-bonds are ~5–10@percent the strength of a C-C covalent bond.
FAQ
What is hydrogen bonding?
An intermolecular force between H bonded to O/N/F and a lone pair on another molecule.
Why is hydrogen bonding important?
It determines boiling/melting points, solubility, protein folding and DNA structure.
Can all molecules form hydrogen bonds?
No — the donor H must be bonded to O, N or F. The acceptor must have a lone pair.
Is hydrogen bonding a chemical bond?
No — it is an intermolecular force, much weaker than a covalent or ionic bond.