Concept 07 / 07Chemical Bonding and Molecular Structure
Hydrogen Bonding and Other Intermolecular Forces
Conditions for hydrogen bonding, intermolecular and intramolecular hydrogen bonds, their effects on boiling points, solubility, viscosity and the structure of ice, and a comparison with dipole-dipole, dipole-induced dipole and London dispersion forces.
A hydrogen bond is an attraction between a hydrogen atom covalently bonded to a small, highly electronegative atom (F, O or N) and a lone pair on another such atom: . The strongly polar X-H bond leaves the hydrogen with a large partial positive charge on a tiny, unshielded nucleus, which can approach the lone pair of Y closely. Hydrogen bonds are much weaker than covalent bonds ( to kJ/mol versus about ) but much stronger than other intermolecular forces.
Types.
Intermolecular hydrogen bonds link different molecules: in water, HF, ammonia, alcohols and carboxylic acids (which form dimers in the vapour and in non-polar solvents).
Intramolecular hydrogen bonds form within one molecule when geometry allows a (usually six-membered) ring, as in o-nitrophenol and salicylaldehyde. They reduce intermolecular association.
Effects.
Boiling points: , HF and boil far higher than expected from the trends of their heavier analogues, as the figure shows ( C versus C). Without hydrogen bonding, water would boil around C.
Isomers: o-nitrophenol (intramolecular H-bond) is more volatile and less water-soluble than p-nitrophenol (intermolecular H-bonds), which is why o-nitrophenol can be separated by steam distillation. Ethanol (C) boils much higher than its isomer dimethyl ether (C).
Solubility: compounds that can hydrogen-bond with water (lower alcohols, sugars, ammonia, amines) dissolve readily.
Viscosity and surface tension are high in extensively hydrogen-bonded liquids such as glycerol and water.
Structure of ice: each water molecule hydrogen-bonds tetrahedrally to four others, forming an open cage-like network with empty space. When ice melts, some hydrogen bonds break and molecules pack more closely, so liquid water is denser than ice (maximum density at C). This is why ice floats.
Biology: hydrogen bonds hold the two strands of DNA together and fix the secondary structures of proteins.
Other intermolecular (van der Waals) forces, in decreasing typical strength:
Ion-dipole: between ions and polar molecules (hydration of ions).
Dipole-dipole: between polar molecules (HCl, acetone); weaker than hydrogen bonds.
Dipole-induced dipole: a polar molecule distorts the electron cloud of a non-polar one.
London (dispersion) forces: between all molecules, from instantaneous dipoles; they increase with molar mass and surface area (polarisability). They are why the noble gases and halogens become liquids and solids as size increases (, gases; liquid; solid), and why n-pentane boils higher than its compact isomer neopentane.
Key Takeaways:
- Hydrogen bond: H on F, O or N attracted to a lone pair on F, O or N.
- Strength about - kJ/mol: between covalent bonds and van der Waals forces.
- Intermolecular H-bonding raises boiling point and solubility; intramolecular H-bonding lowers them.
- Ice is less dense than water because of its open hydrogen-bonded network.
- Dispersion forces grow with molar mass and surface area.
- Pitfall: counting C-H as a hydrogen-bond donor; carbon is not electronegative enough (except in special cases like ).
Brain-Teaser 1: Why is HF a liquid at C (b.p. C) while HCl is a gas (b.p. C), although HCl is heavier?
Solution: HF molecules form strong intermolecular hydrogen bonds (F is the most electronegative element and very small), linking them into zig-zag chains . HCl has only dipole-dipole and dispersion forces. Hydrogen bonding outweighs the larger dispersion forces of HCl. (HF's boiling point is nevertheless below water's because each HF molecule has only one H to donate, forming chains rather than a 3D network.)
Brain-Teaser 2: o-nitrophenol and p-nitrophenol have the same formula. Which is more volatile, and why?
Solution: In o-nitrophenol, the OH hydrogen bonds to an oxygen of the adjacent group within the same molecule (a six-membered ring), leaving little capacity for bonding to neighbours. p-Nitrophenol cannot do this and instead forms hydrogen bonds between molecules. So o-nitrophenol is more volatile and less soluble in water. Where a hydrogen bond forms, inside or between molecules, determines its effect on physical properties.
Brain-Teaser 3: Arrange , , and by boiling point, and explain why group 14 shows no anomaly for the first member.
Solution: : boiling point rises with molar mass because only dispersion forces act, and they grow with size. Methane is non-polar and carbon has no lone pair and is not electronegative enough for hydrogen bonding, so there is no anomaly, unlike , HF and in groups 16, 17 and 15.
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