Hydrogen bonding is a strong type of intermolecular attraction that occurs when hydrogen is bonded to a very electronegative atom and is attracted to another electronegative atom nearby. It is especially important in substances containing N, O, or F because these atoms pull electron density strongly and create large partial charges. Hydrogen bonding explains why water has an unusually high boiling point for such a small molecule.
It also helps determine the shapes and properties of many biological molecules.
A hydrogen bond is not a full covalent bond, but it is stronger than most ordinary dipole-dipole forces and London dispersion forces. In water, each molecule can form hydrogen bonds with neighboring water molecules, creating a temporary network that resists separation. In DNA, hydrogen bonds between complementary bases help hold the two strands together while still allowing them to unzip during replication.
The strength, direction, and number of hydrogen bonds strongly affect melting points, boiling points, solubility, viscosity, and molecular structure.
Key Facts
- Hydrogen bonding usually requires H bonded directly to N, O, or F.
- A hydrogen bond is written as X-H···Y, where X and Y are usually N, O, or F.
- Hydrogen bonds are intermolecular attractions, not the same as covalent bonds within a molecule.
- Stronger intermolecular forces usually mean higher boiling points and melting points.
- Water forms extensive hydrogen bonding, which helps explain its high boiling point, high surface tension, and lower density as ice.
- DNA base pairs are stabilized by hydrogen bonds: A pairs with T using 2 hydrogen bonds, and G pairs with C using 3 hydrogen bonds.
Vocabulary
- Hydrogen bond
- An attraction between a partially positive hydrogen atom bonded to N, O, or F and a lone pair on a nearby electronegative atom.
- Electronegativity
- A measure of how strongly an atom attracts shared electrons in a chemical bond.
- Partial charge
- A small positive or negative charge that forms when electrons are shared unequally in a polar bond.
- Intermolecular force
- An attraction between separate molecules that affects physical properties such as boiling point and solubility.
- Lone pair
- A pair of valence electrons on an atom that is not shared in a covalent bond.
Common Mistakes to Avoid
- Calling every H-containing molecule hydrogen bonded is wrong because the H must usually be directly bonded to N, O, or F.
- Confusing hydrogen bonds with covalent bonds is wrong because hydrogen bonds are attractions between molecules or between distant parts of large molecules, while covalent bonds share electrons within a molecule.
- Assuming larger molecules always have higher boiling points is incomplete because hydrogen bonding can make a smaller molecule boil at a much higher temperature than expected.
- Drawing a hydrogen bond to carbon is usually wrong because carbon is not electronegative enough to create the strong partial positive hydrogen needed for typical hydrogen bonding.
Practice Questions
- 1 Rank these substances from lowest to highest boiling point based on intermolecular forces: CH4, NH3, H2O. Explain which can hydrogen bond.
- 2 A DNA segment contains 8 A-T base pairs and 12 G-C base pairs. How many hydrogen bonds hold these base pairs together in total?
- 3 Methane, CH4, and water, H2O, have similar small molecular sizes, but water boils at a much higher temperature. Explain the difference using molecular structure and intermolecular forces.