Definition and Nature
What is a Hydrogen Bond?: A hydrogen bond is a special kind of force that pulls molecules together. It happens when a hydrogen atom is already bonded tightly to an electronegative atom like fluorine (F), oxygen (O), or nitrogen (N), and then this hydrogen gets attracted to another nearby electronegative atom. It’s not a full bond like a covalent one, but it’s strong enough to hold molecules near each other.
Formation Mechanism: Electronegative atoms, such as oxygen or nitrogen, are very good at pulling electrons toward themselves. When they bond with hydrogen, they pull the shared electrons closer, which gives them a slight negative charge (written as δ⁻). At the same time, the hydrogen loses some electron density and becomes slightly positive (written as δ⁺). This uneven sharing creates a dipole, or a molecule with two different charges.
Dipole Interaction: Because of this charge difference, the partially positive hydrogen atom is naturally pulled toward another nearby atom with a lone pair of electrons, like oxygen or nitrogen. This attraction forms a hydrogen bond, helping to hold molecules close together.
Bond Strength
Relative Strength: Hydrogen bonds are not as strong as the bonds that hold atoms together in molecules—like ionic or covalent bonds—but they are much stronger than simple attractions like Van der Waals forces. So they sit in the middle in terms of strength.
Type of Force: Hydrogen bonds are mostly electrostatic, which means they are caused by attractions between areas of opposite charge. These forces usually happen between polar molecules (molecules that have one side slightly positive and the other slightly negative).
Importance in Biological Molecules
Protein Structure: Proteins are like long chains that must fold into special shapes to work properly. Hydrogen bonds help hold these shapes together, like tiny zippers or buttons that keep the protein folded the right way.
Nucleic Acids Structure: In molecules like DNA and RNA, hydrogen bonds act like glue. They help link the nitrogen bases together in DNA’s double helix and help RNA fold into shapes that are needed for it to function in the cell.
Influence on Physical Properties
Boiling Point Elevation: Substances that have hydrogen bonds, such as water and ethanol, need more energy (heat) to break the bonds between their molecules. That’s why they boil at higher temperatures compared to substances that don’t have hydrogen bonds.
Water’s Unique Properties: A single water molecule can form up to four hydrogen bonds with nearby water molecules. This is why water has a high boiling point, sticks to itself (high surface tension), and has many other unique properties.
Ethanol’s Hydrogen Bonding: Ethanol contains an –OH group (a hydrogen bonded to an oxygen), which allows it to form hydrogen bonds. This gives ethanol a higher boiling point than other small molecules that don’t form these bonds.
Occurrence Between Molecules
Intermolecular Attraction: Hydrogen bonds often form between different molecules. This happens in liquids like water, ammonia, and hydrogen fluoride, where molecules are attracted to each other by hydrogen bonds.
Water Example: In water, the hydrogen atom from one molecule is attracted to the oxygen atom of another molecule. This link between molecules is a hydrogen bond.
Ammonia Example: In ammonia (NH₃), hydrogen atoms from one molecule are attracted to the nitrogen atoms of another molecule, forming hydrogen bonds.
Ammonia Example: In ammonia (NH₃), hydrogen atoms from one molecule are attracted to the nitrogen atoms of another molecule, forming hydrogen bonds.
Occurrence Within a Molecule
Intramolecular Bonding: Hydrogen bonds don’t only happen between different molecules—they can also form within a single large molecule. For example, in huge biological molecules like proteins or DNA, hydrogen bonds help hold certain parts of the same molecule together.
Notable Examples
Water Molecules: A diagram in Raja 5.2 shows how two water molecules stick together through hydrogen bonding, with hydrogen from one molecule attracted to the oxygen of another.
Ammonia Molecules: A diagram in Raja 5.21 shows how ammonia molecules form hydrogen bonds between their hydrogen and nitrogen atoms.
Hydrogen Fluoride Molecules: Raja 5.22 includes a diagram that demonstrates strong hydrogen bonding between HF molecules due to fluorine’s high electronegativity.
Boiling Point Evidence: Water and ethanol have unusually high boiling points, and this is one of the biggest clues that hydrogen bonding is happening between their molecules.
Summary
Key Characteristics: Hydrogen bonds happen when a hydrogen atom is already bonded to an atom like F, O, or N and is then attracted to another electronegative atom that has a lone pair of electrons.
Relative Strength: These bonds are weaker than strong primary bonds (like ionic and covalent), but they are still stronger than other weak forces between molecules.
Functional Significance: Hydrogen bonds are really important! They help give molecules like water and ethanol their special traits and are also essential for how proteins and DNA work in living things.