Overview
This lecture introduces the chemical properties of water, focusing on its molecular structure, its role as a solvent in biochemistry, hydrogen bonding, and the hydrophobic effect.
Structure of Water
- Water is the universal solvent for almost all biochemical reactions.
- Most biochemistry occurs in aqueous (water-based) environments.
- Water molecules consist of one oxygen atom bonded to two hydrogen atoms.
- Oxygen has six electrons in its outer shell, sharing electrons with hydrogen to complete its stable octet.
- Water has two unbonded electron pairs on oxygen, creating a bent molecular shape.
Polarity and Hydrogen Bonding
- Oxygen is strongly electronegative, attracting electrons more than hydrogen.
- This creates a polar molecule: partial negative charge on oxygen, partial positive charge on hydrogen.
- Water molecules can form hydrogen bonds—weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.
- Hydrogen bonds are common in biochemical molecules, not just water, involving elements like nitrogen and sulfur.
Properties of Ice and Liquid Water
- In ice, water molecules form a rigid, open, lattice structure due to stable hydrogen bonds.
- This lattice makes solid ice less dense than liquid water; hence, ice floats.
- In liquid water, hydrogen bonds constantly form and break, allowing molecules to pack more closely, increasing density.
The Hydrophobic Effect
- Hydrophobic molecules do not dissolve easily in water; hydrophilic molecules do.
- The hydrophobic effect strongly influences the structure of macromolecules like DNA, proteins, and lipid membranes.
- Benzene is an example of a hydrophobic molecule with a planar structure due to delocalized electrons.
Key Terms & Definitions
- Aqueous environment — a solution or setting where water is the solvent.
- Electronegativity — an atom’s ability to attract shared electrons in a bond.
- Hydrogen bond — a weak bond formed between a partial positive hydrogen and a partial negative atom (like oxygen or nitrogen).
- Hydrophobic effect — the tendency of nonpolar molecules to avoid mixing with water.
- Polar molecule — a molecule with partial positive and partial negative charges due to uneven electron sharing.
Action Items / Next Steps
- Review the molecular structure of water and practice drawing hydrogen bonds.
- Read about colligative properties, as they will be covered later.
- Study examples of hydrophobic and hydrophilic molecules in biological systems.