3.2.4
Trends in Physical Properties
How chain length, branching, and functional groups influence melting and boiling points.
1. Boiling Point vs. Chain Length
Trend: As carbon chain length increases,
boiling point increases.
- Molecular Mass Increases: More electrons present.
- Surface Area Increases: More points of contact between molecules.
- Result: Stronger London Dispersion Forces (LDF) require more energy to overcome.
2. Boiling Point vs. Branching
Trend: For isomers, as branching increases, boiling point decreases.
Straight Chain (e.g., Pentane)
High surface area. Molecules pack closely (like stacking Pringles). Stronger LDF.
Branched Chain (e.g., 2,2-dimethylpropane)
Sphere-like shape. Reduced surface area. Molecules can't pack closely (like stacking tennis balls). Weaker LDF.
3. Solubility in Water
Trend: As chain length increases, solubility
in water decreases.
Solubility depends on the balance between the Hydrophilic Head (forms H-bonds with water) and the Hydrophobic Tail (non-polar chain).
- Small Alcohols (Methanol, Ethanol): Miscible. The -OH group dominates.
- Large Alcohols (Hexanol+): Insoluble. The long non-polar chain disrupts water's H-bonding network.