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CHE2060 4: Physical properties & interactions 4.1 Physical properties of organic molecules • Solids, liquids & gases • Melting point • Boiling point 4.2 Types of intermolecular interactions • van der Waals interactions • Dipolar interactions • Hydrogen bonding 4.3 Solubility 4.4 Surfactants • Micelles & emulsions Labs Melting point determination Viscosity of organic compounds Distillation of wine Daley & Daley Chapter 4: Physical Properties Solubility “Like dissolves like” Solubility Most biochemical reactions occur in solution: molecules (solutes) are distributed uniformly in their solvents. • single phase – a homogenous solution – best for reactions • multiple phase – solutions that form layers – not great for most rxns Solubility depends on a number of factors: • Increases when solute and solvent interact via intermolecular bonds • Decreases as MW of solutes increases • Increasing temperature increases solubility • Structurally, “like dissolves like” • Polar solvents for polar solutes • Nonpolar solvents for nonpolar solutes Generally, increasing temperature increases solubility. D&D p.190-4 Solubility: nonpolar solvents So nonpolar solvents dissolve nonpolar molecules, but not ionic molecules. Example: mixing fats with salty water Example: mixing fats with solvents like turpentine Wade p66 Solubility: water is a very polar solvent Water, a very polar solvent, and will solubilize ionic & polar compounds, but does not nonpolar solutes. Wade p67 McKee p70 Solubility data This data shows how chain length relates to both boiling point and solubility in water. Name Formula bp (C) Solubility H2O (g/100 mL @ 20C) methanol CH3OH 65 Completely miscible ethanol CH3CH2OH 78.5 Completely miscible 1-propanol CH3(CH2)2OH 97 Completely miscible 1-butanol CH3(CH2)3OH 117.7 7.9 1-pentanol CH3(CH2)4OH 137.9 2.7 1-hexanol CH3(CH2)5OH 155.8 0.59 Two trends: 1. While all of these alcohols have one hydroxyl group, their bps increase because of increasing carbon chain length; ie increasing vdW forces. 2. One hydroxyl group is able to allow short chain lengths (up to 3C) to be watersoluble via H-bonding with water. But one hydroxyl is not sufficient for longer chains. As chain length increases, solubility in water decreases. HCH p.207 Solvents vs. solutes: polarity Notice how structural similarity helps to predict co-solubility. polarity increases D&D p.190-4 Example: solubility vs. structure? Can you predict which molecule is more soluble in pentane? • Drawing Lewis dot structures will help. Cyclohexyl ammonium chloride 4-chlorocyclohexamine 4-chlorocyclohexamine will be more soluble in a non-polar solvent like pentane, because this molecule is the less polar of the two. The first compound is actually a chloride salt of an organic compound. Its N has a + charge and its Cl has – charge. It’s therefore far more polar and so less soluble in pentane, a non-polar solvent. D&D p.190-4