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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