1. 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

2. Intermolecular interactions
Salt bridges
Van der Waals
Dipolar
Hydrogen bonding

3. Types of intermolecular interactions
Intermolecular interactions (or forces) act to hold molecules together and increase melting & boiling points. All involve polarity / charge.
Types of intermolecular interactions include:
van der Waals forces
dipolar attractions
H-bonding
salt bridges
strongest
covalent
H-bonds
dipole-dipole
Van der Waals
Atoms sharing pairs of ve-
Strong dipoles interacting
Two polar bonds interacting
Temporary dipoles in nonpolar bonds
weakest
D&D p.184-9

4. van der Waals attraction
This attractive force occurs when two molecules approach at an optimal distance that allows attraction between the protons & electrons of the two molecules. Most critical in the liquid phase.
When further apart there is no interaction
When closer than optima there is charge-charge repulsion
vdW are weak interactions / strong
when many bonds exist in sum
vdW are temporary & change with molecular “environment”
 +
 +
 +
Non-polar molecules experience vdW because they can induce complementary polarization as they approach on another.
vdW increase with molecular size as “interactable” surface area increases
vdW decrease with branching that
“interupts” interactions.
bp 10°C
bp 36°C
D&D p.184-9

5. van der Waals attraction

6. Dipolar attractions
This attractive force occurs between polar molecules. Similar to vdW, but here the dipolar charges are permanent.
“Nose to tail”
Dipolar attractions increase intermolecular attraction & boiling point.
While ethane & fluoromethane have similar MW, their bps differ (-89°C vs -78°C).
D&D p.186

7. Hydrogen bonds
This attractive force occurs between molecules that have hydrogen bond donors and acceptors.
H-bond donor has a H attached to an electronegative atom (O, N, F)
H bond acceptor has an atom with a lone pair of electrons (O, N, F)
H-bonding also increase intermolecular attraction & boiling point.
While weak, they are strong when present in high numbers.
D&D p.186-9

8. Effect of H-bonding on boiling points
Look at the boiling points of four molecules with similar MWs.
What makes them different?
Molecule
MW (g/mol)
Boiling point (°C)
CH4 16
-161
NH3 17
-33
H2O 18
100 HF 20 19
No H-bonds

9. Water forms more H-bonds in ice than water
Each water molecule can form up to four H-bonds with other waters.
Ice is less dense than water because it of it’s hydrogen-bonding pattern.
In ice water forms a crystalline pattern of H-bonds, hexagonal, that hold molecules as far apart as possible.
So ice is less dense than water.
The H-bonds between molecules in liquid water are temporary & less organized
McKee p70

10. Example: intermolecular interactions & bp
Try ranking these three molecules in order of increasing bp.
Note that their sizes (MWs) are similar.
2-nitrophenol 3-nitrophenol nitrophenol
215°C °C °C
Why?
Well, 2-nitrophenol tends to H-bond with itself rather than others. Intramolecular bonds don’t hold groups of molecules together & increase bp.
D&D p.186-9

11. Example: intermolecular interaction & bp
Try ranking these three molecules in order of increasing bp.
Note that their sizes (MWs) are similar.
cyclohexane 1,4-dioxycyclohexane (aka 1,4-dioxane)
80.74 C bp C bp
Why?
Cyclohexane is completely non-polar. Its only intermolecular interactions are vdW.
Dioxane is a polar molecule and interacts with other molecules via vdW & dipolar attractions. Increased intermolecular bonding results in higher bps.
D&D p.186-9