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. 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

3. Physical properties of simple organic molecules
Physical states
Boiling points & melting points

4. Remember physical properties?
A substance’s (or a molecule’s) physical properties include:
State
Color & odor
Refractive index
Density
Solubility
Melting & boiling points
vaporization or bp
Fusion or mp
solid
liquid
gas
Heat
Heat
Crystalline matrix is held together by inter- molecular forces.
D&D p.175-6

5. Liquid crystals
Liquid crystals are sometimes called a mesophase because they have more order than a liquid, but less than a crystal. Molecules involved are often rod-like and have disc-like arenes.
The more ordered smectic state is due to stronger intermolecular forces.
smectic
nematic
D&D p.178-9

6. Melting point
The temperature at which a solid becomes a liquid. Characteristic of molecule can be used to determine identity & purity.
Four factors influence (and increase) melting point:
Molecular symmetry
Molecular polarity
H-bonding
Molecular weight
Both conformation & orientation also matter
Pentane
mp = - 130°C
2,2-dimethylpropane
mp = -17°C
Benzoic acid = 2 H-bonds
Benzoic alcohol = 1 H-bond
D&D p

7. Even vs. odd chain length
The ability of molecules to pack together tightly influences intermolecular interactions & melting point.
Tighter packing increases melting point
Pure substances also have higher mps. Why?
It’s easier for identical molecules to pack together evenly & tightly.
A mixture of molecules of different shapes & lengths does not pack efficiently, so fewer interactions.
Bruice p98

8. Melting point & shape
So even chains pack tighter than odd. And molecules with spherical shapes have unusually high melting points.
Molecule
Shape
Boiling point °C
Melting point
pentane
linear 36
-130
hexane 69
-95
heptane 98
-91
octane
126
-57
nonane
151
-54
decane
174
-30
tetramethylbutane (8C)
spherical
106
+100
Notice that even-numbered chains have higher mp than odd.
Spherical molecules pack more efficiently than elongated (linear) molecules. The shape of spherical molecules is stable. But the rotation of all bonds in long alkanes gives them shifting shapes that are less likely to settle in a solid (packed & crystalline) form.

9. Examples: melting point
Which has the higher melting point and why?
Hexane or cyclohexane?
Cyclopentanethiol or cyclopentanol?
Cyclohexane, because it has higher symmetry
mp 6.47 C
mp -95 C SH OH
The alcohol because it is able to form
H-bonds.
bp 130 C
bp 140 C
D&D p

10. Similar principles for boiling points
Notice the effects of size, polarity, & ability to H-bond!
I’ve circled acetate, propionate, butyrate & pentanoate…. VFAs!
D&D p.183-5

11. 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

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

13. 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

14. 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

15. van der Waals attraction

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. Solubility
“Like dissolves like”

24. 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

25. 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

26. 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

27. 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 20C)
methanol
CH3OH 65
Completely miscible
ethanol
CH3CH2OH
78.5
1-propanol
CH3(CH2)2OH 97
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:
While all of these alcohols have one hydroxyl group, their bps increase because of increasing carbon chain length; ie increasing vdW forces.
One hydroxyl group is able to allow short chain lengths (up to 3C) to be water-soluble 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

28. Solvents vs. solutes: polarity
Notice how structural similarity helps to predict co-solubility.
polarity increases
D&D p.190-4

29. 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

30. 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

31. Surfactants
Amphipathic lipids
Micelles vs. emulsions

32. Surfactants
Surfactants are a class of molecules with ‘dual’ or ‘two-faced’ physical natures.
One end is polar
The other end is non-polar
So these amphipathic molecules are able to interact with (or be soluble in) both polar and non-polar molecules & solvents.
Soaps are a good example:
:O:
K +1
:O: ..
non-polar tail
hydrophobic -1
polar head group
hydrophilic
So soaps & other amphipathic compounds are able to clean grease from objects by making the grease soluble in water.
Normally grease & water don’t mix…
…but soaps create a molecular bridge between grease & water… surfactants greatly increase solubility!
Bile salts are a surfactant that dissolve lipids in body fluid.
D&D p.194-6

33. Soap scum & ‘advanced’ soaps
So, what’s soap scum? It’s a precipitate formed when metal ions in hard water interact with the carbonate head group of soap & form a precipitate that builds up on surfaces.
Hard water ions: calcium, magnesium, iron Like most precipitates, soap scum is dissolved by acids.. Typical cleaners use HCl.
Some surfactants are less likely to form “scum” precipitates because of their chemical structures & properties.
Sodium dodecanyl sulfate (aka sodium lauryl sulfate):
:O:
polar head group
hydrophilic
:O: S
:O:
non-polar tail
hydrophobic
Na +1
:O: ..
SDS doesn’t precipitate with as many metals…. So less soap scum.
D&D p.194-6

34. Surfactants & lung development
Surfactants can lower surface tension & this helps allow oxygen to gain access to the surface of lung alveoli.
The inner surface of lungs is coated with a solution of lipid in water.
DPPC (dipalmitoylphophatidyl choline) is the lipid of choice.
polar head
non-polar tails
Hyaline membrane disease?
Animals born prematurely haven’t yet developed the ability to produce DPPC, & without the surfactant their lungs aren’t able to inflate & stay inflated.
They can be saved by being given the missing surfactants until they develop their own.
D&D p.194-6

35. Soap bubbles
Soap bubbles are two lipid (soap) monolayers with a layer of water in between them.
The non-polar fatty carbon tails point outward into the air, also non-polar.
Polar head groups face into the layer of very polar water.

36. Lipid structures in water
When mixed with water (or an aqueous solution), amphipathic lipids spontaneously form 3D structures to increase lipid solubility.
Liposomes are formed by closing a bilayer to create a hollow sphere filled with aqueous solution.
Micelles are spherical structures one lipid ‘thick’ with no filling.
Emulsions are micelles that hold non-polar, hydrophobic molecules in their cores where they are solubilized by the surfactant’s tails.
Note that the polar heads are hydrophilic & interact with the water, while the hydrophobic non-polar tails interact only with each other or other hydrophobic molecules (hiding from water).
non-polar molecules
Salad dressing, hollandase sauce, mayonnaise
Emulsions

37. Lipids, emulsions and dairy science
Milk fat droplets have very weird and elaborate structures.
Triacylglcyerol core surrounded by a monolayer of phospholipids.
All enclosed by a phospholipid bilayer studded with proteins.
Processing, like pasteurization and homogenization, changes the structure of milk.

38. Effect of chain length on physical properties

39. Density of organic compounds
The density of organic compounds depends on their ratio of “heavy” and light atoms. For example:
C8H18
C8H14Cl4
The ability of molecules to pack together well (efficiently) also affects density, but is less important than MW and ratio of heavy to light atoms.
density (g/mL)
n-pentane C5H
1-bromobutane C4H9Br
hexane C6H
cyclohexane C6H
diethyl ether C4H10O
1-butanol C4H9OH 0.81
 more dense, sinks
Top: H2O, red food coloring;
Bottom: CH2Cl2
D&D p.197-4
McKee p70

40. 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

41. Lecture 4 key concepts (1):
Matter has four physical states: solid, liquid, gas & plasma.
Do states of matter have definite shape & volume?
Gases – neither
Liquids – definite volume but no fixed shape
Solid – yes to both
Molecular structure determines a molecules physical properties.
Intermolecular forces have major effects on the physical properties of a molecule.
van der Waals
dipolar
hydrogen bonding
Temporary dipolar attractions between neighboring molecules cause vdW.
Melting point is the temperature at which solid & liquid states are in equilibrium.
Boiling point “ “ “ liquid & gas states “
Solubility is favored when the solute interacts more strongly with the solvent than with other molecules of solute.

42. Lecture 4 key concepts (2):
A carbon bonded to some atom of different electronegativity results in a polar bond. Polar bonds cause dipolar interactions.
The interaction between a hydrogen that is attached to an electronegative atom and the nonbonding electrons of an atom in another molecule results in hydrogen bonding.
If the intermolecular forces between solute and solvent are as strong or stronger as the attractions between molecules of the solute or the solvent itself, then the solute will be soluble in the solvent.
The atomic weights of the atoms of a molecule contribute to the density of that molecule. When a molecule has a relatively large number of heavy atoms, its density is high.
Another factor that determines the density of a compound is the strength of its intermolecular attractions. The stronger the intermolecular attractions, the higher the density.
The larger the molecule, the less important are any functional groups in determining that molecule’s physical properties.