Chapter 24
Animal, Vegetable or Mineral,
It’s All Chemistry
Summary of bonding.
Can you answer these?
Intermolecular Forces



“Between” different molecules
Caused by permanent or temporary (induced)
dipoles on molecules
Wide range of strengths explains wide range of
boiling, melting points of covalent materials
Dispersion
weakest -- but ever present
found between all kinds of
molecules and even
between atoms of noble gases
DipoleDipole
Interactions
occur between
polar molecules
Hydrogen Bonds
strongest but limited
to molecules that
contain H atoms
attached to N, O, F.
Relative Strengths of
Intermolecular Interactions
http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml
Dispersion Forces
Electron cloud can fluctuate
Not perfectly symmetrical
about the nucleus.
Temporary dipole induced.
Induces a new dipole in
atoms or molecules that come
close.
EVIDENCE FOR SUCH FORCES
– NOBLE GASES CAN BE LIQUIFIED!
Bigger electron cloud, the stronger the forces.
Higher boiling temperature of gases.
Rn
Xe
Kr
Ar
Ne
Nitrogen

Properties




Chemically unreactive
Boiling point 77 K (-196 °C; -321 °F)
Colorless
Why?



Triple bond
What are strongest intermolecular
forces between N2 molecules?
No low-lying molecular orbitals
NN
N2 is a non-polar molecule. Which
interaction determines its boiling
temperature?
1.
2.
3.
Hydrogen
bonding.
Dipole-Dipole
Interactions
Dispersion forces
Answer Now
Dipole-Dipole Interactions
Formed in molecules with bonds between atoms of
different electronegativities.
examples: C – Cl, C – O, C – F
dispersion forces only
Effect is to increase
“stickiness” of molecules
for each other.
Displayed in differences
of boiling temperatures.
dispersion + D-D forces
Also in solubilities
Hydrogen Bonding




Limited to
H bound to N, O, or F
Among strongest
+
intermolecular interactions
Happens because H is small
and has only 1 electron, and
atom it is bound to is quite
electronegative. H is
essentially a “bare” proton
The most important H-bond:
water
-
+
+
+
+
+
Water

Properties




Fairly reactive, “universal” solvent
Boils at 373 K (100 °C; 212 °F)
More dense as liquid than solid
Can react with itself: 2H2O = H3O+ + OH


Water conducts electricity (but not well)
This reaction goes only to a very small extent (1 molecule in 10
million!)
Why?


Highly polar, so dissolves ionic materials
Extensive hydrogen bonding interactions dissolve
nonionic materials like sugars, lead to high boiling
point, unusual crystal structure
Glucose (a Kind of Sugar)

Properties




Crystalline, molecular solid
Melts, then decomposes on heating
Sticky
Why?



Many hydrogen bonds hold molecules together in
crystal
Many H-bonds add up, so melting point is relatively
high (compared to water, for example)
H-bonds make molecules “sticky”
Atomic-Level Structure of Complex Materials
Determines Properties
Animals & Vegetables
Fats (obtained from animals)
&
Oils (obtained from vegetables)
Covalent Molecules
Whose Melting Points
Are Related To Behavior
in Our Bodies
Minerals
Silicate Mineral Family
Primarily Ionic Material
Different Arrangements of
SiO44- units lead to
stringy, sheet-like
or chunky minerals
Fats and Oils differ in
Their Physical State at Room
Temperature
Chicken Fat &
Marbling in Meat
Solid
Fat
Shortening (Crisco)
Solid
Fat
Butter
Solid
Fat
Margarine
Liquid-Solid mix
“Promise” or “Benacol”
Liquid-Solid mix
Olive Oil
Liquid
Oil
Canola Oil
Liquid
Oil
Fish Oil
Liquid
Oil
Sequence of Melting Temperatures
WHY THIS SEQUENCE? UNDERLYING MOLECULAR STRUCTURES
Heart Healthy Sequence (approximately)
maybe the best for you
Did you read chapter 24
before coming to class?
A.Yes
B.No
Components of Fats and Oils
Fatty Acids
CO2H-(Hydrocarbon tail)
Fats & Oils are Mono-, Diand Tri-glycerides
http://ps100.byu.edu/molecule_sg/fatty_acids.html
+
Glycerol C3H8O3
Acetic Acid
just a simple
example; not
a fatty acid.
Fatty Acids – major
component of fats and oils
Describe the structures:
What molecular groupings
do they have in
common?
▬ Hydrocarbon tail
CO2H ▬
Fatty Acids – major
component of fats and oils
Describe the structures:
n=10
How does one
differ from another?
n=14
n=16
Length of the tail
▬(CH2)nCH3
n=18
What difference does a tail make?
Fats containing
these fatty acids
are solids at
room temperature
Room temperature
How do we get oils?
(lower melting temperatures)
A new family of fatty acids
Its members have
the CO2H ▬ group
How do we get oils?(lower melting
temperatures)
A new family of fatty acids
They have the
hydrocarbon tail.
So What’s Different?
THE TAILS
HAVE KINKS!
Straight chain vs Kinky chain
What difference does a kink
make?
Fats containing
these fatty acids
are solids at
room temperature
Room temperature
1 kink
2 kinks
3 kinks
4 kinks
Oils containing
these fatty acids
are liquid at
room temperature
If you were a fish,
swimming in the cold
North Atlantic, what
would you want flowing
through your veins?
What causes the kinks?
Differences in the Tails

Saturated fatty acids
No kinks

Unsaturated fatty acids
One kink (mono)
More than 1 kink (poly)
each C has 2 H atoms
C▬ C ▬ C ▬ C
(carbon-carbon single bonds)
Some C have only 1 H atom
C▬ C = C ▬ C
(carbon-carbon double bonds)
Kinks occur at double bonds.
(True of Unsaturated Fatty Acids that are found in Nature.)
Why do kinks make a difference?
Molecules without kinks can
snuggle closer together.
RESULT:
more & stronger dispersion forces between tails
stronger hydrogen bonding between CO2H groups
on different molecules
Strong forces mean high melting temperatures
Trans-Fatty Acids –
doing away with the kinks
Saturated Fatty Acid
No Kinks
Unsaturated Fatty Acid
Kink at
Double Bond
Unsaturated Fatty Acid
But no Kink at
Double Bond
Cis vs Trans Double Bonds –
Where are the H atoms?
Cis Double Bond
Gives Kink
Trans Double Bond Has No Kink
Not Naturally Occurring in Foods
occurs during processing of saturated fats
Where do trans fats come from?
Good Fats vs Bad Fats
Good Fats
kinky unsaturated fats
low melting points
don’t clog your arteries
good for your brain
Bad Fats
unkinky fats
saturated & trans-fats
olive oil
cold water fish
high melting points
lard
shortening
prime rib
Wendy’s announced in August that it had
switched to a new cooking oil that
contains no trans fatty acids.
Doughnuts in danger? NYC may ban trans fats
Crisco now sells a shortening
Health officials unveil proposal to bar substance in restaurants
that contains zero trans fats.
MSNBC Sept 27, 2006
Frito-Lay removed trans fats
from its Doritos and Cheetos. Kraft’s took trans fats out of Oreos.
Silicate Minerals
Atomic Arrangements Give Rise to Structural Features
Irregular Chunks
Strings or Fibers
Sheets or Plates
Molecular Ions




Covalent bonding within the ion
Stronger covalent bonds if number of electrons doesn’t match
total nuclear charge  resulting molecule is charged
These charged molecules assemble together in crystal lattice like
ionic materials
Examples: nitrate, silicate, sulfate, chlorate
The basis of Silicate Minerals:
The Silicate ion, SiO44-
Tetrahedral arrangement
of oxygen atoms
around a central silicon
atom
SiO44- - a versatile connector
It is found in minerals
as isolated units
Two tetrahedra
share an oxygen
atom at the connection
Two tetrahedra
share an
oxygen atom
at the
connection
as chains or double chains
or as flat sheets of connected chains
Wait, there’s still more….
Oxygen
Silicon
or as networks
connected equally
in all directions
Positively Charged
Ions (Ca2+, Mg2+)
Formation of Fibers
Double Chain
Strongly-bound unit:
“Submarine sandwich”
Weaker interactions
between units
You can pull apart fibers with your fingers
Formation
of Sheets
silicate sheets
with negative charges
Small Al3+ Ions
Strongly-bound unit
Positively Charged Ions,
K+ and/or Na+
You can pull
apart sheets
with your fingers
Formation
of Irregular
Chunks
Tetrahedron-tetrahedron
connections extend
in 3 directions.
Interactions are
equally strong in all
directions.
You need a hammer
to break crystal
into smaller chunks