Molecular Geometry & Polarity
A. Substance structure will influence physical properties
• Lewis dot structure for molecules
• Molecular Polarity (polar versus nonpolar)
A. Be able to draw
Lewis dot structures
B. molecules have ideal
or non-ideal geometry
C.electronegativity
differences
determines bond polarity
D. Overall Molecular Geometry
determines molecular polarity
These ideas are described on the next two slides
Symmetric versus Asymmetric
Symmetrical arrangement Asymmetrical arrangement
of polar bonds
of polar bonds
bond polarities cancel bond polarities don’t cancel
H
H C
H
H
non-polar
variable
polarity
F
C
F
F
F
••
••
N
H
H
H H O
H
F
F
C
H
H
H H C
H
ammonia
gas polar
••
NO
Net
Dipole
Bond
Dipoles
Cancel
water
F
These
Molecules
Have a
Net
Dipole
Molecular Substances (discrete units)
• Molecular Geometry and Bond Polarity
– Ideal geometries show have variable polarity
– Non-ideal geometries are generally polar
H
H C H
Hideal
variable
ideal non-polar
polarity
F
BUT, these ideal
C
geometries
here
F
F
are polar
F
••
••
••
N
H
H
H H O
H
F
F
C
H
H
H H C
H
••
NO
Net
Dipole
Bond
Dipoles
Cancel
non-ideal
ammonia
non-ideal
water
gas polar
F
These
Molecules
Have a
Net
Dipole
These
Molecules
Have a
Net
Dipole
Draw in bond dipole for each bond
••
C
C
H
C
C
H
O
:
H
H
methane gas
H
H
:
:
••
••
N
H
H
ammonia gas
N
diazene gas
(unstable)
H
:
:
O
O
H
H
water liquid
:
••
••
H
N
N
H
H
:
••
O H
••
:
N
:
H
C
H
formaldehyde gas
O
acetylene gas carbon dioxide gas
ethylene gas
:
••
H N H
••
H
C
:
H
C
:
H
H
:
H
••
H C H
••
••
H
H
nitrogen gas
DIPOLE M OMENTS & M OLECULA R POLARITY
determining the bond polar ity between two disimil ar atoms in a chemical bond. Draw i n all bond dipole moments and the over all dipole moment if the mol ecule i s pol ar .
H
H
C
H
H
C
H
Cl
H
C
H
H
H
H
Cl
Cl
C
H
H
C
C H
H
C
Cl
H
:
:
:
H
N
H
H
N
H
:
N
H
cis
:
H
N
N
H
trans
:
:
H
O
H
H
O
:
:
C
H
Cl
C
trans
cis
O
H
C
H
:
C
Cl
C
N
N
:
H
H
C
Cl
C
O
Molecular Polarity
• Determining the polar nature of molecular substances
– Evaluating Substance Structure
– Physical Observations - Common Sense Approach
• Solubility (solute/solvent interactions) to gauge molecular
polarity
– “Like will dissolve Like”
•
•
•
•
Polar solutes will have highest solubility in polar solvents
Nonpolar solutes will have highest solubility in nonpolar solvents
Polar solutes will have lowest solubility in nonpolar solvents
Nonpolar solutes will have lowest solubility in polar solvents
Molecular substance solubility in water
“Like dissolves Like ”
to gauge Molecular Polarity
••
– Thus sugar molecules must be polar
H O
 H polar
• Methanol CH3OH dissolves in water
– Thus methanol molecules must be polar
• Gasoline -(CH 2)- does not dissolve in water
– Thus gasoline molecules must be NONPOLAR
••
• Sugar dissolves in water

H
C
H
A gasoline molecule is a hydrocarbon made of
repeating –( CH2 )– units is non-polar; NO net dipole
hydrocarbons are non-polar
H
H H
C
C
C
H
H H H
H H H H H
C
C
C C C H
H H H H H
A gasoline hydrocarbon
All dipoles cancel
Nonpolar does not dissolve Polar
H
H O
 H
H
methane
does not dissolve in
water
H
••
H O
 H
n
gasoline
does not dissolve in
water

••
C
••
C
H
H
H

••
Polar physically dissolves in Polar
H
••
 OH 
H
sugar
opposites attract
physical bond
n
dissolves in
water

••
H O
 H
••
C
dissolves in
••
methanol

••
H O
 H
••
C
H
 O

H H
opposites attract
physical bonding

water
The highlighted area is an example of
hydrogen bonding
H
••
dissolves in
••
H O
 H
water
 OH 
••
H
sugar
H O
 H
n
dissolve in
water

••
C

••
methanol
••
C
H
 O

H H

“Like dissolves Like”
Where
everdissolves
there are N Polar
and O atoms, there are polar areas
• Polar
– Vitamin B11 (folic acid)
HO
N
OH
NH2
N
O
O
polar areas
N
NH
N
NH
HO
O
– Vitamin C (ascorbic acid)
O
HO
HO
O
OH
H O
 H

••
HO
••
Water soluble vitamins
“Like dissolves Like”
• Non-Polar dissolves Non-Polar
– Vitamin A, retinol (fat soluble; lipid soluble)
OH
nonpolar areas
– triacylglycerine, a non-polar human body fat (lipid)
O
C-O
O
O
O-C
C-O
Vitamins D, E, and K are fat soluble as well
What would their overall polarity be?
Predict whether the substance is
Polar or Nonpolar
sugar
C6H12O6
polar
baby oil
C20H42
nonpolar
candle wax
C40H82
nonpolar
ethanol
C2H5OH
polar
oxygen
O2
nonpolar
iodine
I2
nonpolar