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 structure
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
1
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
F C
F
F
••
H N H H O
H
Hammonia
water
••
NO
Net
Dipole
Bond
Dipoles
Cancel
gas
These
Molecules
Have a
Net
Dipole
polar
F
F
H C
H
H H C
H
F
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
geometries
F C hereF
are polar
F
••
••
H N H H O
H
Hammonia
non-ideal
non-ideal
water
••
NO
Net
Dipole
Bond
Dipoles
Cancel
gas
polar
F
F
H C
H
H H C
H
F
These
Molecules
Have a
Net
Dipole
These
Molecules
Have a
Net
Dipole
2
Draw in bond dipole for each bond
H
H
H
••
H
C
H
C
C
C
H
C
H
O
C
H
H
H
O
:
:
••
:
:
H
••
H C H
••
H
methane gas
acetylene gas
ethylene gas
carbon dioxide gas
:
:
:
••
••
H
N
H
N
H
H
:
••
••
:
O
N
nitrogen gas
diazene gas
(unstable)
:
••
O H
••
N
H
H
ammonia gas
:
N
:
••
H N H
••
H
O
H
C
H
H
:
H
water liquid
formaldehyde gas
DIPOLE MOMENTS & MOLECULAR POLARITY
determining the bond polarity between two disimilar atoms in a chemical bond. Draw in all bond dipole moments and the overall dipole moment if the molecule is polar.
H
H
C
H
H
H
Cl
Cl
H
C
H
H
H
H
Cl
Cl
C
H
C H
H
C
H
C
H
C
H
O
H
O
Cl
C
Cl
C
C
Cl
trans
cis
:
:
:
H
N
H
H
N
H
:
N
N
N
H
cis
:
H
N
N
:
H
trans
:
H
:
:
O
H
H
O
:
C
C
:
H
H
C
C
H
3
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
δ+ H polar
••
• Sugar dissolves in water
δ−
• 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
4
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
H C
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
δ−
••
H O
δ+ H
H
methane
does not dissolve in
water
H
••
H
gasoline
H O
δ+ H
n
does not dissolve in
δ−
••
C
••
H C
H
water
5
Polar physically dissolves in Polar
H
••
δ− OH δ+
opposites attract
physical bond
n
H
sugar
dissolves in
water
δ−
••
H O
δ+ H
••
C
dissolves in
••
methanol
δ−
••
H O
δ+ H
••
H C δ+ O
H H
opposites attract
physical bonding
−
δ
water
The highlighted area is an example of
hydrogen bonding
H
••
dissolves in
••
H O
δ+ H
water
δ− OH δ+
••
H O
δ+ H
n
H
sugar
dissolve in
δ−
••
C
δ−
••
methanol
••
H C δ+ O
H H
δ−
water
6
“Like dissolves Like”
Where
everdissolves
there are N Polar
and O atoms, there are polar areas
• Polar
– Vitamin B11 (folic acid)
HO
N
OH
NH 2
N
O
O
N
polar areas
NH
N
NH
δ−
HO
••
O
– Vitamin C (ascorbic acid)
O
O
HO
••
H O
δ+ H
HO
Water soluble vitamins
HO
OH
“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?
7
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
8