Molecular Polarity

1
Molecular Polarity

• A Bridge from the Molecular World to Macroscopic
  Properties

2
Definition

• Polar Molecule
• A Molecule with a Positive and Negative Side
• Dipole Moment
• A Measure of Molecular Polarity
• A Nonpolar Molecule will have a Zero Dipole Moment

3
Why is Polarity Important?

• Many Properties Depend on Polarity
• Melting and Boiling Point
• Surface Tension, Viscosity
• Reactivity
• Shapes of Very Large Molecules (Proteins, DNA,
  RNA)
• Solubility (e.g., will it dissolve in water)

4
Requirements

• A Polar Molecule Requires
• Polar Bonds
• And
• A Molecular Shape that Separates the Positive
  from the Negative Side

5
Polar Covalent Bonds

• Bonds between identical atoms such as H-H, F-F
  involve equal sharing of e-
• Bonds between different nonmetal atoms involve
  unequal sharing of e-
• Polar Covalent Bonds have a Partial Charge
  Separation

6
Polar Covalent Bonds

• H IE 1,312 eA -73 kJ/mol
• Cl IE 1,251 eA -348 kJ/mol
• More energy released when e- added to Cl
• e- are unequally shared
• Cl side is slightly -
• H side is slightly

7
Polar Covalent Bonds

• Unequal sharing of e- in a bond called Polar
  Covalent Bond or Polar Bond
• Partial Charge indicated by delta ?/-

?-
?
8
Electronegativity

• Used to Determine Bond Polarity
• Calculated from IE and eA
• ? EN lt .45 Nonpolar Bond
• 1.75 gt ? EN gt .45 Polar Bond
• ? EN gt 1.75 Ionic Bond Atoms

9
Electronegativity

• Electronegativity is the ability of an atom to
  attract electrons in a bond.
• Nonmetals bonded to N, O, F usually have polar
  bonds

10
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
11
Polar, Nonpolar or Ionic Bond?

• O-H

12
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
13
Polar or Nonpolar Bond?

• O-H ? EN 1.4 Polar Bond
• C-H

14
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
15
Polar or Nonpolar Bond?

• O-H ? EN 1.4 Polar Bond
• C-H ? EN 0.4 Nonpolar Bond
• C-O

16
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
17
Polar or Nonpolar Bond?

• O-H ? EN 1.4 Polar Bond
• C-H ? EN 0.4 Nonpolar Bond
• C-O ? EN 1.0 Polar Bond
• H-Cl

18
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
19
Polar or Nonpolar Bond?

• O-H ? EN 1.4 Polar Bond
• C-H ? EN 0.4 Nonpolar Bond
• C-O ? EN 1.0 Polar Bond
• H-Cl ? EN 0.8 Polar Bond
• Remember C-O H-Cl

20
Molecular Shape

• Determined Theoretically from
• Lewis Structure
• VSEPR Shape
• Determined Experimentally from
• Vibrational Exp. (IR and Raman)
• Nuclear Magnetic Resonance (NMR)
• X-Ray Diffraction

21
VSEPR Shapes and Structures
22
VSEPR Shapes and Structures
23
Polar Molecules

• If all atoms attached to central atom are not
  the same
• Or if central atom has 1 or more lone pairs of
  electrons
• The molecule is usually polar

24
Diatomic with Polar Bond
D EN 2.9 - 2.1 .8 (Polar Bond)
d
d-
Polar Molecule and - sides
25
Triatomic with Polar Bonds (CO2)
d-
d-
d
Nopolar Molecule No and - sides
26
Examples

• E.G. Polar HCN, H2O, CHCl3, CH2Cl 2

EN Table
27
Examples

• E.G. Nonpolar BF3, CCl4

28
Quick Quiz

• Polar or Nonpolar?
• CH3OH

Polar !
29
Quick Quiz

• Polar or Nonpolar?
• PF3

30
Quick Quiz

• Polar or Nonpolar?
• PF3

Polar !
31
Quick Quiz

• Polar or Nonpolar?
• CCl2 F2

32
Quick Quiz

• Polar or Nonpolar?
• CCl2 F2

Polar !
33
Quick Quiz

• Polar or Nonpolar?
• C6H6

Nonpolar !
34
In Conclusion

• Polarity Determined from
• Polar Bonds (? EN gt .45)
• Molecular Shape with - sides
• N and O in Molecules often lead to Polar
  Molecules or Regions

35
In Conclusion

• Polarity will be important
• In Determining Intermolecular Forces
• Vapor Pressure, Boiling and Melting Points
• Surface Tension, Viscosity
• Solubility
• Reactivity (Organic Chemistry)
• Large Molecule Structure
• Organic Chem., Biochem., Molecular Biology

36
The End
Y'all Come Back Now, Ya Hear !
37
(No Transcript)
38
(No Transcript)
39
Electronegativity of Some Elements
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.1
O 3.5
F 4.1
Mg 1.3
Na 1.0
Cl 2.9
S 2.4
P 2.1
Si 1.8
Al 1.5
Ca 1.1
Zn 1.7
Back to Examples