CHE 131 Lecture 21, 3/10/2006
Lecture 21, Knowledge and Skills
Lecture 21
• More about MO
• Know what a dipole moment is
• Know the range of dipole moment sizes for molecules
• Know how polar molecules interact with each other or with
external electric fields
• Determine whether a molecule has a dipole moment or not
• Know about induced dipole moments
• Estimate the influence of molecular polarity on physical
properties
Molecular Polarity (9.5)
Intermolecular Forces I (9.6)
London Forces, Dipole-Dipole Interactions
Benjamin S. Hsiao
Department of Chemistry
Stony Brook University
sp3d Hybrid Orbitals (Trigonal Bipyramidal)
sp3d2 Hybrid Orbitals (Octahedral)
1
CHE 131 Lecture 21, 3/10/2006
σ Bonds
Formation of σ Bonds in Trigonal Bipyramidal Molecules
Bonds with the greatest electron density on a line connecting the
atomic nuclei
generic orbital, may represent an
s orbital or any hybrid orbital
s-s type
p-p type
s-p type
s-sp3 type
s-sp2 type
s-sp type
p-sp2 type
p-sp type
p-sp3 type
sp2-sp2 type
sp - sp type
sp3 - sp3 type
etc., including all combinations of s, p, d, and hybrid orbitals
π Bonds
Bonds with the greatest electron density above and below a line
connecting the atomic nuclei
p–p type
d–p type
For example: As(C6H5)5
As
5
For example: PCl5
Molecular Polarity
2
CHE 131 Lecture 21, 3/10/2006
What do polar molecules do?
What do polar molecules do?
Response to External Electric Field
Dipole-Dipole
Interactions
Determine whether a Molecule Has a Dipole Moment
Molecular
Polarity
1. Determine VSEPR structure.
2. Look for molecular symmetry. Determine by inspection if
all existing bond dipole moments cancel each other or not.
3
CHE 131 Lecture 21, 3/10/2006
Determine whether a Molecule Has a Dipole Moment
Structure
A
Example
:N
A
Determine whether a Molecule Has a Dipole Moment
Structure
Dipole Moment
B
Cl
B
F
..
C
A
B
A
A
A
B
A
B
:
C
C
C:
occurs in
interstellar
space
O
A
B
No Net
Dipole
F
B
A
yes
F
Net
Dipole
F
B
no
N:
B
A
Example
B
B
no
C
F
F
..
A
B
C
..
B
A
C
A
A
Determine whether a Molecule Has a Dipole Moment
A
Determine whether a Molecule Has a Dipole Moment
Example
Structure
Structure
Example
A
B
B
B
A
A
B
B
B
Si
C
CH3
B
B
A
C
B
B
A
C
B
A
B
B
B
B
B
A
C
..
A
B
B
..
C
B
B
H3C
H3C
B
C
B
B
No Net
Dipole
CH3
B
B
B
Net
Dipole
B
:
B
B
A
B
C
B
B
B
A
B
4
:
B
B
C
C
A
B
B
:
:
A
B
B
Net Dipole
Moment
No Net Dipole
Moment
CHE 131 Lecture 21, 3/10/2006
Determine whether a Molecule Has a Dipole Moment
Example
Structure
Net Dipole
Moment
Types of Intermolecular Forces
No Net Dipole
Moment
B
B
B
A
B
B
B
C
B
B
B
B
A
B
(dispersion forces)
C
B
B
Attractions between
all types of atomic
and molecular
species
B
B
A
C
C
B
B
C
B
A
B
C
B
B
A
B
C
C
C
B
B
A
C
B
C
London Forces
induced dipole — induced dipole interaction
London Forces
Helium
2+
2+
Momentary situation,
temporary dipole
Average situation
Attraction
2+
2+
neighboring atom
2+
2+
induced dipole
Polarization
Polarizability: ability to have a dipole induced >> heavier rare gases
5
CHE 131 Lecture 21, 3/10/2006
Dipole — Dipole Interactions
Intermolecular Forces in Methane
long-range order in solids
short-range organization in liquids and gases
Boiling Points of Nonpolar Substances
Table 9-6, p.412
6
CHE 131 Lecture 21, 3/10/2006
Molecule of the Week: Water
Structures & Boiling Points
High boiling point
High melting poit
High heat of vaporization
High heat of fusion for a
small molecule
High heat capacity,
liquid
solid
High dipole moment
High dielectric constant
High surface tension
Very small size
Density increases upon
melting, with
maximum at 4 oC
Determine the hybridization around the central atom in SF5+
a. sp, b. sp2, c. sp3, d. sp3d, e. sp3d2
The correct answer is D.
7
100 oC
0 oC
44 kJ/mol
6 kJ/mol
4.18 J/deg.g
2.03 J/deg.g
1.86 D
80
72 dynes/cm