Download Chapter Outline Measuring Polarity Examples Permanent Dipole

2/26/2014
Chapter Outline
Polar Bonds and Polar Molecules
 5.1 Molecular Shape
 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)
 5.3 Polar Bonds and Polar Molecules
» What Makes a Molecule Polar?
» Dipole Moments




5.4
5.5
5.6
5.7
Valence Bond Theory
Shape and Interactions with Large Molecules
Chirality and Molecular Recognition
Molecular Orbital Theory
Polar Bonds
and Polar Molecules (cont.)
 Requirements for Polar Molecule:
• 1. Molecule must contain polar bonds (i.e.,
covalent bond between atoms with ΔEN).
• 2. Orientation of polar bonds results in charge
separation from one part of the molecule to
another.
Polar bonds… but linear shape results in partial
charges canceling out; nonpolar!
Measuring Polarity
 Dipole moment (μ):
Bond Dipole:
•Separation of
charge within a
covalent bond.
• Measured value defining extent of separation
of + and − charge centers in a molecule.
(Units = debyes (D); 1 D = 3.34 × 10−30 coul∙m )
Polar Molecule:
•Vectors of bond
dipoles sum >
zero.
Polar!
© 2012 by W. W. Norton & Company
Examples
Permanent Dipole Moments
Determine whether or not the following molecules are polar (a) CH2O
(b) CHCl3
(c) CCl3F
© 2012 by W. W. Norton & Company
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Chapter Outline
The Orbital Overlap Model of Bonding
 5.1 Molecular Shape
 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)
 5.3 Polar Bonds and Polar Molecules
 5.4 Valence Bond Theory
H-H
H-F
» Orbital Overlap and Hybridization.
» Hybridization and Molecular Geometries
 5.5 Shape and Interactions with Large Molecules
 5.6 Chirality and Molecular Recognition
 5.7 Molecular Orbital Theory
End to end overlap = sigma () bond
Problem: the available s and p-orbitals are at 90o
angles, not at the predicted 109.5o!
Predicted Bonding and VSEPR Geometry for CH4
109.5
Lewis
Structure
o
Electron Group Geometry
around the central C atom
is tetrahedral.
Valence Bond Theory (hybrid orbitals)
• A quantum mechanics-based theory of bonding that assumes covalent bonds form
when half-filled orbitals on different atoms overlap or occupy the same region of
space.
Hybridization Rules – will be upgraded as we go
New orbitals are constructed from pre-existing s, p,
and d-orbitals = hybrid orbitals
• In order for the bonding to match the VSEPR geometry, atomic orbitals must be
combined into new “hybrid” orbitals that do result in the correct geometry.
1. Hybridize the CENTRAL ATOM ONLY (others
as needed)
2. Only use valence shell electrons
3. The number of hybrid orbitals formed = number
of atomic orbitals used
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sp3 Hybridization = Tetrahedral EG
For CH4, we need 4 hybrid orbitals, so 4 atomic
orbitals are required as follows: (s + p + p + p) = sp 3
Needed to form
4 sigma bonds
Hybridization Rules – cont’d
Other Examples – NH3 and H2O
1. Hybridize the CENTRAL ATOM ONLY (others
as needed)
2. Only use valence shell electrons
3. The number of hybrid orbitals formed = number
of atomic orbitals used
4. Hybrid orbitals get 1 electron for a -bond, 2
electrons for a lone pair.
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