UNIT 6. Theories of Covalent Bonding and Intro to Organic Chemistry VSEPR Theory, Molecular Shapes, and Valence Bond Theory. Electron Domains . Electron domains are the regions in the molecules where it is most likely to find electrons.
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Theories of Covalent Bonding and Intro to Organic Chemistry
VSEPR Theory, Molecular Shapes, and Valence Bond Theory
The Octet Rule
Lewis Structures of Covalent CompoundsFollow these steps in order.
1. Decide which atoms are bonded.
2. Count all valence electrons.
3. Put two electrons in each bond.
4. Complete the octets of the atoms attached to the central atom except H, which takes a duet.
5. Put any remaining electrons on the central atom.
6. If the central atom has less than an octet, form double or triple bonds.
Putting Formal Charges on Lewis Structures
The formal charge of any atom in a compound or ion may be calculated using the following:
FC = # of valence electrons – number of bonds – number of nonbonding electrons
FC of O = 624 = 0
FC of O = 616 = 1
FC of O = 632 = +1
Three completely equivalent Lewis structures can be drawn for the nitrate ion, NO3.
Reality is a blend of the three. There are no double bonds in the nitrate ion, but each bond is more stable than just a single bond. All three structures are resonance structures.
From Lewis Structure to Electron Domain Geometry via VSEPR
Bond Angles when a Nonbonding Electron Pair or Multiple Bond is Present
Bond Angles for Atoms in Organic Molecules: Effect of Nonbonding Electron Pairs
Example: O in an ether
4 electron domains
tetrahedral geometry
Bond angles on O are <109.5° because the two e pairs take up extra space.
Example: C in an alkane
4 bonding domains
tetrahedral geometry
Bond angles 109.5°
Example: N in an amine
4 electron domains
tetrahedral geometry
Bond angles on N are <109.5° because the e pair takes up extra space.
Lewis Structures – Explain bonding as a sharing of electron pairs and geometry through VSEPR.
VB Theory  A more quantitative approach to explaining bonding. Here bonds are explained by the overlap of orbitals on the two atoms in the bond.
The periodic table may also be used to determine the electron configuration of the elements.
Bonds occur from the overlap of atomic orbitals.
Cl: 1s22s22p63s23p5
H: 1s1
The bond in HCl is formed by the overlap of the H 1s orbital with the Cl 3p orbital.
How would you describe the ClCl bond?
The bond in HH is formed by the overlap of the two H 1s orbitals.
Bonds formed by endtoend overlap are called sigma (σ)bonds.
Valence Bond Theory – Hybrid Orbitals
Describing bonds as the overlap of s and p orbitals explains some geometries, but certainly not the tetrahedral geometries (e.g. H2O).
What orbitals are overlapping here?
Valence Bond Theory – Hybrid Orbitals
Since the orbitals (s, p, d, f, etc.) are all mathematical solutions to the Schrödinger wave equation, it is true that linear combinations of these orbitals are also solutions to the Schrödinger wave equation.
In other words, we may mix s, p, and d orbitals to make new, hybrid orbitals that are also valid.
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
2p
2s
C ground state (the orbitals of C are what determine the geometry of HC≡CH.)
Orbital energy
Two e are available for bonding, but the geometry is wrong.
1s
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
Energy is used to promote one 2s e to a 2p orbital.
2p
2s
Orbital energy
A 2s orbital and a 2p orbital mix to make two new orbitals. The other two 2p orbitals are unchanged.
1s
Consider HC≡CH. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
2p
sp
The 2s orbital and one 2p orbital mix to form two sp hybrid orbitals. The energy of mixing is more than paid back when the CH and C≡C bonds are formed.
Orbital energy
1s
C
C
C
C
C
Hybrid orbitals have a small lobe and a large one. The large lobe allows more overlap and, therefore, the formation stronger bonds. The energy needed to make the hybrid orbital is paid back, with interest, in the formation of stronger bonds.
Consider H2C=CH2. VSEPR says the molecule is linear.
How can hybrid orbitals explain the geometry?
One 2p orbital remains unhybridized.
2p
sp2
Orbital energy
Mixing the three orbitals gives three sp2 hybrid orbitals with the same energy.
1s
What orbitals overlap to form each of the CH bonds?
Consider CH4. How can hybrid orbitals explain this tetrahedral geometry?
sp3
Orbital energy
Mixing the four orbitals gives four sp3 hybrid orbitals with the same energy.
1s
The VSEPR geometry tells you the hybridization: Tetrahedral VSEPR geometry sp3 hybrid orbitals.
What orbitals overlap to form each of the CH bonds in methane?
PCl5 has the shape of a trigonal bipyramid.
P ground state:
3s
3p
3d
Energy is used to promote 3s e to 3d:
Hybridize:
sp3d
3d
Trigonal bipyramid VSEPR geometry sp3d hybrid orbitals.
SF6 has the shape of an octahedron.
S ground state:
3s
3p
3d
Energy is used to promote 3s and a 3p e to 3d:
Hybridize:
sp3d2
3d
Octahedral VSEPR geometry sp3d2 hybrid orbitals.
Use Lewis structures to get the electron domain geometry, and that will lead to the bond angles and hybridization.
# of e e domain bond angle hybridization
domains geometry
2 linear 180° sp
3 trigonal 120° sp2
planar
4 tetrahedral 109.5° sp3
5 trigonal 90°,120°,180° sp3d
bipyramidal
6 octahedral 90°,180° sp3d2
Hybridization and Bond Angles in Larger Molecules
Just identify the geometry/hybridization around each atom in succession.
sp2, trigonal planar geometry, bond angle is >120° (due to double bond)
H : O :
 
H — C — C — O — H acetic acid
 ¨
H
..
sp3, bent geometry, bond angle is <109.5°
What orbitals overlap to form each of the bonds in acetic acid?
sp3, tetrahedral geometry, bond angle is 109.5°
Hybridization and Bond Angles in Larger Molecules
H

H — C — C ≡ C — H propyne

H
sp, linear geometry, 180°
sp3, tetrahedral geometry, 109.5°
H

H — C— C ≡ C — H

H
What orbitals overlap to form each of the bonds in propyne?