1. Structure and Properties of Organic Molecules
Organic Chemistry, 8th Edition L. G. Wade, Jr.
Chapter 2 Lecture
Structure and Properties of Organic Molecules

2. Lecture 3: Orbital Theory, Molecular Shapes (VSEPR) and Hybridization
Wave Properties of Electrons
Wave Interactions
LCAO
Valence Bond Theory
Sigma Bonding
Pi Bonding
Molecular Shapes and Hybridization
Bond Rotation in Organic Chemistry

3. Wave Properties of Electrons
Electrons have wave-like properties
Orbitals are 3-D standing waves
Standing wave vibrates in fixed location (Like a guitar string)
Traveling wave is like an ocean wave
Chapter 1

4. Wave Properties of Electrons
Orbitals are described by a wave function
Wave function, , is a mathematical description of size, shape, and orientation.
Amplitude may be positive or negative.
Node: Amplitude is zero.
Chapter 1

5. Wave Interactions Linear combination of atomic orbitals
on different atoms produce molecular orbitals (MO’s)
Lead to Bonding: Covalent Bonds
on the same atom give hybrid orbitals (sp, sp2, sp3)
Different geometries for molecules

6. LCAO Linear combination of atomic orbitals Conservation of Orbitals
Number of new orbitals = initial number of orbitals
Waves that are in phase add together: Amplitude increases
Waves that are out of phase cancel: Zero Amplitude

8. Valence Bond Theory A bond occurs when atomic orbitals overlap.
Overlapping orbitals are like overlapping waves.
Only constructive interference results in a bond.
Chapter 1

9. Sigma Bonding
An MO that is cylindrically symmetrical along the line connecting the nuclei is referred to as a sigma bond (σ ─ bond)
A bond may be formed by s-s, p-p, s-p or hybridized orbital overlaps.
Every single bond connecting two atoms contains a sigma bond (σ ─ bond)

10. Bonding Region
Electrons are close to both nuclei.

11. Bonding Molecular Orbital
Two Hydrogens, 1s constructive overlap

12. Anti-Bonding Molecular Orbital
Two Hydrogens, destructive overlap

13. H2: s—s Overlap
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Chapter 2 13

14. Cl2: p-p overlap
Constructive Overlap along the same axis forms a sigma bond.

15. HCl: s-p overlap

16. Pi Bonding Pi bonds form after sigma bonds.
Sideways overlap of parallel p-orbitals.

17. Pi Bonding
Electron density found above and below line connecting two nuclei
Overlap is parallel; not linear.
Π-bonds are not cylindrically symmetric
Π-bonds are not as strong as σ-bonds.

18. Multiple Bonds
A double bond (2 pairs of shared electrons consists of a sigma bond and a pi bond
A triple bond (3 pairs of shared electrons consists of a sigma bond and 2 pi bonds)

19. Molecular Shapes and Hybridization
Bond angles cannot be explained with simple s and p orbitals.
Valence shell electron-pair repulsion (VSEPR) is used to explain the molecular shape of molecules.
Hybridization is LCAO within one atom, just prior to bonding.
Hybridized orbitals are lower in energy because electron pairs are farther apart.

20. sp Hybrid Orbitals 2 VSEPR pairs Linear electron pair geometry.
180° bond angle.
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Chapter 2 20

21. The Bonding of BeH2
The bond angle in BeH2 is 180º and the geometry is linear.
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Chapter 2 21

22. sp2 Hybrid Orbitals 3 VSEPR pairs
Trigonal planar e- pair geometry 120o bond angle
Chapter 1

23. sp3 Hybrid Orbitals 4 VSEPR Pairs
Tetrahedral e- pair geometry o bond angle

24. Summary of Hybridization and Geometry
Hybrid Orbitals
Hybridization
Geometry
Approximate Bond Angle 2
s + p = sp
linear
180⁰ 3
s + p + p = sp2
trigonal
120⁰ 4
s + p + p + p = sp3
tetrahedral
109.5⁰
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Figure: 02_UNT02.jpg
Chapter 2 24

25. Rotation of Single Bonds
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Ethane is composed of two methyl groups bonded by the overlap of their sp3 hybrid orbitals.
There is free rotation along single bonds.
Chapter 2 25

26. Bonding in Ethylene
Ethylene has three sigma bonds formed by its sp2 hybrid orbitals in a trigonal planar geometry.
The unhybridized p orbital of one carbon is perpendicular to its sp2 hybrid orbitals, and it is parallel to the unhybridized p orbital of the second carbon.
Overlap of these two p orbitals will produce a pi bond (double bond) that is located above and below the sigma bond.
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Figure: 2-17 page 51 in the 8th edition
( un.jpg in IRC DVD)
Chapter 2 26

27. Rotation Around Double Bonds
Double bonds cannot rotate.
Compounds that differ in how their substituents are arranged around the double bond can be isolated and separated.
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Chapter 2 27

28. Skill Building: Practice Problems
Problem 2-1 thru 2-7