《Organic Chemistry 5th Edition (L.G. Wade JR.)》wade02

Organic Chemistry, 5th Edition L. G. Wade, Jr.

Chapter 2 Structure and Properties of Organic Molecules

Jo Blackburn Richland College, Dallas, TX Dallas County Community College District 2003, Prentice Hall

Wave Properties of Electrons Standing wave vibrates in fixed location. Wave function, , mathematical description of size, shape, orientation Amplitude may be positive or negative Node: amplitude is zero_+Chapter 2

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Wave Interactions Linear combination of atomic orbitals between different atoms is bond formation on the same atom is hybridization.

Conservation of orbitals Waves that are in phase add together. Amplitude increases. Waves that are out of phase cancel out. =>Chapter 2 3

Sigma Bonding Electron density lies between the nuclei. A bond may be formed by s-s, p-p, s-p, or hybridized orbital overlaps. The bonding MO is lower in energy than the original atomic orbitals. The antibonding MO is higher in energy than the atomic orbitals. =>Chapter 2 4

H2: s-s overlap

=>Chapter 2 5

Cl2: p-p overlapConstructive overlap along the same axis forms a sigma bond.

=>Chapter 2 6

HCl: s-p overlapQuestion: Draw the predicted shape for the bonding molecular orbital and the antibonding molecular orbital of the HCl molecule.Answer: See bottom of page 42 in your text. =>Chapter 2 7

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

=>Chapter 2 8

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 two pi bonds.

=>Chapter 2 9

Molecular Shapes Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory. Hybridized orbitals are lower in energy because electron pairs are farther apart. Hybridization is LCAO within one atom, just prior to bonding. =>

Chapter 2

sp Hybrid Orbitals

2 VSEPR pairs Linear electron pair geometry 180° bond angle=>Chapter 2 11

sp2 Hybrid Orbitals 3 VSEPR pairs Trigonal planar e- pair geometry 120° bond angle

=>Chapter 2 12

sp3 Hybrid Orbitals 4 VSEPR pairs Tetrahedral e- pair geometry 109.5° bond angle

Chapter 2

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Sample Problems Predict the hybridization, geometry, and bond angle for each atom in the following molecules: Caution! You must start with a good Lewis structure! NH2NH2 CH3-C C-CHOO CH3 C _ CH2

=>Chapter 2 14

Rotation around Bonds Single bonds freely rotate. Double bonds cannot rotate unless the bond is broken.

=>Chapter 2 15

Isomerism Molecules which have the same molecular formula, but differ in the arrangement of their atoms, are called isomers. Constitutional (or structural) isomers differ in their bonding sequence. Stereoisomers differ only in the arrangement of the atoms in space. =>Chapter 2 16