Chapter2-5 - Chapter 2 Hydrocarbon Frameworks: Frame...

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Chapter 2 Hydrocarbon Frameworks: Alkanes Chem 211 B. R. Kaafarani 1
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Hydrocarbons romatic liphatic Aromatic Aliphatic H H Alkanes Alkynes Alkenes H H H H H H H H Chem 211 B. R. Kaafarani 2 C C H H H H C C H H C C H H
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Models for Chemical Bonding The Lewis model of chemical bonding predates the idea that electrons have wave properties. There are two other widely used theories of bonding that are based on the wave nature of an electron. 1. Valence Bond Theory Molecular Orbital Theory 2. Molecular Orbital Theory Chem 211 B. R. Kaafarani 3
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Models for Chemical Bonding 1. Valence Bond Theory: Constructive interference between electron waves of two half-filled atomic orbitals is basis of hared lectron ond shared-electron bond. olecular rbital heory 2. Molecular Orbital Theory: Derive wave functions of molecules by combining wave functions of atoms. Chem 211 B. R. Kaafarani 4
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2.3. Bonding in H 2 : The Valence Bond Model Electron pair can be shared when half-filled orbital of one atom overlaps in phase with half-filled orbital of another. 1 s 1 s H H in-phase overlap of two half-filled ond of H H H pp hydrogen 1s orbitals bond of H 2 Bond: orbitals overlap along internuclear axis. Cross section of orbital perpendicular to internuclear axis is Chem 211 B. R. Kaafarani 5 a circle.
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Valence Bond Model of H 2 : Figure 2.4 Chem 211 B. R. Kaafarani 6
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2.4. Bonding in H 2 : The Molecular Orbital Model Main Ideas Electrons in a molecule occupy molecular orbitals (MOs) just as electrons in an atom occupy atomic orbitals (AOs). Two electrons per MO, just as two electrons per AO. Express MOs as combinations of AOs. Chem 211 B. R. Kaafarani 7
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MO Picture of Bonding in H 2 Linear combination of atomic orbitals (LCAO-MO) method expresses wave functions of molecular orbitals as sums and differences of wave functions of atomic rbitals wo AOs yield two MOs orbitals. Two AOs yield two MOs Bonding combination Antibonding combination MO = (H) 1s + (H') 1s ' MO = (H) 1s - (H') 1s Chem 211 B. R. Kaafarani 8
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Fig. 2.6: Energy-Level Diagram for H 2 MOs antibonding * MO 1 s 1 s AO AO bonding Chem 211 B. R. Kaafarani 9 MO
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2.5. Introduction to Alkanes (C n H 2 n +2 ). The simplest lkanes Methane Ethane and Propane Alkanes: Methane, Ethane, and Propane ethane H H Methane (CH 4 ) CH 4 Ethane (C 2 H 6 )C H 3 CH 3 Propane (C 3 H 8 H 3 CH 2 CH 3 Chem 211 B. R. Kaafarani 10 bp bp -160 160 ° C bp bp -89 89 ° C bp bp -42 42 ° C
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2.6. sp 3 Hybridization and Bonding in Methane Structure of Methane Tetrahedral Bond angles = 109.5 ° Bond distances = 110 pm ut structure seems inconsistent with But structure seems inconsistent with electron configuration of carbon. Chem 211 B. R. Kaafarani 11
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Electron configuration of carbon 2 p Only two unpaired electrons. Should form bonds to only two hydrogen atoms.
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This note was uploaded on 01/06/2012 for the course CHEM 211 taught by Professor Bilalkaafarani during the Spring '11 term at American University of Beirut.

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Chapter2-5 - Chapter 2 Hydrocarbon Frameworks: Frame...

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