lecture 6 - 2 is not a stable entity • When we applied...

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318M – Krische, Lecture 6: W - 09/10/08, Ch. 1, Covalent Bonding and Shapes of Molecules Today, we discussed hybrid atomic orbitals (hybrid AO’s) and the molecular orbitals (MO’s) that result from their combination. MO can be viewed as in phase and out of phase combinations of AO’s. We started off by considering a very simple case: the combination of two hydrogen atoms to provide elemental hydrogen, H 2 . The bond strength of H 2 is 104 Kcal/mol. When two H-atoms approach, their 1s orbitals overlap. In phase and out of phase combinations result: the sigma-bonding MO and the sigma-antibonding MO, respectively. Since each H atom contributes a single electron (recall 1s 2 electronic configuration), we only populate the sigma-bonding MO. Thus, each electron in the sigma-bonding MO contributes about 52 Kcal/mol. A similar analysis allowed us to understand why He
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Unformatted text preview: 2 is not a stable entity. • When we applied this approach to the construction of methane (CH 4 ), we obtained 90 o bond H-C-H bond angles. This was inconsistent with the observed 109 o bond angle. To understand bonding in methane, we were required to invoke hybrid AO’s. • For a detailed discussion on hybrid AO’s, see on-line handout Ch.1 part III. • Using hybrid AO’s, we constructed methane (CH 4 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ). On your own, try propene CH 3 CHCH 2 ) and allene (CH 2 CCH 2 ). Notably, by considering the formation of pi-bonds, we were able to understand why all atoms are coplanar for ethylene. • We conclude that geometry, which can be predicted using VSEPR, enables the prediction of hybridization. Be prepared to assign geometry and hybridization for organic molecules....
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