Lecture 7 - Lecture 7 Organic Chemistry I Prof. Jonathan L....

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Organic Chemistry I 310/318M Pre-Health Professionals Unique numbers: 54410, 54435, 54440, 54445, and 54655 Prof. Jonathan L. Sessler Lecture 7
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Combining VB & MO Theories • VB theory views bonding as arising from electron pairs localized between adjacent atoms. These pairs create bonds. • Further, organic chemists commonly use atomic orbitals involved in three hybridization states of atoms ( sp 3 , sp 2 , and 2p ) to create orbitals to match the experimentally observed geometries. • How do we make orbitals that contain electrons that reside between adjacent atoms? For this, we turn back to MO theory.
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Combining VB & MO Theories • To create orbitals that are localized between adjacent atoms, we add and subtract the atomic orbitals on the adjacent atoms, which are aligned to overlap with each other. • Recall, that this combination is only possible if the starting orbitals are of the correct symmetry, and works best if they are close in energy. • This math takes place, whether or not we add electrons to the orbitals, but only has meaning once we do.
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• Consider methane, CH 4 . The sp 3 hybrid orbitals of carbon each point to a 1 s orbital of hydrogen and, therefore, we add and subtract these atomic orbitals to create molecular orbitals. • One resulting MO is lower in energy than the two atomic orbitals, and is called a bonding σ orbital. The other is higher in energy and is antibonding. • Typically, in Organic I, we show only the bonding MOs, of which there are 4 in this case.
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Combining VB & MO Theories • Figure 1.17 Molecular orbital mixing diagram for creation of any C-C σ bond. Here, we are looking at one part of a hybrid orbital ( sp 3 , sp 2 , or sp ) interacting with another hybrid orbital.
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Combining VB & MO Theories • This approach is used to create C-H σ bonds. •C H 3 CH 3 contains 1 C-C σ bond and 6 C-H σ bonds.
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Combining VB & MO Theories • A double bond uses sp 2 hybridization. • Consider ethylene, C 2 H 4 . Carbon and other second-period elements use a combination of sp 2 hybrid orbitals and the unhybridized 2 p orbital to form double bonds.
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Combining VB & MO Theories • Figure 1.21 MO mixing diagram for the creation of any C-C π bond. Note this new symmetry designation!
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Ethylene H H H H 121.4 o 1.34 Å Hint: Put in Lewis dots if confused! H H H H angles now understood σ bond from sp 2 C & 1s H 3 e - from carbon = 6 e - 1 e - from hydrogen= 4 e - 10 e - in sigma system CC Ca. 120 deg. Ca. 120 deg. Ca. 180 deg.
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• We notice that above σ bond diagram leaves us with 1 more orbital (the p z AO) and 1e - per carbon atom. General idea of separating σ lone pair framework from π framework. This works because they are different symmetries. high energy (empty) lower energy (full with 2e - ) We will pay special attention to the π framework because π bonds need not be localized.
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Molecular Orbital Treatment of Ethylene - As We Will Do It In Org. I First, we start with the sigma system, using the lowest MO as a VB approximation for the total bonding sigma framework.
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Lecture 7 - Lecture 7 Organic Chemistry I Prof. Jonathan L....

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