The co is much closer in energy to the lone

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Unformatted text preview: d to look to the acceptor ­donor orbitals of the interaction pair. Regardless of what R is, the donor is always the same  ­ the lone pair on the in ­ring oxygen. Similarly the acceptor is always the σ* of the C ­R bond. It is the relative energy of this σ* orbital of the C ­R bond that varies. As the diagram above shows, the σ* orbital of the C ­O bond is lower in energy than the σ* orbital of a C ­H or C ­C bond. When R is C or H this energy gap between the σ* and the oxygen lone pair is too large for any acceptor ­donor interactions to take place, thus there is no added stereoelectronic stability to having a C ­C or C ­H bond axial. The C ­O σ* is much closer in energy to the lone pair and the corresponding interaction between the C ­O σ* and the lone pair overcomes any steric effect of having the oxygen axial. A similar interaction can be seen in the conformations of esters. Methyl acetate has two conformations that it can exist in, a Z and an E conformation (shown below). Although sterics would indicate that the E isomer would be more stable, the Z isomer is predominately preferred. Once again an acceptor ­donor interaction is responsible for this observation. In the Z isomer the donor is the lone pair on the ester oxygen and the acceptor is the σ* of the C ­O bond. In the E isomer the acceptor changes to the σ* of the C ­C bond. Once again the σ* of the C ­O bond is much closer in energy to the lone pair on oxygen and thus there is added stability for the ester when it is in the Z conformer. Dunbar, K and Petrik, I UIUC Chemistry 436 III. Molecular Orbital Theory Applied to SN1/SN2 Reactions: SN1 Reaction: The reaction above is a standard SN1 reaction, where the leaving group (X ­) leaves forming a carbocation (Step 1), and then the nucleophile attacks the empty p orbital on this carbon (Step 2). Applying MO theory to each of the steps in the reaction we can generate the diagram shown below. Step 1:...
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