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This makes the a good acceptor orbital of electron

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Unformatted text preview: ould expect the fluorine atoms to be anti with respect to one another, however a gauche conformation is experimentally observed. Without using molecular orbital theory it is quite difficult to provide an explanation for this experimental observation. From the previous section, we know that the σ* of the C ­F bond is low in energy because of the large difference in electronegativities of carbon and fluorine. This makes the σ* a good acceptor orbital of electron density and makes the σ orbital a poor donor. Thus when the C ­H bond is bond is anti ­periplanar to the C ­F bond you can get a donor ­acceptor interaction between the σ of the C ­H bond and the σ* of the C ­F interaction that you cannot get when the fluorines are anti ­periplanar. Molecular orbital ­based conformational effects can be thought of as donor ­acceptor interactions. Generally speaking the donor orbitals can be lone pairs or bonding molecular orbitals and the acceptor orbitals are anti ­ bonding orbitals. Furthermore lone pairs are always better donors than bonding orbitals. Thus CH2FNH2 would Dunbar, K and Petrik, I UIUC Chemistry 436 have a larger preference for the gauche conformation than CH2FCH2. The potential interaction between the lone pairs of the nitrogen and the σ* of the C ­F bond is shown in the diagram above. This same explanation can be used to explain the anomeric effect. The lone pair on the oxygen in the ring is the donor and the acceptor is the σ* orbital of the axial group on the adjacent carbon. This donor ­acceptor interaction is very poor when the axial atom has an electronegativity close to that of carbon and in these cases sterics will determine the orientation of the R group. If the group is more electronegative than carbon, the donor ­ acceptor interaction is pronounced and an added stability is achieved when the group is axial. In order to understand why non ­electronegative atoms do not experience the anomeric effect, we nee...
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