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| Conformations or Conformational Isomers (Rotational Isomers) |
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| Consider an ethane molecule (H3C-CH3). Infinite number of arrangements are possible if one methyl group is allowed to remain stationery and the other rotated through the C-C axis depending on the angle of rotation. This angle is called the dihedral angle or the angle of torsion and the different arrangements are called rotational isomers or conformations or conformers. |
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| For e.g., for ethane, two extreme conformations are possible. |
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| The eclipsed conformation |
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| Considering the three dimensional model of ethane, the single carbon atom and the three hydrogen atoms at the back are exactly covered by the those at the front. The atoms at the back are not visible. Hence the conformation is known as eclipsed conformation. |
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| The staggered conformation |
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| All atoms are symmetrically seen and appear symmetrically situated at maximum possible distances. |
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| fig.12.3 - Two rotational isomers or Conformations of ethane |
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| It is difficult to separate the eclipsed and the staggered conformations in ethane due to the free rotation about the carbon carbon single bond. However, if there are bulky tertiary groups around the single bond, then separation is easy as rotation is hindered. |
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| The staggered form is usually more stable than the eclipsed forms. This is because, in the eclipsed form, the atoms or groups are closer and hence tend to repel each other. This repulsion is called non bonded interactions and it raises the energy and makes the eclipsed form unstable. In the case of staggered form, the atoms or groups are as far away from each other as possible and hence there is no repulsion. Hence the energy is less and this conformation is more stable. |
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| Ball and stick models are used to visualize conformations in three-dimensional space. For two dimensional space representations, Sawhorse convention and Newman Projection Formulae are used. |
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