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| Symmetric Elements and Chirality |
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| The presence of a single chiral center indicates chirality of a molecule. Another way to recognize chirality is to detect the presence of some symmetry elements. |
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| A molecule will not be chiral if it possesses: |
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 a plane of symmetry (s) |
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a
center of symmetry ( ) |
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an n fold alternating axis of symmetry (Sn or Cn) where n is an even number. |
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| These three are called elements of symmetry. |
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| Plane of symmetry |
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| A plane that divides an object into two identical halves is called plane of symmetry. Its also called mirror plane as it cuts a molecule into two parts, where one is the mirror image of the other. Molecules having such a plane are always inactive due to internal compensation. For e.g., meso tartaric acid has a plane of symmetry |
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| Mesotartaric acid |
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| There are two symmetric carbon atoms and they are therefore achiral. Hence meso tartaric acid is optically inactive. |
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| Center of symmetry |
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| The center of symmetry is an imaginary point in the molecule. If a line is drawn from an atom or a group of the molecule to this imaginary point and then extended to an equal distance beyond the point, it meets the mirror image of the atom or group. For e.g., trans-1,4-dimethyl-diketopiperazine has a center of symmetry and therefore optically inactive. |
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| If a line is drawn from methyl group on carbon 1 to the center of symmetry and extended beyond this point by an equal distance it meets the methyl group at carbon 4, therefore it is optically inactive. |
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| Alternating axis of symmetry |
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| When a structure is rotated through an angle of 2p/n about an imaginary axis and then reflected across a plane perpendicular to the axis, an identical structure results. |
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