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| Chirality |
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| A compound must be chiral for it to exhibit optical activity. Chiral is derived from the Greek word cheir meaning hand. A chiral molecule is one that is not super imposable on its reflection. |
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| For e.g., your left hand is a mirror image of your right hand. They are not super imposable. Therefore your hands are chiral in nature. |
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| Chiral compounds that are mirror images of each other and not super imposable are called enantiomers (enantiomers means "opposite" and meros means "part" in Greek). The phenomenon of compounds exhibiting chirality is known as enantiomerism. |
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| Physical properties of enantiomers are similar. However they rotate the plane-polarized light in opposite directions. |
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| Chiral compounds have a chiral center usually a carbon atom bonded to four different atoms or groups. As soon as any two of the four atoms or groups become identical the chirality and hence the optical activity is lost. For e.g., |
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| Lactic acid |
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| is chiral (all four groups around the carbon are different) |
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| But |
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| Glycollic acid Propionic acid Isobutyric acid |
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| Glycollic acid, propionic acid and isobutyric acid have two similar atoms or groups and hence not chiral and not optically active. |
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