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Chirality

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The term chirality originates from Greek word (cheir) meaning hand and it describes drugs which have the property of molecular asymmetry and therefore exist as a pair of non-superimposible mirror images. Chirality is a useful concept in physical and life sciences, especially when applied to a molecular level. Chirality manifests itself in both molecules and crystals and its origin lies clearly in molecular architecture.

Generally the phenomenon of chirality exists in organic molecules. However, it is found in some inorganic compounds. In some molecules the carbon atoms remain attached to four different atoms or groups. This arrangement makes the whole molecule asymmetric. Chirality cab be recognized at different length scales from subatomic particles to atoms and molecules.

Chirality Definition

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An object is chiral if it cannot be superposed by pure rotation and translation on its image formed by inversion through a point. Chirality is a molecular phenomenon that has a significant impact in a pharmaceutical content. A chiral molecule is defined as a compound that is not superimposible on the molecule that forms its mirror image. 

Generally, the phenomenon of chirality exists in organic molecules. However, it is also found in some inorganic compounds. In some molecules the carbon atoms remain attached to four different atoms or group. This arrangement makes the whole molecule asymmetric. Molecules of this types differ in their three dimensional configurations and exist in two forms, which are mirror images of each other. 

Many objects are non superimposible on their mirror images. For example, left hand and right hand look similar but if left hand on the right hand. They do not coincide. The objects which are non superimposible on their mirror image are said to be chiral and this property is known as chirality. While the objects which are superimposible on their mirror images are called achiral.

Racemisation

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A mixture containing two enantiomers in equal properties will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture or racemic modification. A racemic mixture is represented by prefixing di or before the name, for example butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known as racemisation.

Racemisation could occur prior to passage through a chiral reaction path and this phenomenon could be responsible for a number of as yet undiscovered literature errors. The methoxide ion causes the racemisation of chiral methyl phosphine esters and reaction of nucleophiles with such species could be accompanied by racemaisation caused by the leaving group released as the reaction progresses.

Optical activity

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Optical activity refers to all phenomena associated with the differential interaction of left versus right circularly polarized radiation for a sample possessing chirality or handedness, at the molecular level. Optical activity is a measurable molecular property resulting from the interaction of radiation and matter whereas molecular chirality is more fundamental property with a basis in pure geometry, which is associated with the molecule independent of its interaction with radiation.

Optical acitivity refers to the phenomena in which the plane of polarization of linearly polarized light rotates gradually as the light beam proceeds through materials with low symmetry. Its the ability of a sample to rotate a plane of polarized light. A solution consisting of a mixture of enantiomers at a ratio other than 50:50 can rotate plane polarized light to either the right or the left. A rotation to the right is designated (+) a rotation to the left is designated (-). Earlier nomenclature used dextrorotatory or levorotatory.