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Stereo Isomerism

In stereochemistry, atoms and functional groups are oriented in three dimensions to define the structure and orientation of the molecules.
In stereochemistry, atoms and functional groups are oriented in three dimensions to define the structure and orientation of the molecules. Despite containing the same molecular and structural formulas and functional groups, stereoisomers differ by the three-dimensional orientation of their atoms and groups within the molecule. Stereoisomers differ in their physical, chemical, physicochemical, and biochemical properties because of the orientation of the functional group and the geometry of the molecule. A stereoisomer can be divided into three classes according to its symmetry and energy.

Conformational Isomers

Geometrical Isomers

The molecular formulas for maleic acid (m.p. 130°C) and fumaric acid (m.p. 287°C) are the same, but they differ in how they add up their functional groups. There are physical and chemical differences between them. Such isomerism is called geometric isomerism.

Rotation around a double bond within a carbon-carbon structure is restricted. If two atoms or groups exist on the same side, they are called cis (Latin for same) and when they appear on the opposite side, they are called trans (Latin for across). In a cyclic skeleton containing a double bond, diastereoisomerism is termed π diastereoisomerism while in noncyclic compounds with a double bond, diastereoisomerism is termed σ diastereoisomerism.

Optical Isomers

According to Biot, many organic and inorganic compounds can rotate polarized light in the path of a passing plane in opposite directions but with an identical amplitude in the solution form. Compounds that rotate polarized light can exhibit optical isomerism. When at least one asymmetric carbon atom is present in a molecule, optical isomerism will be possible. Chiral carbon refers to carbon atoms linked to four chemically different functional groups. It is possible to have two optically active isomers with one asymmetric carbon. Dextrorotatory light (Latin, dexter: right) is known as the isomer that rotates to the right, while laevorotatory light (Latin, leaves: left) shows rotation to the left. Even though both isomers are mirror images of each other, they cannot be superimposed. Enantiomers are paired up in enantiomorphs, and they are referred to as enantiomorphs. Enantiomers do not possess planes or symmetry centers. Examples include:



In an enantiomer mixture in which both enantiomers are present in equal concentrations, a rotating plane-polarized light from a laevo isomer will be cancelled by a dextro rotating isomer, so the mixture will be inactive. This type of mixture is called a racemic mixture. A racemic form is obtained by converting an enantiomer to a racemic form. Resolution is the process of separating enantiomers from racemic mixtures. In a molecule with more than one asymmetric carbon atom, the formula represents the maximum number of optically active isomers that may be generated

N = 2n

N - Numerical values for optically active isomers

n - Carbon atoms that are asymmetric.

In terms of boiling point, melting point, solubility, and rotation of plane-polarized light, enantiomers are identical. Chemically, these reagents, solvents, and conditions are similar to achiral reagents. The rates of enantiomers' reactions with chiral reagents, solvents, and catalysts differ. Considering that tartaric acid contains two asymmetric carbon atoms, it will have four optically active forms.



Identical and symmetrical forms (I) and (II) exist. The upper half of these forms is the mirrored image of the lower half. Internal compensation renders the molecule optically inactive. Meso-isomers are homogeneous and symmetrical stereoisomers. Despite their mirror images, (III) and (IV) cannot be superimposed. These forms are enantiomeric.

If you compare (III) with (I) or (IV) with (I), you are not comparing enantiomeric pairs. The mirror image does not superimpose the other. Each of their molecules consists of one identical half and two mirror images of the other. Stereoisomer diastereomers do not mirror each other's images and are not superimposable. Physically and chemically, they differ in their properties, with both achiral and chiral reagents. There is also a difference in the rate of reaction and in the product.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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