Different Types of Isomeric Impurities Present in Pharmaceutical Products

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Importance of Relative Response Factor in Impurities Quantification
August 24, 2020
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Isomers are complexes with identical molecular formulas but different arrangements and activities. Isomers of similar compounds have likely to own dissimilar chemical or physical properties which can have some significant association.

Isomeric impurities generated in drug because of conversion within the critical process parameters (CPP) during drug synthesis. The presence of geometric isomer impurities in drugs is a challenging task for scientists.

Depending on the shape of difference amongst the structures, it’s possible to categorized isomers in two forms of isomerism: structural isomerism and stereoisomerism, which can be divided into further subtypes.

Structural Isomerism:

These structural isomers have a similar molecular formula but dissimilar structures i.e. linked differently to every other with different sequences and other ways. There are several subtypes of structural isomerism:

  1. Positional isomers: These isomers have the identical functional groups but at different sites of comparable carbon chain.
  2. Functional group isomers: These isomers also named as functional isomerism, occurs when the atoms form different functional groups.
  3. Chain isomers: These isomers are linked to the main chain of carbon atoms in numerous ways, which can be branched or straight.
  4. Metamerism: These isomers are uncommon and is restricted to molecules having a divalent molecule equivalent to O or S and alkyl groups.
  5. Tautomerism: This isomerism is due to reciprocal conversion of two isomeric forms with different functional groups.
  6. Ring-Chain isomerism: This isomer is an open chain molecule and also the other is cyclic molecule.

Stereo Isomerism:

Stereoisomerism has the identical formula but differ within the arrangement or alignment of atoms in the area. It can further categorize as:

  1. Geometric Isomerism: it’s shown by molecules during which their position in space are fixed to every other due to the presence of a ring structure or a double bond.
  2. Optical Isomerism: Two or more molecules that have the identical molecular arrangement but differ within the optical activity are optical isomers. It again subdivided as: Diastereomers: These are optical isomers which don’t seem to be mirror images of each other. Enantiomers: These are optical isomers that are non-superimposable mirror images of each other.

Optical isomers will rotate plane polarized light in opposite directions.

  • Dextrorotary (d-isomer): A type that rotates plane polarized light to the right.
  • Laevorotary (l-isomer): a species that rotates plane polarized light to the left.
  • A racemic mixture (containing equal amounts of both optical isomers) won’t rotate plane polarized light since the optical effects of the both isomers cancel one another out.

The dextrorotary isomer of substance can have potent pharmacological or toxicological activity from that of laevorotary isomer and hence undesired optical isomer is taken into account as chiral impurity of drug substance.


  1. S. Ahuja, Chiral Separations by Chromatography, Oxford, New York, 2000.
  2. S. Ahuja, Impurities Evaluations of Pharmaceuticals, Dekker, New York,
  3. Graham Solomons, T. W., Fryhle C.B.,  Snyder S.A. Solomons’ organic chemistry. 12th Edition. John Wiley & Sons Incorporated, 2017.

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