Isomerism

=Structural Isomerism=


 * Structural Isomers ** (also called **constitutional isomers**) are molecules that have the same molecular formula (same number of each type of atom) but a different structure (or structural formula). An example would be 2,3-dimethylpentane and heptane, both having the molecular formula C7H16. Because of their different structure, these isomers have different physical properties (m.p., b.p., solubility etc), and may have different chemical properties (particularly if the structural isomers have different functional groups such as CH3COCH3 and CH3CH2CHO).

=Geometric Isomerism= G eometric Isomerism results most commonly from // Carbon-Carbon double bonds //. The important property which introduces the feature is the inability of the Carbon atoms to rotate relative to one another about the double bond. This is due specifically to the Pi bond but I won't discuss this part of the subject further on this site. The lack of rotation means the same groups can be attached in different ways to achieve diastereomers. The molecules have identical connectivity so can't be described as structural isomers. T here are two ways to name compounds of this kind. When two of the four groups attached to the carbons involved are hydrogen, the // cis // and // trans // notation can be used. When the other two constituents are on the // same side // of the molecules name is given the prefix 'cis'. Otherwise if the constituents are on // opposite sides // the prefix is 'trans'.

=Optical Isomerism=

Optical Isomers or Enantiomers
Optical isomers (like geometric isomers) are examples of stereoisomers. The enantiomer and its mirror image are non-identical. All amino acids, (except the simplest amino acid, glycine), are optically active. This means they contain an **asymmetric**, or **chiral**, carbon atom. This is a carbon atom which has four different groups attached.

To show the different enantiomers of a molecule it is necessary to draw a 3-dimensional structure. For any enantiomer the structure of the mirror image can be drawn by swapping **any two** groups.

Enantiomers have identical physical properties (melting point, solubility etc) BUT differ in that they **rotate** plane polarised light in opposite directions. This rotation of polarised light is measured by placing a sample in a piece of equipment called a polarimeter.

Optical isomers also have identical chemical properties except in their reactions with other optical isomers. This has important consequences in biochemistry where only one of the optical isomers of each amino acid occurs commonly. In fact the biological behaviour of optical isomers can be profoundly different. This is because in biological systems most receptor sites are asymmetric and therefore will only combine with (react with) one of the pair of enantiomers. This has important consequences in the design of drugs eg the drug methorphan is one optical isomer that can be used as a cough suppressant, while its mirror image is an addictive drug. The drug thalidomide, prescribed as a treatment for morning sickness in early pregnancy in the 1960s, tragically caused the development of serious birth defects (badly deformed limbs, or none at all). In fact, only one of the two optical isomers of thalidomide appear to cause these birth defects, although it could be that once ingested each isomer readily changes into the other form. Today thalidomide is being used successfully as a treatment for leprosy (although not for pregnant women).

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