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Optical Isomerism

In organic chemistry the idea of optical isomerism is all about compounds having similar physical and chemical properties but differ towards the behavioural patterns as far polarized lights are concerned but it obviously revolves around few factors which determines the optical isomerism in organic molecules. 

The behaviour towards polarized light by molecules are specific for optical isomers and the phenomenon is optical isomerism.


Define Optical Isomers

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We can define optical isomers as when the mirror image of one molecule is not superimposable on that molecule. This is the second type of stereoisomerism which has more subtle 3D relationship among the atoms than is the case in geometric isomerism.
  • The criteria for optical isomerism are as follows
  • The presence of at least one asymmetric carbon is essential
  • The optical rotation is due to asymmetric carbon
  • No superimposing of mirror image
Any carbon atom which is bonded to four different groups is called an asymmetric carbon and is also known as chiral carbon.
The non-superimposable mirror images are called enantiomers or optical isomers and these often arise when a molecule contains a carbon with four different groups attached to the central carbon atom. 

Optical Isomers
The enantiomers of 1 chloro 1 methoxyethane.

Optical isomers are said to exhibit the property of handedness by analogy to the relationship between left and right hands. The mirror images will be identical to a view of left orientation but if placed over one another then cannot be superimposed and hence they are enantiomers.

The old concepts for optical isomerism highlights the dextro isomers, laevo isomers, and meso isomers.

The isomer which rotates the plane of polarised light to clockwise (right) direction is known as dextro isomers. 

Dextro Isomers
The isomers which rotates the plane of polarised light to anticlockwise (left) direction is known as laevo isomers. 

Laevo Isomers
Any compound with two or more asymmetric carbon atoms but have a plane of symmetry is called the meso compound. 

Meso Isomers

Optical Isomerism in Coordination Compounds

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The optical isomerism in coordination compounds was suggested by Van’t Hoff and he specified that three dimensional shape of organic molecules provides an insight into the phenomenon of optical isomerism. He pointed out that many of the compounds which show this phenomenon possess an asymmetric carbon atom. The property of optical isomerism should be associated with any molecule which is dissymmetric by virtue of forming a mirror image not super-imposable. Any dissymmetric structure has no plane or centre of symmetry but does possess certain symmetry axes.

If the metal atom is considered as the centre of the complex then we can simply place the molecules bound to it as the corners of an octahedron. Werner’s theory also derives from complexes of cobalt (III) where the cobalt atom is shown with characteristics coordination number of six. The essential condition for optical isomerism in coordination compounds is the same as for organic molecules or where the complex should lack a plane or centre of symmetry.

Example: a complex of $MX_3ABC$ where three dissimilar ligands A, B, and C occupy the three corners of a triangular face of the octahedron. The number of different ligands needed to produce dissymmetry in complexes is greatly reduced if these are of chelating type.

The nitrogen atoms of one molecule can occupy two coordination positions and form a five membered heterocyclic ring containing the metal atom. Optical isomerism is an important aspect of the chemistry of coordination compounds because ligand interchange often occurs readily in solution and when a pure complex is dissolved the solution may contain a variety of compounds including isomers of the original solid state structure.

The differences between isomers are exist crystallographically, spectroscopically and analytical differences as well. In coordination position isomerism the distribution of ligands between the coordination centres differs and each of these the two cations exists in a number of isomeric forms. These are more evident when the cation and anion of a salt are both complex in nature. 

Coordination Position Isomerism

Optical Isomerism

The two isomers differing in the distribution of ligands between the cation and anion

$[Co (NH_{3})_{6}[Cr (ox)_{3}]$  and $[Cr(NH_{3})_{6}][Co(ox)_{3}]$

The same metal might be become the coordination centre in both cation and anion.

$[Cr (NH_{3})_{6}][Cr (SCN)_{6}]$  and $[Cr(NH_{3})_{4}$ $(SCN)_{2}][Cr(NH_{3})_{2}$ $(SCN)_{4}]$

Optical Isomerism in Complexes

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The optical isomerism in complexes and complex compounds can exhibit the optical activity if they do not possess any centre and plane of symmetry. Optical isomerism is found to exist among complex species having coordination numbers of four or higher numbers.

The transition metal complexes of the penta phenyl cyclopentadienyl ligand have understood to have shown same features but they tend to be rather insoluble and quite likely that the addition of alkyl groups to the phenyl rings would increase the solubility of the complexes formed. If such substitution is not symmetrical then further complication begins and the recent synthesis of the penta phenyl cyclopentadienyl ligand suggest that this may become a much observed ligand. 

Optical Isomerism in Complexes
The cis form and the other ‘trans’ form.

Optical Isomers in Complexes
The cis form and the other ‘trans’ form.

Optical Isomerism Examples

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  • 2 amino propanoic acid compound has amino group attached to the carbon number 2. This is a naturally occurring amino acid where two enantiomers are formed across a mirror line.
Out of the two enantiomers only the clockwise form of alanine is the naturally occurring form. The formula of the compound is $CH_{3} CH (NH_{2}) (COOH)$

The common practice of checking the polarisation of plane for the rest of amino acids, which although have the same group arrangement but only the alkyl group is changed. It’s always better to work only with the active enantiomers. 

  • Lactic acid or 2 hydroxy propanoic acid is again a compound with four different groups attached to the central carbon.
The arrangement ensures two enantiomers. $CH_{3} CH (OH) (COOH)$. Out of the two enantiomers only the clockwise form is taken for all kinds of studies.

  • Another compound that could be considered for optical isomerism is Butanol or more specifically Butan-2-ol.
The compound $CH_{3} CH (OH) (C_{2}H_{5})$ has four different groups attached to the central carbon with hydrogen, methyl, and ethyl and hydroxy attached to central carbon atom. This compound shows two enantiomers of clockwise and anti-clockwise forms of Butan-2-ol. 
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