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Stereoisomerism

In organic chemistry the manner in which molecules are arranged or the spatial relationship these molecules provide gives a lot of ideas about how the physical aspects and few of chemical properties will lead into.

A lot of things go into the stereoisomerism of molecules and that could include the configuration aspect, conformational aspect, the manner in which they are constituted and the overall composition of the molecule. All of these combine to give us an idea of a specific isomerism pattern better known as stereoisomerism.

 

Define Stereoisomers

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This is a type of isomerism where the compounds in question are considered to be stereoisomers if they vary only in their components spatial arrangements. In stereoisomerism the four significant factors which impart the overall impact on the molecule. These are composition or the type of atoms are there in the molecule, constitution or the manner in which the atoms are connected, configuration or the arrangement of the atoms in and around the main chain, and conformation or the manner in which it describes the arrangement of atoms in three dimensional array.

Conformation isomerism is all about how the atoms are rotated around a single bond or a couple of bonds. These can be arranged either in equatorial form or axial form. 

Equatorial BrAxial Br

Configurational isomerism is all about cis and trans isomerism where the functional groups are either attached in same side of the main chain or on either side of the main carbon chain. If the attachment of functional groups are in the same side then we are looking at cis form of isomers. The dipole moment of such molecules are found to be high and hence have higher boiling and melting points than the other form.

In case the functional groups are attached on either side of the main carbon chain then we are looking at trans form of isomer. The dipole moment of such molecules is zero and the boiling point as well as melting point is low. The configuration of any molecule is fixed and cannot be changed as well as it restricts the free rotation.

Trans isomer

Trans Isomer

Cis isomer 

Cis Isomer

Rotation can be restricted even when two adjacent carbon atoms of a chain are attached by a double bond where there is a pi ($\Pi $) and sigma ($\sigma $) bond. This pi ($\Pi $) bond can hold these functional groups in one plane and while they can be either on same side (cis) or opposite side (trans). 

Butene Isomers

Stereoisomerism in Organic Chemistry

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When we consider isomerism, we come across many such molecules where two or more compounds show equal number of atoms with same molecular formula but they differ from each other in both physical as well as in some specific chemical properties and are hence termed as isomers and the phenomenon as isomerism.

Isomerism is of two types and it can either be considered from structural or from the spatial arrangement or simply stereoisomerism. When isomerism is caused by the different arrangement of atoms or groups in space the overall phenomenon is grouped under stereoisomerism. So any compounds which have same structural formula but eventually differ in the configuration then we look at the spatial arrangement or rather the three dimensional arrangement of atoms which characterises a particular compound.

Geometric isomerism which basically a form of stereoisomerism is also called cis-trans isomerism are observed from a restricted rotation about a double bond comprising a pi (p) and sigma (s) bond or may be just around a single bond of a cyclic form. In alkene the geometric isomerism gives an insight how it works in and around the double bond.

The carbon atoms of the carbon-carbon double bond are hybridised in $sp^2$ form. The double bond between the carbon-carbon comprises a pi (p) and sigma (s) bond. The sigma (s) is formed by the $sp^2$ hybrid orbitals overlapping, while the pi (p) bonds are formed by the p orbitals overlapping. The presence of the pi (p) bond restricts the molecule in one position and so C=C bond along with four other atoms which are attached to these carbon atoms remain in one plane and their position gets fixed in the space.

The rotation around the C=C bond is not achievable as this might lead to the breaking of sidewise pi (p) bond. The restricted rotation about the C=C double bonds is the main cause of the geometric isomerism in alkenes.

1-butene

1-butene

Cis-2-butene

Cis-2-butene

Stereoisomerism in Coordination Compounds

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The common geometrical isomers of coordination compound complex are found to be octahedral, tetrahedral and square planar. Usually the geometrical isomers have the same kind of bonding but have different spatial arrangement of the participating atoms. In octahedral coordination complex we can see six coordinates, in tetrahedral coordinate complex there are four coordinates, and finally in the square planar coordinate complex there are overall four coordinates. 

Octahedral Complex

In each of the coordinate complex, each of the bonding is found to be same while the bonds with tapering and dots are shown to represent the three dimensional array structure of the complex. 

Tetrahedral Complex

As each of the bonding is same no isomers could be possible in octahedral coordinate complex which has overall 5 ligands of type and a single ligand of another. 

Square Planar Complex
 
No isomers are visible in case of tetrahedral coordinate complex as well as square planar form as these complexes has overall three ligands of one particular type and a single ligand of another type.

Types of Stereoisomerism

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Stereoisomerism in which the atoms are connected in same order but are different in spatial arrangement is of two types. Diastereomers and enantiomers are considered from image and mirror image point of view. Enantiomers are form of stereo isomeric forms where the images and mirror images are not super imposing.

Diastereomers are basically any stereoisomers that are not enantiomers. These are not related to any image or mirror image stereoisomers. For any molecule which has different orientation can result in quite different properties of molecules. When we consider the chiral object or centre we can identify the mirror image which is different from the original form.

The mirror image cannot be super imposed. Whenever two chiral object interact there is an energetic difference. Any molecule or molecules which are not super imposable are considered to be chiral and if they can be super imposed then are termed as achiral.

Chiral Molecule         Achiral Molecule

We can identify the chirality present in a molecule by just identifying the absence or presence of symmetry. Any molecule having the symmetry plane are not considered as chiral or achiral, while molecules lacking this aspect are bracketed as chiral. But there is an exception as well. Compounds with a centre of symmetry are also considered as achiral.

Stereoisomerism in Carbohydrates

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Sometimes we come across compounds which are achiral but they comprise of chiral carbon atoms. Such compounds are termed as meso compounds. The maximum number of stereoisomers for any compound is calculated as $2^{n}$, where n is the number of chiral atoms. 
Diastereomers
Enantiomers 

Identical Compounds

Although this compound has three stereoisomers in all the chiral atoms are just two. These type of meso compounds that we are observing here are generally formed due to an internal symmetry plane which bisects a minimum of two or more in some cases in symmetry the disposed chiral centres. In order to understand and identify the number of stereoisomers in carbohydrates we use Fischer projection method. The molecule is $[HO_{2} CCH (OH) CH (OH) CH (OH) CO_{2}H]$

As we know the maximum number of stereoisomers for any compound is $2^{n}$, so $2^{3}$ = 8. 

Stereoisomers

The compound shown here has only four stereoisomers, while the stereoisomers 1 and 3 are the meso forms as it follows the internal symmetry plane rule. The stereoisomers 2 and 4 are basically the enantiomers. While the enantiomers are found to have same energy values but diastereomers can have highly varying energy limits. That is why the separation of diastereomers is considerably easier than enantiomers.

Stereoisomerism in Monosaccharides

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Isomerism is for compounds with same molecular formula and different structural form. Monosaccharides show many types of isomerism.
Aldose are monosaccharides bearing an aldehyde H-C=O group at one of the end, which gives the glucose, galactose mannose and ribose forms.
The carbons of a monosaccharide molecule are numbered beginning from aldehyde carbon of an aldose or even from hydroxy-methyl carbon which is next to or closer to ketonyl carbon of a ketose. Ketose are monosaccharides bearing a ketone group (C=O) at a position other than carbon number one.

Similarly for aldose ketose isomerism the existence of an aldose and a ketose in different structural form with same chemical formula shows the difference in spatial arrangement which is why it’s considered under stereoisomerism. 

Glucose or Aldohexoses
Fructose or Ketohexose
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