Stereochemistry of organic compounds-i

Stereochemistry• Stereochemistry, a subdiscipline of chemistry,

involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. An important branch of stereochemistry is the study of chiral molecules.

• Stereochemistry is also known as 3D chemistry because the prefix "stereo-" means "three-dimensionality".

• The study of stereochemistry focuses on stereoisomers and spans the entire spectrum of organic, inorganic, biological, physical and especially supramolecular chemistry.

Isomerism

The concept of isomerism is an important feature of the study of organic compounds. It relates to the existence of different compounds which have the same molecular formula but different properties.

Such compounds are called isomers. The difference in properties of isomers is due to the difference in the relative arrangements of atoms

in their molecules.

There are two types of isomerism:• Structural isomerism

• Space or stereo isomerism

Structural isomerism

Structural isomerism, or constitutional isomerism (per IUPAC), is a form of isomerism in which molecules with the same

molecular formula have bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for

constitutional isomers.

There are various types of structural isomerism: Chain isomerism: In chain isomerism, or skeletal isomerism, components

of the (usually carbon) skeleton are distinctly re-ordered to create different structures. Pentane exists as three isomers: n-pentane (often called simply "pentane"), isopentane (2-methylbutane) and neopentane (dimethylpropane).

position isomerism (regioisomerism): In position isomerism a functional group or other substituent changes position on a parent structure.

Functional isomers are structural isomers that have the same molecular formula (that is, the same number of atoms of the same elements), but the atoms are connected in different ways so that the groupings are dissimilar. These groups of atoms are called functional groups, functionalities. Another way to say this is that two compounds with the same molecular formula, but different functional groups, are functional isomers.

Metamerism: This type of is due to the difference in the distribution of carbon atoms on either side of functional group in compounds having the same molecular formula.

Tautomerism: This is a special type of functional isomerism where the isomers exist simultaneously in dynamic equilibrium with each other. The isomers involved in equilibrium are called tautomers.

Stereoisomerism

The isomeric compounds which differ from each other only in the relative arrangement of atoms in space but resemble one another with respect to which atoms are linked to which other atoms are

called stereoisomers and the phenomenon is known as stereoisomerism.

There are three types of stereoisomerism:

• Optical isomerism

• Geometrical isomerism

Optical isomerism: In this the isomeric compounds resemble one another in their chemical reactions and most of the physical properties but differ in their effect on polarized light.

Molecular chirality

• A molecule is considered chiral if there exists another molecule that is of identical composition, but which is arranged in a non-superposable mirror image. The presence of an asymmetric carbon atom is often the feature that causes chirality in molecules.

Types of optical isomerism 1. Enantiomerism 2. Diastereomerism

Enantiomerism : An enantiomer is one of the two stereoisomers that are mirror images of each other that are non-super imposable and which rotate the plane polarized light equally but in opposite directions. The phenomenon of mirror image isomerism is called enantiomerism.

Fundamental condition of enantiomerism

Enantiomeric molecules are always non-super imposable mirror images of each other. The non-super imposability of mirror images invariably arises due to chiral nature of the molecules. A molecule is termed as chiral if it has no plane or centre of symmetry and is therefore, non-super imposable on its mirror image. Therefore, chirality is the fundamental and only condition of enantiomerism.

Characteristics of enantiomers

1. They have identical physical properties such as m.p, b.p, densities, solubility, refractive index.

2. They have identical chemical properties except in reactions with other optically active compounds.

3. They have different biological properties.4. When equal quantities of enantiomers are mixed together it

results in the formation of an optically inactive form called racemate.

Representation of stereoisomersThe flying wedge formulae

Diatereoisomerism

• Diastereomers (sometimes called diastereoisomers) are a type of a stereoisomer. Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other. When two diastereoisomers differ from each other at only one stereocenter they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two.

• Diastereomers differ from enantiomers in that the latter are pairs of stereoisomers that differ in all stereocenters and are therefore mirror images of one another.

Enantiomers of a compound with more than one stereocenter are also Diastereomers of the other stereoisomers of that compound that are not their mirror image. Diastereomers have different physical properties (unlike enantiomers) and different chemical reactivity.

Characteristics of Diastereomers:1. They show similar but not identical chemical properties.

2. They have different physical properties such as m.p, b.p, density, solubility, refractive indices.

3. On account of different physical properties, they can be rather easily separated through fractional crystallization, fractional distillation, chromatography.

Erythro and Threo isomers • If similar groups are on same side in the fischer projection formula,

the diastereomer is known as erythro isomer.• If the two similar groups are on the opposite sides in the projection

formula, the diastereomer is known as threo isomer.

Meso compound

A Meso compound or Meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more chiral centers it is not chiral. A Meso compound is "super posable" on its mirror image.

Racemization

• In chemistry, Racemization refers to the conversion of an enantiomerically pure mixture (one where only one enantiomer is present) into a mixture where more than one of the enantiomers are present. If the racemization results in a mixture where the enantiomers are present in equal quantities, the resulting sample is described as racemic or a racemate.

• Racemization can be brought about by one of the following:

By action of heat By action of chemical reagents Autoracemisation

Comparison of internal and external compensation

External compensation This is due to mirror image

relationship between structures of the molecules of two enantiomers mixed together.

In this the molecule of one enantiomer cancels the rotation of the other enantiomer.

The racemate forms is a mixture of two isomeric compounds.

It is a reversible process and the racemate mixture can be resolved into enantiomers.

Internal compensation

This is due to mirror image relationship between the structures of two halves of the same molecule.

One half of the molecule counter balances the rotation of the other half.

Meso or internally compensated compound represent a single pure compound.

It is a permanent effect and it is not possible to resolve into optically active forms.

Rectus and sinister configuration

• Identify each of the chiral centers (most commonly an sp3 C with 4 different groups attached)

• Then at each chiral center.... Assign the priority (high = 1 to low = 4) to each group attached to the chirality center based on atomic number.

• Reposition the molecule so that the lowest priority group is away from you as if you were looking along the C-(4) σ bond. If you are using a model, grasp the lowest priority group in your fist.

• Determine the relative direction of the priority order of the three higher priority groups (1 to 2 to 3)

• If this is clockwise then it is the R-stereoisomer (Latin; rectus = right handed) 

• If this is counter-clockwise then it is the S-stereoisomer (Latin; sinister = left handed)

Assignment

Explain the following terms: 1. polarised light 2.optical activity 3.specific rotation 4.structural isomerism 5.enantiomers 6.diastereomers 7.meso compounds

What is resolution? Give two methods for this purpose with examples.

What is meant by internal and external compensation? How do they differ from each other?

With the help of suitable examples, explain the terms threo and erythro isomers.

Explain what is Walden inversion? Explain necessary and sufficient condition of optical activity?

Cis-alkene has higher boiling point than trans- alkene. Why? Write short note on isomerism exhibited by maleic and

fumaric acid?