Stereochemistry of organic compounds

Isomerism Isomerism is the phenomenon whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organizations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry.

Structural Isomers have different structural formulae because their atoms are linked together in different ways. It arises owing to: arrangement of Carbon skeleton e.g. The formula C4H10 represents two possible structural formulae, butane and methylpropane:

Position isomerism- e.g. propan-1-ol and propan-2-ol

Functional Isomerism e.g. the molecular formula C2H60 represents both ethanol and methoxymethane.

Ring chain isomerism Cyclic alkanes are isomeric with alkenes, e.g. cyclopropane and propene

Stereoisomers Stereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not superimposable. There are two types: 1) Geometric Isomerism e.g. cis-but-2-ene and trans-but-2-ene

2) Optical isomerism -Involves an atom, usually carbon, bonded to four different atoms or groups of atoms. They exist in pairs, in which one isomer is the mirror image of the other. e.g. butan-2-ol

CHIRAL STRUCTURES In spite of their many similarities, there is a fundamental difference between a pair of hands that can be observed by trying to place your right hand into a left-hand glove. Your hands have two important properties: (1) each hand is the mirror image of the other, and (2) these mirror images are not superimposable. The mirror image of the left hand looks like the right hand, and vice versa, as shown in the figure below.

A very large number of organic molecules such as 2-chlorobutane and lactic acid are known to have chiral structures as they are not superimposable on their mirror images.

Achiral structures A molecule is achiral if it is superimposable on its mirror image. Most achiral molecules do have a plane of symmetry or a center of symmetry. Achiral molecules that contain a stereocenter are called meso. The molecules discussed in the previous section are achiral because they possess either a plane of symmetry or a center of symmetry.

Enantiomers Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are non-superimposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule. Chiral molecules with one or more stereocenters can be enantiomers. It is sometimes difficult to determine whether or not two molecules are enantiomers. For introductory purposes, simple molecules will be used as examples. More complex examples will be given later.

Diastereomers Diastereomers are stereoisomers that are not mirror images of one another and are non-superimposable on one another. Stereoisomers with two or more stereocenters can be diastereomers. It is sometimes difficult to determine whether or not two molecules are diastereomers. For introductory purposes, simple molecules will be used as examples

DIFFERENCE BETWEEN DIASTEREOMERS AND ENANTIOMERS? 1) Diastereomers of a molecule are not mirror images of each other, 1) enantiomers are mirror images. 2) Molecules with more than one stereocenter can be Diastereomers if they are not mirror images of each other. 2) If there is only one stereo center, then that molecule has enantiomers. 3) Diastereomers have different physical and chemical properties 3) enantiomers have similar physical and chemical properties except, they have different optical properties towards the plane polarized light.

Erythro and Threo Terms used for diastereomers with two adjacent chiral Carbons, without symmetric ends. For symmetric molecules, use meso or d,l

Diffrence between internal and external compensation Internal compensation External compensation 1)This is due to the mirror-image relationship between the structures of the molecules of two enantiomers mixed together. 1)This is due to the mirror-image relationship between the structures of the two halves of the same molecules 2) The racemate formed is a mixture of two isomeric compounds. 2) Meso or internally compensated compound represents a single pure compound. 3) It is possible to have complete or partial external compensation of rotation so as to cause complete or partial racemisation. 3) Partial internal compensation is not possible. 4) It is reversible process and the racemic mixture can be resolved into (+) and (-) enantiomers. 4) It is permanent effect and it is not possible to resolve an internally compensated molecule into optically active forms.