Optical Isomerism- Reema

Optical Isomerism

1. OPTICAL ISOMERISM In the study of carbon compounds there are large number of compounds in which not only the number but also the nature of atoms constituting the molecules is same and yet these compounds differ from each other in their physical and chemical properties. Such compounds are called as isomer s and the phenomenon is called Isomerism

2. STRUCTURAL ISOMERISM STERO ISOMERISM CONFIGURATIONAL ISOMERISM CONFORMATIONAL ISOMERISM OPTICAL ISOMERISM GEOMETRICAL ISOMERISM

3. STEREOISOMERISM Stereoisomers have the same structural arrangement of atoms (same molecular formula, same atomic bonding), but they differ as to how these atoms or groups of atoms are oriented in space Stereoisomerism is of two types: 1.Conformational isomerism Stereoisomers that interconvert rapidly at room temperature about the sigma bond are called conformational isomers.

4. 2. Configurational isomerism Stereoisomers that have identical constitutional structures i.e., the component atoms or group of atoms are arranged in the same order, but they differ as to how these atoms are oriented in space are called configurational isomers. These isomers do not readily interconvert and thus can be separated.

5. Polarimeter The instrument that measures the rotation of plane polarized light is called a polarimeter It consists of a light source (usually sodium lamp), monochromater filter, two Nicol prisms and sample tube (polarimeter tube).

6. The magnitude of the observed rotation of an optically active compound depends upon a number of factors 1. Nature of optically active compound. 2. Concentration of the optically active compound in the solution. 3.Path length of the sample cell (polarimeter tube) through which the light passes 4. Nature of solvent in which the optically active compound is dissolved. 5. Wavelength of the monochromatic light. 6. Temperature at which the rotation is recorded. If the concentration of the sample solution was doubled (the number of molecules of sample which the light beam encounters is doubled) then the observed rotation would be twice the original rotation. Chemists have evolved a new term called specific rotation To correlate the relationship between the path length, concentration and observed rotation.

7. Specific rotation : It is defined as the number of degrees of rotation observed when light is passed through an optically active compound placed in a tube having a path length of decimeter (10cm) and concentration of l gram per milliliter. It is dependent on the temperature and wavelength of light used [ α ]= α /cl Where [ α ] =specific rotation α = observed rotation C=concentration of the sample l =length of the polarimeter tube Lousis Pasteur is considered to be founder of field of stereo chemistry.

8. ENANTIOMERISM According to van't Hoff's fundamental hypothesis of stereochemistry, when four different atoms or groups bonded to carbon are imagined to be directed towards the four corners of a regular tetrahedron, at the center of which lies the carbon atom itself, such a carbon was described as asymmetric and is usually indicated by an asterisk (C*). This asymmetric carbon atom also known as chiral carbon is now known as chirality center. Two different structural arrangements are possible for such a carbon compound. Consider the example of lactic acid .( α -hydroxypropionic acid).

9. Such optically active stereoisomers, which are non-superimposable mirror image of each other, are classified as enantiomers or enantiomorphs. Molecules that exist as non-superimposable mirror images of each other are chiral (ky-ral)or dissymmetric. Chirality is one of the essential and foremost condition required for the existence of enantiomer i.e., if a compound whose molecules are chiral then they can exist as enantiomers or vice-versa. Just as any object has only one mirror image, similarly any chiral molecule will have only one enantiomer

10. Chiral The word chiral is derived from the Greek word cheir meaning 'hand', referring to the ' handness ‘ of the molecule. Each hand is therefore chiral and exists as a pair of enantiomers. Some other examples of chiral are human feet, gloves, shoes etc. Molecules or objects that are superimposable on their image are achiral. Objects like table, chairs, spoon, fork etc. all look like the same in the mirror, hence achiral.

11. Consider the example of lactic acid in which the hydroxyl group is replaced by another similar group-say methyl (isobutyric acid) Chirality A chiral centre is an atom bonded tetrahedrally to four different atoms or groups. A chiral centre is usually a carbon atom but may be N, P, S, Si etc. A chiral centre is usually indicated with an asterisk(*)

12. Stereogenic centre or stereo centre is defined as an atom bearing groups of such nature that interchange of any two groups spatially will produce an stereo isomer(enantiomer or diastereomer). Diastereomer 1.Consider 2-bromo-3-chlorobutanoic acid it as 2 asymmetrical carbon atoms Therefore 2 2 =4 optically active forms are possible. CH 3 CH 3 CH 3 CH 3 H Cl H H Cl Cl H H Br Br H Br H H Br COOH COOH COOH COOH I II III IV 2.Stereo isomers which are optical isomers but not mirror images are called as Diastereomer. C l

13. Properties of Diastereomer They have different physical properties such as M.P, B.P, density, solubility etc. They can be separated from each other but techniques like fractional distillation fractional crystallization chromatography etc. Erythro and threo enantiomers Stereoisomers I &II are pare of enantiomers similarly III&IV are also enantiomers. The pairs of enantiomers with similar groups on the opposite sides of the carbon chain are called as threoenantiomers. III&IV The pair of enantiomers with similar groups on the same side of the carbon chain are called as erythroenantiomers I & II.

14. Elements of Symmetry There are four types : 1. Simple or proper axis of symmetry. 2. Plane of symmetry.

15. 3.Centre of symmetry. 4.Alternating axes of symmetry or improper axis.

16. Enantiomers physical and chemical properties: 1. Identical physical property but differ in the direction of plane polarized light but extent of rotation is same. 2. They have identical chemical properties except towards optically active reagents. 3.When equal amount of enantiomers are mixed together an optically in active mixture known as racemic mixture denoted by ( ±) Or dl is obtained. Meso compounds : Molecules which are optically inactive even though they contain stereogenic carbons are known as meso compounds.

17. METHODS OF REPRESENTING CHIRAL MOLECULES Common methods of representing molecules are: Wedge and dashed wedge formula or the perspective formula: In this representation a solid wedge depicts a bond projecting from the paper towards the viewer A broken or dashed wedge depicts a bond projecting away from the viewer A simple or continuous line represent a bond that lines in the plane of the paper. Example: bromochloroiodomethane compare it with ball and stickmodel.

18. 2.Fisher projection formula It is not always convenient to draw three-dimensional formulas of molecules as the number of chiral center increases. Scientist developed a two dimensional projection formula called Fisher Projection formulas for carbohydrates and amino acids, which is nowadays being used to represent many chiral molecules

19. 3.Newman projection formula: This involves viewing the molecule along the carbon-carbon bond. The front carbon is represented by a point and the three lines emanating from it in the form of ‘y’ or λ represent the atoms/groups bonded to it. The back carbon is represented by a circle and the three atoms/groups bonded to it by three equally spaced radii, which emanate from the circumference of the circle.

20. 4 . Sawhorse projection formula Another simple method for representing the three-dimensional formula on paper. The molecule is viewed slightly from above and sideways. The bond between the two carbon atoms is represented by a diagonal line, which is slightly elongated. The left hand bottom end carbon is considered to be towards the viewer, while the right hand top end carbon is away from the viewer. Resolution of racemic mixture Since the two enantiomers in a racemic mixture have identical physical and chemical properties these can not be separated by usual methods like fractional distillation ore fractional crystallization. The separation of (+) and(-) components is termed as resolution. Following methods are used for separation: 1.Mechanical separation. 2.Biochemical Method 3.Chemical method.

21. Walden inversion: 1.Inversion of configuration is the conversion of a molecule into another which has the opposite relative configuration. 2.When an atom or a group directly linked to an asymmetric carbon atom is replaced. 3.The reaction may procced with the inversion of configuration. 4.The phenomenon was first observed by walden and hence such a reaction is known as walden inversion. E.g.: (+)-malic acid can be converted to(-)- malic acid by treating it with PCl 5 followed by AgOH.