![• Specific rotation [α], is the rotation caused by a sample at a
concentration of 1.0 g/mL in a sample tube of length 1.0 dm
(10.0 cm). The value of specific rotation depends on the
concentration of sample, its structure, wavelength of light
source, temperature, length of the sample tube, and solvent.
• The temperature t (in oC) and wavelength λ of light source are
indicated as superscript and subscript respectively while writing
the specific rotation.](https://image.slidesharecdn.com/isomerism-1-200409061411/75/Isomerism-PART-1-Optical-Isomerism-26-2048.jpg)
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Isomerism PART-1 (Optical Isomerism)
This document discusses stereochemistry and isomerism. It defines constitutional and stereoisomers, and describes different types of constitutional isomers like chain, position, functional, and tautomeric isomers. It also discusses configurational isomerism including optical isomers like enantiomers and diastereomers. Chirality and chiral centers are explained. Methods to represent 3D structures in 2D like Fischer projections are introduced. The document also covers topics like optical activity, polarimetry and racemic mixtures.
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Isomerism PART-1 (Optical Isomerism)
- 1.
- 2. ISOMERISM • The compoundshaving same molecular formula are called isomers. • two compounds show different properties even if they have same atoms or groups present and have same covalent bonding. • Bond connectivity. It describes the sequence in which different atoms or groups are bonded to each other in a molecule. • The isomers that differ in their bond connectivity are known as constitutional isomers/ structural isomer. • study of three-dimensional nature (spatial arrangement) of molecules is known as stereochemistry.
- 4. Constitutional Isomers • Constitutionalisomers have same molecular formula but different bond connectivity, that is, the sequence in which atoms are bonded in a molecule, is different. • Chain isomers • Position isomers • Functional isomers • Metamers • Tautomers. • Chain isomers: These isomers differ in the way the carbon atoms are bonded to each other in a carbon chain. • Eg: Butane and 2-methylpropane (C4H10)
- 5. • Position isomers.These isomers differ in the position of functional group in a carbon chain Functional isomers. These isomers have same molecular formula but they differ in the nature of functional groups Eg: alcohols and ethers with same molecular formula
- 6. • Metamers. Theseisomers have same functional group but they differ in the arrangement of alkyl groups around the functional group.
- 7. Tautomers. These arethe two forms of same compound, which arise due to migration of a hydrogen atom in a compound. The two forms are readily inter-convertible and exist in dynamic equilibrium with each other. This phenomenon is known as tautomerism.
- 8. • Examples ofstereochemistry • (–) Adrenaline has more hormonal activity than (+) adrenaline. • (–) Nicotine is more poisonous than (+) nicotine. • One stereoisomer of Limonene has smell of lemon while the other smells like oranges.
- 9. CONFIGURATIONAL ISOMERISM: Thecarbon is tetravalent in nature and the non-planar tetrahedral geometry in carbon compounds gave birth to the concept of stereochemistry. The arrangement of different groups or atoms in space is termed as configuration. Stereoisomers :These differ from structural isomers, in that structural isomers have different structural formulas, while stereoisomers have the same general structural formula but different orientations in space.
- 10. • There aredifferent kind of stereoisomers: • a. Optical isomers (e.g. enantiomers). • b. Geometrical isomers (cis, trans isomers). • c. Conformational isomers (conformers). (Diastereomers) • Optical isomerism: Optical isomers are named like this because of their effect on plane polarized light. Light is a wave motion that contains oscillating electric and magnetic fields. The electric field of ordinary light oscillates in all planes. However, it is possible to obtain light with an electric field that oscillates in only one plane. such light is called plane polarized light or simply polarized light.
- 11. Enantiomers: They areoptical isomers, with two stereoisomers that are related to each other by reflection. They are mirror image of each other that are non super imposable. • Compounds that are enatiomers of each other have same physical properties, except for the direction in which they rotate polarized light. Diastereomers: The stereoisomers of a compound, which are not mirror images of each other are termed as diastereomers. The diasteromers exist only when a compound has two or more stereogenic centres. • Conformational isomers (conformers): Its a form of isomerism that describe the phenomenon of molecules with the same structural formula but with different shapes due to rotation about one or more bonds.
- 12. CHIRALITY • The term‘chiral’ is taken from Greek word cheir meaning handedness. • An object that is not superimposable on its mirror image is called chiral and is unsymmetrical while the object that is superimposable (exactly overlaps its image) on its mirror image is called achiral and is symmetrical.
- 13. CHIRALITY: ENANTIOMERS ANDDIASTEREOISOMERS • A molecules in which carbon is attached to four different groups or atoms is said to be Chiral in nature. • The carbon is said as stereogenic center. Chiral molcule dont have plane of symmetry. These molecules are optically active. • A molecule with one stereogenic centre always exists in enantiomeric forms.
- 14. • Meso compounds.A compound having stereogenic centres but optically inactive (achiral), due to presence of a plane of symmetry, is termed as meso compound or a meso form. • In meso compounds the plane divides the molecule into halves that are mirror images of each other. This internal compensation makes meso compounds optically inactive. The achiral molecule CH2BrCl has a plane of symmetry, but the chiral molecule CHBrClF does not.
- 15. Fischer Projection Representation of3-D Structures in 2-D • The three dimensional structure of carbon compounds is due to their tetrahedral geometry. • A Fischer projection is represented by an additive cross (+), where horizontal line represents the bonds above the plane and vertical line represents the bonds below the plane.
- 16. Writing the Fischerprojection: Step 1. The structure of a compound, whose Fischer projection is to be written, is first written in a vertical manner. The carbon with the lowest rank, as per IUPAC nomenclature, is written on the top. Step 2. The chiral carbon in the structure is assumed to lie in the plane of the paper while the groups attached vertically and horizontaly to chiral carbon are assumed to lie below and above the plane of paper, respectively.
- 17. For example, forLactic acid CH3CH(OH)COOH, the Fischer projections 2-Hydroxypropanal
- 18. Fischer projection of2-bromo-3-chlorobutane- compound with more than one chiral centre. Step 3. A Fischer projection can be rotated through 180° in the plane of the paper since this does not change the configuration, that is, spatial arrangement of groups around chiral carbon. A rotation by 180° does not involve the interchange between horizontal and vertical lines
- 19. Number of Stereoisomersof a Compound: The number of stereoisomers possible for a compound is related to the number of chiral centres (n) present in it as Number of stereoisomers = 2n Where n is the number of stereogenic or chiral centre(s) present in the compound. Eg: Lactic acid CH3CH(OH)COOH with one chiral centre, possible stereoisomers (21 = 2),
- 20. Eg: 2-bromo-3-chlorobutane whichhas two chiral centres. stereoisomers (22 = 4) Lactic acid 2-bromo-3-chlorobutane
- 21. Threo and Erythroprefixes: Fischer projection of a stereoisomer having two different chiral centres (may or may not be on the adjacent carbon atoms), if two similar groups or atoms are on the same side of carbon chain, the isomer is designated as erythro. On the other hand, if two similar groups or atoms are on opposite sides of the carbon chain, the isomer is designated as threo.
- 22. OPTICAL ACTIVITY: Ordinarylight consists of light waves vibrating in all planes. When passed through a ‘Polaroid lens’ or ‘Nicol prism’ light moves in one plane only and is called plane- polarized light (PPL). An unsymmetrical or chiral compound, placed in the path of plane-polarized light, will rotate the plane of light. The compounds exhibiting such a behaviour are termed optically active.
- 23. A compound rotatingthe plane of polarization towards right (clockwise) is termed dextrorotatory (+), whereas a compound rotating the plane of polarization towards left is termed laevorotatory (–). The optical activity is measured in the laboratory by polarimeter. Polarimeter consists of a light source (usually D–line of sodium), two polaroid lenses (one polarizer and one analyzer), and a sample tube placed between polarizer and analyzer lenses. If the sample tube is empty and polarizing axes of two lenses are parallel, the intensity of light reaching the observer is ‘maximum’.
- 24. • Once thesolution of sample is placed in the tube, the emerging light is observed from the analyzing lens. If there is no change in the intensity of light, the sample is said to be optically inactive. • If the sample rotates the plane of polarization, the intensity of the light observed through analyzer lens is found to decrease and the sample is said to be optically active. • The analyzer lens is then rotated to observe the maximum intensity of light. • If the rotation of the plane, towards right, the sample is dextrorotatory, if rotation is towards left, laevorotatory.
- 26. • Specific rotation[α], is the rotation caused by a sample at a concentration of 1.0 g/mL in a sample tube of length 1.0 dm (10.0 cm). The value of specific rotation depends on the concentration of sample, its structure, wavelength of light source, temperature, length of the sample tube, and solvent. • The temperature t (in oC) and wavelength λ of light source are indicated as superscript and subscript respectively while writing the specific rotation.
- 27. Racemic mixture: Amixture containing equal amounts of a pair of enantiomers is termed as racemic mixture (also called racemic modification). A racemic mixture is optically inactive. As the enantiomers exhibit same specific rotation in opposite directions, the specific rotation values of two isomers cancel each other, resulting in zero optical activity. The racemic mixture is denoted by (+). For example, (+) lactic acid.