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STEREOCHEMISTRY

STEREOCHEMISTRY. Learning Objectives. Why Stereochemistry? Stereo isomers Optical Activity – Optical Isomers Optical Rotation Chirality-Chiral atom-Chiral molecules Enantiomers Specifying Configuration of Chiral carbon Racemic Mixture. Stereo isomers. Are we same ?. The Answer is- NO.

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STEREOCHEMISTRY

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  1. STEREOCHEMISTRY

  2. Learning Objectives Why Stereochemistry? Stereo isomers Optical Activity – Optical Isomers Optical Rotation Chirality-Chiral atom-Chiral molecules Enantiomers Specifying Configuration of Chiral carbon Racemic Mixture

  3. Stereo isomers

  4. Are we same? The Answer is- NO

  5. Some objects are not the same as their mirror images. Orientation in 3D space is different , however the connectivity of atoms is same-relative position is different.

  6. Optical Activity-Optical Isomers Biot in early 19th Century discovered Solutions of chiralcompounds rotate plane-polarized light and the molecules are said to be optically active. Plane-polarized light that passes through the solution ofachiralcompounds remain in that plane.

  7. Plane polarized light is rotated in solutions of optically active compounds • Rotation of plane polarized light is measured with polarimeter. • Rotation of plane polarized light is expressed in degrees, is . • Clockwise rotation is called dextrorotatory. • Counterclockwise rotation is called levorotatory.

  8. Optical Rotation • Angle of rotation primarily depends on Concentration(g/mL), temperature, wavelength of light, path length (l in dm) etc. • Optical Rotation , α = [α] .c.l Where [α] is proportionality constant known as specific rotation • [] = observed rotation/(path length x concentration)= /(l x C) = degrees/(dm x g/mL)

  9. Example 1: 200 g of an organic compound was dissolved in 1000 mL of solvent and is placed in 25 cm polarimeter tube where it rotates the plane polarized light by +238˚. Calculate specific rotation of of the sample. • Solution: We know, α = [ α ].c.l Therefore, [α ] = α/ c.l (c=200/1000) = 238/0.2x25 = 47.6˚ ans Can you find a mistake in this?

  10. Question:- If optical rotation of a given sample is +356˚ at concentration c then calculate the optical rotation of the sample when concentration is reduced to half of its initial value.Select the correct option. The options are A) 2˚ B) 4˚ C) 6˚ D) - 4˚

  11. Solution • Since sp. rotation and length of tube remains constant , therefore, α/α’ = c/c’ = c/0.5c = 2 α’ = 356/2 = 178˚ No option has this digit. Can you find the MYSTERY?

  12. Chiral - Chiral Atom - Chiral Molecule • Chiral - Non Superposable Mirror Image • Chiral Atom - An atom with which four different groups are attached. • Chiral Molecule - A molecule having non superposable mirror image.

  13. Chiral Atoms

  14. Chirality and Enantiomers • Chirality arises • Due to absence of symmetry in a molecule • The chiral C is referred to as • A chiral center • A stereogenic center • An asymmetric center • Chirality is a molecular property • Due to presence of a chiral center/Chiral axis/chiral plane

  15. QUESTION:- How many chiral centers are present in the given molecule ? 1. 2. 1 3. 4. 5. 6. 0 2 3 2 2

  16. 7. 8. 2 0 9. 10. 3 4

  17. Identify chiral and achiral molecules in each of the following pair of compounds. (Wedge and Dash representations)

  18. Solution

  19. Enantiomers - Description • Stereo isomers that are related as object and its mirror image. • Chiral molecules can have enantiomers. • Similar physical and chemical properties except their optical rotation.

  20. Examples of Enantiomers • Molecules containing one stereo center have a nonsuperposable mirror image – enantiomer.

  21. Properties of Enantiomers • Identical in every respect • Chemical properties are identical • Spectroscopic properties are identical • Physical properties are identical • Except • Direction of rotation of plane polarized light • Biological properties

  22. Configuration of chiral carbon Absolute Configuration- CIP Rule • The groups attached to chiral carbon are ranked in priority sequence and compared. • The relationship of the groups in priority order in space determines the label applied to the configuration.

  23. Racemic Mixtures • An equi-molar mixture of pair of enantiomers does not rotate the plane polarized light therefore optically inactive. • The process of conversion of enantiomer into a racemic mixture is known as racemisation. • Separation of a Racemic mixture into individual enantiomers is resolution.

  24. Retention Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. Example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid.

  25. Consider the replacement of a group X by Y in the following reaction and identify the configuration: A= Retention B= Inversion A+ B = Racemisation and optically inactive

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