1 / 27

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.

deacon-holmes
Download Presentation

STEREOCHEMISTRY

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related