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

The Answer is- NO

Stereoisomer OpticalGeometricalConformational Orientation in 3D space is different, however the connectivity of atoms is same-relative position is different.

Biot in early 19 th Century discovered Solutions of chiral compounds rotate plane- polarized light and the molecules are said to be optically active. Plane-polarized light that passes through the solution of achiral compounds remain in that plane.

 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.

 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)

 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?

The options are A) 2˚ B) 4˚ C) 6˚ D) - 4˚

 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?

 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.

 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

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

Solution

 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.

 Molecules containing one stereo center have a nonsuperposable mirror image – enantiomer.

◦ 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

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.

 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.

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.

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