STEREOCHEMISTRY
Why Stereochemistry?Stereo isomersOptical Activity – Optical IsomersOptical RotationChirality-Chiral atom-Chiral moleculesEnantiomersSpecifying Configuration of Chiral carbonRacemic Mixture
Learning Objectives
Stereo isomers
Are we same?
The Answer is- NO
Some objects are not the same as their mirror images.
Stereoisomer
Optical Geometrical Conformational
Orientation in 3D space is different , however the connectivity of atoms is same-relative position is different.
Biot in early 19th 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.
Optical Activity-Optical Isomers
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)
Optical Rotation
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?
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.
The options areA) 2˚ B) 4˚ C) 6˚ D) - 4˚
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.
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?
Solution
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.
Chiral - Chiral Atom - Chiral Molecule
Chiral Atoms
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
Chirality and Enantiomers
1. 2. 1
3. 4.
5. 6.
QUESTION:- How many chiral centers are present in the given molecule ? 0
3 2
2 2
9. 10.
7. 8.20
43
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.
Enantiomers - Description
Molecules containing one stereo center have a nonsuperposable mirror image – enantiomer.
Examples 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
Properties of Enantiomers
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.
Configuration of chiral carbon• R• S
Absolute Configuratio
n• D• L
Relative Configuratio
n
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.
Racemic Mixtures
RetentionRetention 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-olis 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
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