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