ORGANIC

TRANSFORMATIONS

Presented by: Zia Ur

Rehman

STEREOCHEMISTRY

Chemistry In Three Dimensions

ISOMERS

Different compounds having same molecular formula but

different structures.

Structural Isomers / Constitutional Isomers:

Distinguishable by two dimensional structure

The sequence in which the atoms are bonded to each other in the molecule.

e.g. C4H10

butane C C C C

H

H

H

H

H

H

H

H

H

H

n-butane

C

CH3

H3C

H

CH3

iso-butane

Stereoisomers:

Distinguishable by three dimensional structure

Isomers that differ in the way the atoms are oriented in space, but not in which atoms are bonded to which atoms.

e.g. cis-2-butene trans-2-butene

HC C

H3C CH3

H HC C

H3C H

CH3

Conformational Isomers

Stereoisomers which are interconvertible simply by rotation of one part of the molecule with respect to other about a single bond.

Eclipsed conformation:C-H bond opposite to each other

Staggered conformation:C-H bonds at maximum distance

C

H

H

H

C

H

H H

EclipsedC

H

H

HC

H

H H

Staggered

Stereoisomers which are not interconvertible without breaking and making the bonds, and can be isolated

Configurational Isomers

A chiral object is that which has non superimposable mirror image.

Chirality

An atom with four different groups attached

to it, is called chiral atom and also calledchiral center.

Chiral Center

Any object that has a plane or point of symmetry is achiral (not chiral).

An atom with four different groups are not attached to it, is called achiral atom.

Achiral

C

H

Cl

H

CH3

Enantiomers are stereoisomers that are non-superimposable on their mirror images.

Enantiomers

Diastereomers are stereoisomers that are not mirror images of each other – they are stereoisomers that are not enantiomers.

Diastereomers

C C

H

CH3

H3C

H

C C

CH3

H

H3C

H

The property of enantiomers to rotate the plane polarized light

towards left or right.

Compounds that rotate plane polarized light to the right

(clockwise) are called dextrorotatory, (d) or (+).

compounds that rotate plane polarized light to the left

(counter-clockwise) are called levorotatory (l) or (-).

Optical Activity

Light that has been passed through a nicol prism or other polarizing medium so that all of the vibrations are in the same plane.

plane polarized light

non-polarized polarized

Polarimeter

It is used to measure thr optical activity of a compund.

Specific Rotation

The angle of rotation of plane polarized light by a 1.00 gram per cm-3 sample in a 1 dm tube. [α ]D (D = sodium lamp, λ = 589 mμ).

α[ α ]D = where α = observed rotation l * d

l = length (dm)

d = concentration (g/cc)

(+)-alanine [ α ]D = +8.5

(-)-lactic acid [α ]D = -3.8

Drawing Enantiomers

Fischer Projection:

Newman Projection:

R,S-System

The atom of higher atomic number has the higher priority.

1

2 3

4

(S)-2-bromobutane

Erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite side.

Erythro,Threo-System

Syn describes groups on the “same” face while anti describes groups on “opposite” face.

Syn,Anti-System

If the two groups with the higher priorities are on the same side of the double bond, that is described as the (Z)- isomer.

If the two groups with the higher priorities are on opposite sides of the double bond, then this is the (E)- isomer.

E,Z-System

Contains chiral center but their mirror images are non superimposeable.

Meso Compound

Cl HCH3

Cl HCH3

H ClCH3

H ClCH3

An equimolar mixture of a pair of enantiomers is called racemate or racemic mixture.

It is denoted by symbol ( ).

Racemic Mixture

+-

C COOH

H3C

HCl

C COOH

H3C

ClH

+

+_ - -Chloropropanoic acid

A stereoselective reaction can produce multiple stereoisomers theoretically, but more of some produced than others.

Stereoselective Reaction

Prominent

A stereospecific reaction produces different stereoisomer products from different stereoisomer reactants.

Stereospecific Reaction

Since most of the natural (biological) environment consists of enantiomeric molecules (amino acids,nucleosides, carbohydrates and phospholipids are chiral molecules), then enantiomers will display different properties. Then, in our body:

Biological Significance of Chirality

Food Technology

D-Amino acid is purposeful in fermented products such as

cheeses, yoghurt, wine or viniger.

“L” form of glutaminic acid is the carrier of taste such as

meat, brothy or umami taste.

In plant essential oils

In fruit flavor

Pharmaceuticals

Polymers Chiral stationary phases in high performance liquid

chromatography (HPLC).

Asymmetric catalysis applications.

Chiral liquid crystals in ferroelectrics and nonlinear optical

devices (cholesteric liquid-crystalline oligomers).

Very soluble, optically active, potentially ion exchangeable, and

thermally stable chiral polymers are synthesized (poly(hydra-zide-

imide)s containing ethyl L-lysine ester).

Pressure-Sensitive Touch Panel Based on Piezoelectric Poly(L-

lactic acid) Film.