DR. PRAVIN S. JOGI M.Sc., NET, Ph.D.

HEAD & ASSISTANT PROFESSOR

DEPARTMENT OF CHEMISTRY

JANATA MAHAVIDYALAYA, CHANDRAPUR

ISOMERISM

Same MF with different chemical

structure……

Eg

C2H6O

C

H

H

H

C O

H

H

H

CH

H

H

OC

H

HH

CLASSIFICATION OF ISOMERS

ISOMERS

STRUCTURAL OR

CONSTITUTIONAL ISOMER

• CHAIN

• POSITIONAL

• FUNCTIONAL

• METAMERS

• TAUTOMERS

STEREOISOMERS

• OPTICAL

• GEOMETRICAL

• CONFORMER

WHY NEED TO STUDT STEREOCHEMISTRY ?

The properties of many drugs depends on

their stereochemistry:

Pt

Cl

Cl

NH3

NH3

CIS

Pt

H3N

Cl

Cl

NH3

TRAN

ANTINEOPLASTIC DRUGSNON-ANTINEOPLASTIC DRUGS

Cl

O

NHCH3

R-KETAMINE

Cl

O

NHCH3

R-KETAMINE

ANASTHETICHALLUCINOGEN

The difference in odor between caraway seeds and mint leaves arises

from two stereoisomers of carvone due to different arrangement of

atoms at the carbon (*)

DIFFERENCE BETWEEN STRUCTURAL AND STEREOISOMER

C

H

H

H

C O

H

H

H C

H

H

H

C O

H

H

H STRUCTURAL

STEREOISOMERSC

COOH

CH3

OHHC

COOH

CH3

HHO

• Tetrahedral carbon, Asymmetric carbon

C

H

HH

HBr

Cl

I

F

C

A

B

C

DC DC

A

B

CONVERTION OF TETRAHEDRAL STRUCTURE INTO FISCHER

PROJECTION FORMULA

C

A

B

DC

OBSERVER

A

B

DC

Horizontal-near to observer

Vertical-away from observer

C OHH

CHO

CH2OH

C

CH2OH

CHO

OHH

C

CHO

HOH2C

H

OH

CONVERTION OF 3D INTO PLANER STRU.

CONFIGURATION

•An absolute configuration refers to the spatial arrangement of

atoms of a chiral molecular entity (or groups) and its

stereochemical description. It is determine by X- ray

crystallography.

C

CHO

CH2OHHHO

C

CHO

CH2OHHOH

Methods of determining relative configuration.

Method-1D ,L- configuration: RELATIVE CONFIGURATION

Method-2R ,S- configuration: ABSOLUTE CONFIGURATION

RELATIVE

CONFIGURATION

C

CH2OH

OH

CHO

H C

CH2OH

H

CHO

HO

(+)-GLYCERALDEHYDE (-)-GLYCERALDEHYDE

D and L Assignments

CHO

H OH

CH2OH

D-(+)-glyceraldehyde

Chapter 6 16

COOH

H2N H

CH2CH2COOH

L-(+)-glutamic acid

*

Penultimate carbon is the stereocenter farthest away from the carbonyl group. If the higher priority group is on the left, then (L), if on the right then (D) sugar.

CHO

H OH

HO H

H OH

H OH

CH2OH

D-(+)-glucose

*

*

*

*

*

What is the meaning of……

1.D-(d)-Glucose

2.L-(d)- Glucose

3.D-(l)-Glucose

4.L-(l)-Glucose

CONFIGURATION AND THE R-S

CONVENTION

Rule 1

The atoms directly attached to the stereogenic center are ranked according to atomic

number. The higher the atomic number, the higher the priority

Rule 2

If a decision cannot be reached with rule 1, work outward from the

stereogenic center until a decision is made. Example of ethyl and methyl

below.

Rule 3

Multiple bonds are treated as if they were an equal number of single bonds.

Which group has the higher priority, isopropyl or vinyl?

Assign the configuration (R or S) to the following enantiomer of 3-methyl-hexane

CCH

3

CH2CH

2Br

H

CH(CH3)

2

CH2

O

O

H

H

Example: Name the following compounds.

C

H

Br

CH2CH

3

CH3

CH3Br

CH3H

CH3H3C

ClHH Br

CLASIFICATION OF STEREOISOMERS

1.OPTICAL ISOMERS

2. GEOMETRICAL ISOMERS

3. CONFORMATIONAL

ISOMERS

Optical activity – When a substance rotates

the plane of plane polarized light. (1815 by

Biot)

Plane polarized light – Light that has been

passed through a nicol prism or other

polarizing medium so that all of the vibrations

are in the same plane.

non-polarized polarized

POLARIMETER – An instrument used to measure optical activity.

light source sample tube

polarizer analyzer

DEXTROROTATORY – when the plane of polarized light

is rotated in a clockwise direction when viewed through

a polarimeter.

(+) or (d) do not confuse with D

LEVOROTATORY – when the plane of polarized light is

rotated in a counter-clockwise direction when viewed

through a polarimeter.

(-) or (l) do not confuse with L

The angle of rotation of plane polarized light by an

optically active substance is proportional to the number

of atoms in the path of the light.

Stereoselective and stereospecific

reactions.* *

CH3CH=CHCH3 + Br2 → CH3CHCHCH3

Br Br

2-butene 2,3-dibromobutane

2 geometric isomers 3 stereoisomers

cis- and trans- (S,S)-, (R,R)-, and (R,S)-

meso-

C C

H

CH3

H3C

H

TRAN OR E 2-BUTENE

CH3

BrH

BrH

CH3

MESO-2,3-DIBROMOBUTANE

Bromiation

Br BrCH3

HBr

HBr

CH3

A reaction in which stereochemically different molecules react differently

is called a stereospecific reaction. In this case the cis- and trans-

stereoisomers give different products.

C C

CH3

H

H3C

H

CIS OR Z 2-BUTENE

CH3

BrH

HBr

CH3

CH3

HBr

BrH

CH3

S,S-2,3- DIBROMOBUTANE

R,R-2,3 -DIBROMOBUTANE

Bromiation

Br Br

PRODUCT IS RACEMIC MIXTURE

HOMOTOPIC ATOMS

C C

H

H

H H

HH

C C

H

H

H Cl

HH

C C

H

H

H H

HCl

H By ClH By Cl

ENANTIOTOPIC ATOM

C C

H3C

CH3

H H

HH

C C

H3C

CH3

H Cl

HH

C C

H3C

CH3

H H

HCl

H By ClH By Cl

R-2-CHLOROBUTANES-2-CHLOROBUTANE

n-BUTANE

PRODUCTS ARE ENANTIOMERS OF EACH OTHER

C C

H

HH3C

H

H by ClH by Cl

C C

H

ClH3C

H

C C

Cl

HH3C

H

PRODUCTS ARE DIASTEREOMERS OF EACH OTHER

DIASTEREOTOPIC ATOM

THANK

YOU