1. Stereochemistry & stereoisomers
Mr. Sonaji V. Gayakwad
Asst. professor
Dept of chemistry
Mrs.K.S.K. College,Beed

2. Stereochemistry The Arrangement of Atoms in Space or
three dimensional structure of atoms.
Stereoisomerism: is one aspect of stereochemistry.
Isomers are compounds that have the same molecular formula but with different structures.
There are two main classes of isomers:
1- Structural isomers (or constitutional isomers)
2- Stereo-isomers

4. I-Constitutional isomers:
These are compounds whose atoms are connected differently . Different connections among atoms which may be due to difference in: A- Skeleton of carbon or B- Functional groups or C- Position of functional groups
I-Constitutional isomers: 4

6. These are compounds whose atoms are connected in the same order but with different geometry or arrangements. Types of Stereoisomers are: A- Optical isomers (e.g. enantiomers & configurational diastereomers) B- Geometric isomers or Cis &trans isomers or cis &trans stereomers (both in alkenes and cycloalkanes)
II-Stereoisomers: 6

8. Enantiomers :
These are non-superimposable mirror image stereoisomers.
Diastereomers: Steroisomers which are not enantiomers are called diastereomers.
A- configurational diastereomers:
These are non-superimposable non-mirror image stereomers.
B- cis-trans diastereomers:
These contain substituents on same side or opposite side of double bond or ring (cyclic structure).

9. Cis-trans diastereomers
Isomers
constitutional
isomers
stereoisomers
Optical isomers
or Enantiomers
Diastereomers:
Configurational &
Cis-trans diastereomers 8

10. Optical Isomerisms Optical Isomerisms:
It manifests itself by its effect on plane-polarized light.
Polarizers are used to produce plane-polarized light (e.g. polaroid film, nicol prism).
Optical Activity
Any compound that has the ability to change the direction of plane polarized light or to rotate it, is said to be optically active compound.
Optical isomers are optically active substances.
The rotation itself is called optical activity. 10

11. The diffrence between ordinary or plane-polarized light:
A beam of ordinary light is vibrating in all possible planes perpendicularly, but
Plane-polarized light is vibrating in only one of these possible planes. 11

12. Measurement of optical activity
Polarimeters are used to measure the optical activities

13. Measurement of optical activity
Plane polarized light passing through an optically active solution is rotated by a certain number of degrees alpha (α) called the “observed rotation”.
If α found to be to the right (clockwise rotation), the optically active compound is designated as dextro-rotatory with the symbol (+).
If α found to be to the left (counter clockwise rotation), is termed levo-rototatory with the symbol (-). 13

14. The observed rotation (α) depends upon:
The concentration of the solution (C)
The length of the polarimeter tube (L)
The temperature (T)
The wavelength of the light (λ)

15. Optical activity and structure of compounds

16. Optical activity and structure of compounds
Chiral carbon atoms
4 different substituents on carbon, then it is no longer superimposable on its mirror image and we say that carbon is chiral .
Carbon with 1,2,3 different atoms or groups attached can be superimposed on its mirror image and is achiral.
Optical activity and structure of compounds

17. A molecule is chiral if two mirror image forms are not superimposable upon
one another.
A molecule is achiral if its two mirror image forms are superimosable
The chiral centre is usually indicated by an asterisk (*)
A molecule with a single chiral carbon must be chiral
But, a molecule with two or more chiral carbons may be chiral or it may not. 3

18. Bromochlorofluoromethane is chiral
It cannot be superimposed point for point on its mirror image. Cl Br H F 4

19. Bromochlorofluoromethane is chiralCl Cl Br Br H H F F
To show nonimsuperposability, rotate this model 180° around a vertical axis. 4

20. a carbon atom with four different groups attached to it also called: w
The chiral carbon atom
a carbon atom with four different groups attached to it
also called:
chiral center; chiral carbon asymmetric center
asymmetric carbon stereocenter
stereogenic center w x y z C 12

21. Chirality and chiral carbons
A molecule with a single stereogenic center is chiral.
2-Butanol is an example. H C
CH3
CH2CH3 OH 13

22. Examples of molecules with 1 chiral carbon
CH2CH3
CH2CH2CH2CH3
CH3CH2CH2
one chiral alkane 13

23. Examples of molecules with 1 chiral carbonOH
Linalool, a naturally occurring chiral alcohol 13

24. Examples of molecules with 1 chiral carbon
CHCH3
1,2-Epoxypropane: a chiral carbon
can be part of a ring
attached to the chiral carbon are: —H
—CH3
—OCH2
—CH2O 13

25. Enantiomers rotate light in equal amounts in opposite directions.
(+) Dextrorotatory (Latin dexter is "right")
(-) Levrorotatory (Latin levus is "left")
A mixture consisting of equal parts of any pair of enantiomers is called a racemic mixture (or racemic modification) and is designated by (+/-).
A racemic mixture does not rotate plane-polarized light because (+)-rotation caused by one enantiomer is canceled by rotation in the opposite direction by the (-)-enantiomer. A solution of a racemic mixture of enantiomers is optically inactive. 25

26. In racemic mixtures of drugs, the better fitting enantiomer is called the eutomer (Eu) while the lower affinity enantiomer is called the distomer (Dist).
In racemic mixtures of drugs, the distomer should be viewed as an impurity comprising 50% of the mixture. An impurity that is by no means inert. Several implications of racemic drug treatment should be considered:
Side effects
Antagonist
Metabolized to unfavorable metabolite
Metabolized into a toxic metabolite

27. (Cahn-Ingold-Prelog R/S system)
Assignning Absolute Configuration
(R) & (S) Configuration
(Cahn-Ingold-Prelog R/S system)
In the R,S system, groups are assigned priority using the Cahn-Ingold-Prelog system just as in the E,Z system for naming alkenes.
To assign (R) or (S) configuration to a chiral carbon:
1. Rank the 4 atoms (groups) attached to the carbon .
2. Project the molecule so that the group (atom) of lowest priority is to the rear.
The most probable atoms used are:
H=1, C=6, N=7, O=8, F=9, S=16, Cl=17, Br=35
Br> Cl> S> F>O >N> C> H
3. Select the group (atom) of highest priority and draw a curved arrow toward the group (atom) of next lowest priority. (assign priority in order of decreasing atomic number).
4. Clockwise orientation (arrow direction) is R. Counterclockwise arrow direction is S. 27

28. A compound with n chiral carbon atoms can have a maximum of 2n stereoisomers.
Example: a compound has 2 chiral carbons and 22 (= 4) stereoisomers. A compound with 3 chiral carbons and 23 (= 8) stereoisomers.
Diastereomers
Stereoisomers which are not mirror-image isomers are called diastereomers. Diastereomers have different chemical and physical properties.
Diastereoomers:
Possess > 1 chiral center
Inversion of 1 chiral center produces a compound that is not a mirror image

29. I-Assigning priorities and determining R or S of compounds containing one chiral carbon

30. I-Assigning priorities and determining R or S of compounds containing one chiral acrbon
It can then be done in the conventional manner. You should note, however, that in the drawing below, connecting the priorities in the original Fischer projection gives the same rotation as in the drawing on the right (both are S). This method will always work if the lowest priority group is oriented either up or down on your Fischer projection.

31. If the groups are oriented improperly in the original drawing, the Fischer can be “rearranged” using the following set of rules:
1-Exchanging any two groups around a Fischer projection ("one exchange") generates the enantiomer of the original compound, and
2-Exchanging groups twice ("two exchanges") regenerates the original stereochemistry.

32. In the example shown above, the original molecule (R configuration) is re-drawn with two of the groups "exchanged" so that the hydrogen (the lowest priority group) is placed in the "top" position; this new molecule now has S configuration.
The second exchange regenerates the original R configuration.
A third exchange would again generate S, a fourth, R, etc.
An example of converting a drawing into a Fischer, and using it to assign configuration is shown below:

33. II-Assigning priorities and determining R or S of compounds containing multiple chiral carbons

34. For compounds with multiple chiral centers, written as extended Fischer projections, assignments can be made in the same manner, as shown below.
In the following compound,

35. The top carbon is R, and, rearranging the bottom carbon,
Enantiomer
The top carbon is R, and, rearranging the bottom carbon,
Identical

36. III-The two molecules shown below are identical at one center, and identical at the other, and are therefore identical.

37. Practice Problems

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