Stereoisomerism is defined as “ isomerism in
compounds having equal number of same
types of bonds with identical connectivity ,
but differing in the spatial arrangement of
Stereo isomers are divided into two classes
When a substance rotates the plane of plane polarized light.
Plane polarized light:-
Light that has been passed through nicol prism or other
polarizing medium so that all of the vibration are in the same
polarimeter – an instrument used to measure optical activity.
light source sample tube
• There are many molecules with two or more stereogenic centers that have
• Every diastereomer can have a mirror image, which constitutes two stereoisomers.
• A molecule with two stereogenic centers will have up to two diastereomers and each
will have a mirror image. This means there is a maximum total of four stereoisomers
for a molecule with two stereogenic centers.
• This observation can be extended to all molecules that have stereogenic centers. For
a given number of stereogenic centers (say n) there will be a maximum of 2n
• A molecule with two stereogenic centers has 22 or 4 stereoisomers.
• A molecule with 4 stereogenic centers will have a maximum of 24 or 16
• If a molecule has 9 stereogenic centers, the maximum number of stereoisomers will
be 29 or 512 stereoisomers.
• A molecule with 28 stereogenic centers; 228 means 2.684 x 108 stereoisomers (that's
more than 268 million stereoisomers) for one constitutional isomer of a single
Enantiomers: Mirror-image stereoisomers.
Diastereomers: Stereoisomers which are not
I is a stereoisomer of III & IV.
II is a stereoisomer of III & IV.
I is not a mirror image of III or IV.
II is not a mirror image of III or IV.
I is a diastereomer of III & IV.
II is a diastereomer of III & IV.
Diastereomers often have similar, but not identical,
chemical properties. They have different physical
properties, and can be separated from one another
ordinary physical means.
+ A method of representing stereogenic centers without
using wedged or dashed bonds.
Horizontal bonds come from the stereogenic center
Vertical bonds go from the stereogenic center away from
R And S Configurations From Fischer
To determine the R,S configuration of each.
• interchange pairs of groups around the stereogenic center of one
until both projections are identical.
• Since each exchange inverts the configuration anodd number of
exchanges means the configurationswere opposite; an even
number means they are the same.
“Racemization is the process by which 50 % of an
optically active compound ( dextro or levo) may
be converted to its enantiomer.”
Method Of Racemization
Consider an optically active halide. The process of racemization would
involve the removal of halide ion from the sp3 carbon. This is possible
in a solvent which can stabilize the carbocation.
Resolution Of Racemic Mixture:
“The separation of enantiomers from
racemic mixture is called resolution of
Following special methods have been used for
separation of enantiomers from racemic mixture:
1. Mechanical method
2. Biological method
3. Chemical method
4. Chromatographic method