Fischer's D and L notation Pseudo asymmetric center Cis-Trans isomerism E-Z system

1. STEREOCHEMICAL
ASPECTS
PREPARED BY- MOHAMAD HAIDER
ROLL NO.- 04/MPH/DIPSAR/19
DEPARTMENT OF PHARMACEUTICAL CHEMISTRY, DIPSAR

2. CONTENTS
 Fischer’s D and L notation
 Pseudo asymmetric centers
 Cis-Trans isomerism
 E-Z system

3. Fischer’s D and L notation
 The D-L system corresponds to
the configuration of the molecule: spatial
arrangement of its atoms around the
chirality center.
 D-L system tells us about the relative
configuration of the molecule, compared
to the enantiomers of glyceraldehyde as
the standard compound.

4.  Compounds with the same relative
configuration as (+)-glyceraldehyde are
assigned the D prefix, and those with the
relative configuration of (-)-glyceradehyde
are given the L prefix.
 It only relates the stereochemistry of the
compound with that of glyceraldehyde,
but says nothing about its optical activity.

5.  In assigning the D and L configurations of
sugars, we could directly look for the OH
group of the bottom asymmetric carbon in
the Fischer projection. If it's located on the
right, we designate it with D, and vice
versa.

6. Pseudo Asymmetric Centers
 A pseudo asymmetric centre is found in a
meso molecule (a molecule with multiple
stereocenters that is superimpossible on
its mirror image) where a plane of
symmetry runs through a streogenic
centre and that stereogenic centre’s two
substituents are constitutionally the same
but configurationally different.

7.  Example:
1.
 Other examples:

8. Cis-Trans Isomerism
 Cis-trans isomerism (also known as
geometric isomerism) is a form of
stereoisomerism. These isomers occur
where there is restricted rotation
somewhere in a molecule, generally in
carbon-carbon double bond.
 Cis isomer is an isomer where two of the
same atoms are on the same side of the
double bond in a molecule.

9.  A trans isomer consists of molecule with
two same atoms but here the atoms lie in
the opposite side of the double bond.
 Example: Cis-2-butene has both methyl
groups on the same side of the molecule.
Trans-2-butene has the methyl groups on
opposite sides of the molecule. Their
structural formulas are as follows:

10.  There are two requirements:
1. Rotation must be restricted.
2. There must be two nonidentical
groups on each doubly bonded carbon.
 When there is free rotation about the C–C
bond, the two models represent exactly
the same molecule; they are not isomers.
Two structural formulas that look different
can be drawn, but because of possibility
of this free rotation about single bonds,
these two structures represent the same
molecule:

11.  The presence of a double bond
does not necessarily lead to cis-trans
isomerism as shown:

12. E-Z System
 The traditional system for naming the
geometric isomers of an alkene is to name
them as cis or trans. But if there are three
different groups (or four), then the
cis/trans approach is insufficient to
describe the different isomers.
 For such alkenes E-Z system is use. It is
based on the same set of ‘Priority rules’
on which the R-S is based. These priority
rules are often called the Cahn-Ingold-
Prelog (CIP) rules.

13.  The general strategy of the E-Z system is
to analyze the two groups at each end of
the double bond. At each end, rank the
two groups using the CIP priority rules.
Then, see whether the higher priority
group at one end of the double bond and
the higher priority group at the other end
of the double bond.
 If they are on the same side it will be Z
isomer (Z, from German zusammen =
together) and on opposite sides, E isomer
(E, from German entgegen = opposite).

14. • In simple cases, such as 2-butene, Z
corresponds to cis and E to trans.
However, that is not a rule as indicated
in the following example:

15. THANK YOU