Here is a presentation of Isomerism (Optical Isomerism) as per the Pharmacy PCI syllabus.

1. ISOMERISM-2
ABSOLUTE AND RELATIVE
CONFIGURATION
Dr Akhil Nagar
RCP-IPER
Shirpur

2. ABSOLUTE CONFIGURATION (R AND S
CONFIGURATION): configuration to stereoisomers is
important and configuration to (+) and (–) isomers, sign of optical
rotation. The actual configuration of the molecules, that is,
absolute configuration is assigned by a set of sequence rules CIP
rule. These sequence rules give nomenclature of stereoisomers in
terms of R and S configurations.
Sequence rules:
Rule 1. Write the stereoisomer in terms of Fischer projection
formula.
Rule 2. Identify the atoms attached to chiral carbon, Now assign
priority order in accordance with their atomic numbers.

3. The chiral carbon is attached to –CH3, –NH2, –OH, and an H atom. The
groups –CH3, –NH2, and –OH are attached through C, N, and O atoms
respectively. The atom with the highest atomic number is assigned the
highest priority (1) and the atom with lowest atomic number is assigned
the lowest priority (4).

4. Rule 3. If in a molecule different groups are attached to chiral
centre through similar atoms, priority order cannot be assigned on
the basis of atomic number of the atoms directly attached to chiral
centre. In such case, next set of atoms with higher atomic number
present in groups, and priority is assigned at the first point of
difference in accordance with the atomic number.

5. Rule 4. In case of groups having multiple bond, for assigning
priority order a multiple bond (double or triple) is considered to
be bonded to an equivalent number of similar atoms through
single covalent bonds.

6. Assigning R and S Configuration: According to Sequence rules, (1–4),
priority order is assigned to different groups and atoms attached to chiral
carbon. The group or atom with least priority is placed at a position
vertically downward in the Fischer projection. Then the arrangement of
rest of the three groups or atoms is considered according to their
decreasing priority order.

7. Relative Configuration (D- and L- Nomenclature): The term relative
configuration is used when a molecule is assigned configuration with
respect to glyceraldehydes.
In glyceraldehydes if –OH
group is present on the right
hand side, it is assigned D
configuration

8. If –OH group is on the left
hand side, it is assigned L-
configuration
Compounds prepared from or converted into D-glyceraldehydes
belong to D-series. Similarly, compounds prepared from or
converted into L-glyceraldehydes belong to the L-series.

9. CHIRALITY IN A MOLECULE WITH NO STEREOGENIC (CHIRAL)
CENTRE
The most common cause of asymmetry in a molecule is presence
of an asymmetric carbon (stereocentre). The molecules with one
stereocentre exist in two steroisomeric forms, which are optically
active.
Tartaric acid (a molecules with two stereocentres), meso structure
exhibits a plane of symmetry and does not show optically activity.
There are compounds where stereocentre (chiral carbon) is not
present yet they exhibit optical activity. These molecules possess
asymmetry for entirely different reasons.
Eg: Allene and Biphenyls.
Allenes, CH2=C=CH2, the two double bonds are present on the
same carbon.

10. The central carbon atom is sp hybridized and linear, while the two outer
carbons are sp2 hybridized and trigonal.
For effective overlap the p orbitals of terminal CH2 groups will
overlap with p orbitals of central atom. Since the two p orbitals on
central carbon are mutually perpendicular it clearly indicates that p
orbital on each terminal CH2 are perpendicular

11. In substituted biphenyl, the two phenyl rings are not present in same
plane. The substituents present adjacent to bond joining two benzene
rings restrict the rotation around this bond and the molecule is said to
be conformationally locked. The molecules are highly strained and
cannot achieve symmetry because of the steric hindrance or strained
structure. Thus, the substituted biphenyls exist in enantiomeric forms,
which are optically active.