Different methods used for determination of absolute configuration and correlative methods has been discussed

1. Introduction to methods used for
determination of Absolute
Configuration
Prepared by
Dr. Krishna swamy G
Faculty
DOS & R in Organic Chemistry
Tumkur University

2. Configuration
Three dimensional arrangements of atoms or groups in a molecule with respect to
each other is known as configuration.
If the molecule exhibit isomerism and not rotatable through C-C bond then it leads
to Configurational isomers.

3. Two systems have been developed to study the configuration of organic
compounds.
(1) Absolute Configuration
(2) Relative configuration
(1) Absolute configuration: Precise Three dimensional arrangements of atoms
or groups around an asymmetric carbon atom or chiral centre
a
bc
d
a
b
d
c
Regular Tetrahedron

4. (2) Relative configuration: There are two types
Intramolecular Intermolecular
Applied within the molecule based on
the relative position of two
substituents i.e. same or opposite side
Examples are cis / trans, syn / anti
and erythro/threo
Applied between the molecule based
on the relative position of substituents
around chiral centre.
Examples are R / S, D / L
COOH
H
HOOC
H
COOH
HOOC
H
H
cis trans
Relative configuration

5. If they are achiral, relative configuration is sufficient to describe them uniquely.
If they are chiral and has two chiral centre along with relative configuration, then
absolute configuration is necessary to describe them uniquely.
COOH
H
HOOC
H
COOH
HOOC
H
H
cis trans
Relative configuration


2n
= 22
= 4

6. cis trans
(1S,2R)
(1R,2S) (1S,2S) (1R,2R)
Relative configuration
Absolute configuration

7. Hence for a molecule with multiple chiral centre configuration is determined in
two stages.
1st stage: Determine the relative configuration
2nd stage: Assign absolute configuration at a particular chiral center by suitable
method.
There are three methods are available at present for determination of absolute
configuration.
(1) Method based on optical rotation, Circulardichroism (CD) and Optical
Rotatory Dispersion (ORD).
(2) Method based on anomalous X-ray scattering
(3) Method based on crystals used as probes.

8. Once the configuration of a few molecules are determined by any of the above
methods, many other molecules configuration may be known by correlation.
For correlation generally two groups of methods are known
(1) Chiroptical method- In this correlative method spectroscopic measurements
of chiroptical properties such as optical rotation, CD and ORD are used as an
tool for the elucidation of configuration of chiral compounds.
(2) Non chiroptical method- In this correlative method for configuration
assignment will be done through chemical transformation

9. Types of polarized light

10. When passing linearly polarized light (Lpl) of a given wavelength through a
chiral sample the phenomenon of optical rotation i.e. rotation of the plane
polarized light can be observed.
Two enantiomers generate rotation of equal magnitude but of opposite sign and
therefore can be easily distinguished.
Optical rotation

11. The specific rotation of a compound is calculated using the following formula:

12. Elucidation of configuration of chiral compounds will be done based on optical
rotation by using Empirical rules as follows
 Distance rule
 Rule of shift
 Rule of optical superposition
 Mill’s rule

13. According to Fresnel, Linearly polarized light mathematically and graphically
represented as a superimposition of left and right beams of circularly polarized
light which are enantiomeric to each other.
Optical Rotatory Dispersion (ORD)
Change of specific rotation (or) molar rotation with range of wavelength of light
is ORD.

14. In optically inactive medium, the two light component migrate at the same
velocity, so the resultant light exhibits the properties of Lpl does not change its
orientation i.e. no rotation occurs.

15. If Lpl passes through a chiral sample, both the components lcpl and rcpl are
slowed down relative to prior velocities but to a different extent. Consequently
resulting linearly polarized light appears rotated relative to the incident beam by
certain angle (alpha) which is called optical rotation.


16. There are two types of ORD spectra
(1) Normal plain curves which are characterized by a monotonic increase of
specific rotation while wavelength is decreased.
(2) Anomalous spectra are characterized by monotonic decrease of specific
rotation with increasing wavelength.

17. Normal plain ORD spectra
The plain ORD curve are normally obtained if no selective absorption by the
sample. Examples are hydrocarbons and alcohols.

18. Whereas anomalous behavior can be observed if molecule possesses a chiral
carbon along with chromophore that has a lambda maximum in the Uv / Vis
region.
The anomalous ORD is also described as a Cotton Effect (CE)curve.
A CE is called +ve if the rotation magnitude first increase with a decrease in
wavelength.
A CE is called –ve if the rotation magnitude first decrease with a increase in
wavelength.

21. Circular dichroism (CD) is the differential absorption of left and right circularly
polarized light and is most frequently applied for the assignment of the absolute
stereo structure.
When the linearly polarized light passes an optically active medium in a spectral
region where absorption takes place, left- and right-circularly polarized rays do
not only propagate with different velocities, but they are also absorbed by a chiral
sample to a different extent (i.e., ΑlCpl ≠ ΑrCpl). Thereupon, the incident Lpl is
converted into elliptically polarized light (Epl).
Ellipticaly polarised

22. L - AR
L - R) c l
 c l
 c lw.k.t
 = difference between molar extiction coefficient of left & right

23. The measurement of the ∆ε value as a function of the wavelength λ leads to a
CD spectrum. The maximum of the CD curve coincides with the wavelength of
anomalous ORD crossover, and the sign of COTTON effect in ORD spectrum
corresponds to that of CD.

24. Elucidation of configuration of chiral compounds will be done based on ORD and
CD spectrum by using Empirical rules as follows
 α-Axial halo ketone rule
 Quadrant rule
From this ORD/CD spectra elucidation of absolute configuration is done by
comparison of configuration of known compound with closely related
compounds.

25. When incident x-rays of moderately high energy are either scattered normally
by an atom / absorbed and reemitted at slightly different phase and intensity,
this effect is called anomalous scattering.
When a parallel beam of x-rays strikes, scattering takes place at A & B and two
set of differentiated wave fronts starting exactly with same phase cause
interference patterns at photographic plates p and p’. Since the phase difference
of the two interfering waves depends solely on the distance travelled by them
from both ends, the interference patterns are identical for the enantiomers A-B
and B-A.
A B
P P'
AB
P P'
Anomalous x-ray scattering

26. If one of the nuclei say B is a heavy atom and absorbs partially at the frequency
of x-ray used i.e. secondary wave front scattered from B undergoes a phase lag.
The two ends of the molecule A and B can now be differentiated by the x-ray as
regards their absolute position in space.
In this diagram, the diffracted rays from B has not only travel a longer distance
to reach p’ but has already suffered a phase lag so that the difference in phase
with respect to the rays from A is noticeable.

27. On contrary, the scattered rays from A in the diagram have to travel a longer
distance than the rays from B to reach p but because of an initial lag at B, the
ultimate phase difference when both scattered wave front reach the photographic
plate decrease to that extent.
If the phase lag is expressed in terms of reduced velocities, the dotted line
represents the new path and the two interference pattern at circled p and p’ are
now substantially different. The enantiomers are thus differentiated and an
analysis of the two interference patterns will give their absolute configuration.

28. For reliable determination of absolute configuration requires good quality
crystals and the compound should contain at least one heavy atom (heavier than
Na) for observing an appreciable amount of anomalous scattering.