the topic contain basic about of dissymmetry and the chiroptical properties.

1. STEREOCHEM

2. CONTENTS
Molecular dissymmetry and chiroptical properties:
• Linearly and circularly polarised light.
• Circular birefringence and circular dichroism.
• ORD and CD curves.
• Cotton effect and its applications.
• Octant rule and the axial α-haloketone rule with
applications.

3. Molecular dissymmetry and chiroptical properties:

4. SYMMETRY
• In order to study the symmetry of a molecule
certain operations such as rotation and
reflection are performed
• By doing so we get an arrangement
• These are symmetry operations which posses
element of symmetry

5. Symmetric molecules
• Any element of
symmetry present
in the molecule
Asymmetric molecules
• Does not have any
element of
symmetry present
Dissymmetric molecules
• No plane and no
centre of
symmetry
• May or may not
have axis of
symmetry
• Necessary for
optical activity

6. • Plane polarised light passing through medium
• Optical rotation
• Optical rotation of enantiomers

7. Linearly and circularly polarised light

8. Linearly polarised light
• Electric field vector changes its magnitude
sinusoidal in a plane along the direction of
propagation

9. Circularly polarised light
• Magnitude of vector remains constant but its
direction changes

11. • ER = EL ( RCP = LCP same
amplitude)
• The vector of LP wave (E)
at A is maximum (ER + EL)
• Decreases at B (ER and EL
rotates in opposite
direction)
• Becomes zero at ER and EL
oppositely directed
• Negativity increases at B’
• Negative maximum at A’
• The vector oscillates in a
plane xy (projection line-
ABOB’A’) along x-axis and
y-axis being the line of
propagation.

12. • CP ray is generated from
two concurrent LP waves
of equal magnitude with
plane of polarisation at
right angle to each other
and has phase difference
of /2
• OA when LPv is
maximum, LPH is nil
• OB when LPV and LPH
has intermediate values
• OC when LPv is nil, LPH is
maximum
• Thus describes an helix.

13. Optical activity has two important phenomena
Circular birefringence and circular dichroism
• Circular dichroism and circular birefringence are caused by the asymmetry of the
molecular structure of matter. The optical activity of solutions of biological
macromolecules provides information about the structural properties of the
macromolecules.
nl  nr
Causes optical rotation
kl  kr
Causes elliptically polarised ray

14. Specific rotation
• It is defined as the observed angle of optical
rotation α when plane-polarized light is passed
through a sample with a path length of 1 decimeter
and a sample concentration of 1 gram per 1
milliliter.
• Where, α –Angle of Rotation in degrees l –path
length is in decimeters d – Density of liquid is in g
100 ⁄ ml 1 T– Temperature.
• α is positive the medium is dextrorotatory
• α is negative the medium is levorotatory

15. Circular Birefringence
• Some material poses a
special property: the
two circularly polarised
component has
different refractive
index
• Er and El no longer
oscillates in orginal
plane AOA’ but BOB’
inclined at angle α

16. Calculation
• The angle of rotation is given by α =
• the specific rotation([α]T
λ) =
• Molecular rotation [φ]T
λ =

17. Circular dichroism
• Some material poses a special property: they
absorbs two circularly polarised component to
different extents.

18. • If absorption LCP>RCP (i.e.
Kl> Kr)
• New position are OEl and
OEr with resultant vector
as OB
• The flatten of helix whose
projection of plane is an
ellipse
• Major axis AOA’
• Minor axis COC’

19. Calculation
• The ellipticity is defiened by an angle 𝛹 (in radian) so that tan𝛹 represents
the ratio of minor to major axis of the ellipse.
For small difference of Kl and Kr, 𝛹 = ¼ (Kl-Kr) ……… (1)
• Specific epllipticity of medium [𝛹]T
λ = ……………….. (2)
• Relation of absorption coefficient (K) and molecular extinction coefficient (e)
I = Io x e-kl= Io x 10-C’l …………………….(3)
Io – initial intensity of light
I – intensity after travelling through length l and C’ refers to concentration in gmol/l
• K = 2.303 x  x C’ = 2.303 x  x C x 1000/M …………………………(4)
• Specific ellipticity in terms of molecular extinction coefficient
Combining equations (1) (2) (3) and (4),
[𝛹]T
λ = (l - r)
• Molecular ellipticity []λ
T is expressed in same way as molecular rotation of
a medium by introducing a factor M/100
[]λ
T = (l - r) = 3300 x Δ

20. ORD and CD curves

21. ORD is a technique in which optical activity is due to
rate of change of specific rotation with wavelength of
light

22. • With improvement in instrumental technique
one can now measure molecular ellipticity or
differential dichric absorption and plot [] or
Δ versus wavelength to give CD spectrum
• ORD and CD curves are equivalent and
respective to their implication in
stereochemistry; like ORD,CD can also be
positive and negative

23. application
• Use of plain curve
• CD and ORD curves with cotton effect
1. Functional group analysis
2. Position of a functional group
3. Determination of configuration.

24. Octant rule and the axial α-haloketone rule with applications.

25. The axial α-haloketone rule
Rule applies when there is an axial halogen next
to the keto group of a cyclohexanone moeity
Carbonyl at the head of the chair closet to the
observer; if the halogen appears at
• Right – the compound shows strong positive
cotton effect
• Left – the compound shows strong negative
cotton effect

26. • Alternate way is to project the molecule into four
quadrant

27. Applications
Determination of absolute configuration of (-)-trans-2-decalone (R=H)
On bromination it gives (+)-trans-2-bromo-1-decalone
with Br at axial position (R=Br)
For R=Br, XIa give positive cotton effect and XIb gives
negative cotton effect
The configuration of (-)-trans-1-decalone corresponds
to XIa.

28. Octant rule
• Sign of cotton effect of chiral cyclohexanone
derivatives with their absolute configuration
• The space around carbonyl group is divided
into eight sectors (octant)with the help of
orthogonal planes about x,y,z axis
• This rule only applies to substituted
cyclohexanone

30. Contribution of the substituent in different sector towards the
cotton effect
• Substituents lying in the co-ordinate planes make
no contribution
• Substituents lying on the back upper left and
back lower right, then octant make positive
contribution. (Positive rotation).
• Substituents lying on the back upper right and
back lower left then the octants make negative
contribution. (Negative rotation).
• Substituents at C2 axial and C6 axial contribute
more than C2 equatorial and C6 equatorial

31. Applications
• Conformation
(+)-3-methylcyclohexanone (the ketone shows
positive cotton effect by the accordance with
principles of conformational analysis)
According to octant rule, XVa gives positive cotton
effect and XVb gives negative cotton effect

32. • Configuration
Trans-10-methyl-2-decalone
The absolute configuration of (+)-enantiomer shows
positive cotton effect
The mirror image would show negative cotton effect.

33. • Steriodal ketones

34. References
• D. Nasipuri. Stereochemistry of organic
compounds: Principles and applications. New
age international limited, Publishers.