This document discusses optical rotatory dispersion (ORD), which is defined as the rate of change of specific rotation with a change in wavelength. ORD is used to determine the structures of carbonyl compounds. Key points covered include: the principles of plane polarized light, optical activity, specific rotation, and circular birefringence; factors that affect specific rotation; Cotton effects and curves; and the octant rule for establishing absolute configuration from ORD data.

1. BY
JAGADEESH BABU
M.PHARM

2.  LIGHT:
 It is an electromagnetic radiation consists of both
vibrating electric and magnetic vectors perpendicular to
each other & having the wavelength of 4000-7700
angstroms.
 Study of light is called Optics
 WAVE LENGTH:
 The distance between successive crests or peaks of a
wave. It is denoted by “ λ ”.
 CHROMOPHORE:
 An atom or group whose presence is responsible for the
colour of a compound

3. OPTICAL ROTATORY DISPERSION (ORD)
 DEFINITION:
 Rate of change of specific rotation with change in
wave length is called as ORD.
 It is used for structural determination of carbonyl
compounds.
 FUNDAMENTAL PRINCIPLES OF ORD:
 Plane polarized light.
 Optical activity
 Specific rotation.
 Circular Birefrengence.
 Optical Rotation.

4.  Light from ordinary lamp consists of waves vibrating in
many different planes.
 When it is passed through polaroid lense it is found to
vibrate in one plane is said to be plane polarised light
or polarised light.

5.  The compounds which are having the ability to
rotate the plane of polarised light are called
optically active compounds.
 This property of compound is called optical
activity.
 It is measured by Polarimeter.
 Compound which rotates plane of polarised light
to right(clock wise) is called
DEXTROROTATORY & is denoted by (+) sign.

6.  If the compound rotates plane of polarised light towards left
side (anti clock wise) is called LEVOROTATORY & is denoted
by (-) sign.
 Image of Polarimeter .

7.  When a ray of monochromatic polarized light
strikes a solution, several phenomenon’s
occurs like –
 1. Reflection on the surface.
 2. Refraction.
 3. Rotation of plane polarization
 4. Absorption.

8.  Enantiomers are optically active compounds.
 Optically active molecules have different
refractive indices, and different extinction
coefficients for L and R circularly polarised light.

9.  For a compound to be optically active it must be
devoid of the following properties –
 1. Plane of symmetry (σ)
 2. Center of symmetry (i)
 3. Alternating rotation – reflection axis of
symmetryor An improper axis ( s )

10. 1. PLANE OF SYMMETRY -: (σ)
It is the plane, which divides a molecule such
that one half forms the mirror images of other
Ex: Mesotartaric acid
2. CENTER OF SYMMETRY -: (i)
It is the point in the compound from which a
line drawn from one side & extended equally in
the opposite side reaches the same group.
Ex: Diketo dimethyl piperazine

11. 3. ALTERNATING ROTATION –
REFLECTION AXIS OF SYMMETRY OR AN
IMPROPER AXIS ( S ):-
A molecule possess n-fold-alternating axis
of symmetry, if then rotated through an
angle 3600/n about the axis followed by a
reflection in a plane perpendicular to the
axis, the molecule is same as original one.
Ex. 1, 2, 3, 4 – tetramethyl cyclobutane.
The isomer that rotates the plane polarized
light to the left is called levoisomer (-) & to
the right is called as dextroisomer (+)

12. According to Fresnel, a plane polarized light may be
considered as the combination of two circularly polarized
light of which one is right circularly polarized light (RCPL)
& other is left circularly polarized light (LCPL) which are in
equal & opposite in nature.
A circularly polarized light (CPL) is one whose plane of
polarization rotates continuously & in the same sense
around the axis of the polarization of the wave & it may be
described as right handed screw or helix twisting around
the direction of propagation, where LCPL wave describe the
left handed screw.

13. THE FIGURE BELOW REPRESENTS HOW THE
ELECTRIC VECTOR OF RCPL (ER) & THAT OF LCPL
(EL) COMBINED TO GIVE A PLANE POLARIZED
WAVE (E)
E
E1 ER
Plane of polarization RCPL+LCPL = PPL

15. If Refractive index is same for two circularly
polarized light then it vibrate in opposite direction
with the same angular velocity.
In the below image we can observe the change of E

16. SPECIFIC ROTATION
• It is defined as the rotation produced by a solution
of length 10cm & unit concentration (1gm/ml) for
given wavelength of light at the given temperature.
It is denoted by α.
IT DEPENDES ON VARIOUS FACTORS LIKE:
• Nature of substance.
• Length of the column.
• Conc. of the sol.
• Temp of the sol.
• Nature of the solvent.
• Wavelength of the light used.

17.  If two equal & opposite beams of CPL & PPL
passes through an optically active compound it
result in characteristic phenomenon is called
Circular Birefringence.

18.  When a plane polarized light(PPL)t is passed
through optically active compound due to it’s
Circular Birefrigence results in unequal rate of
propagation of left & right circularly polarized
rays.
 This unequal rate of propagation of both left &
right circularly polarized light deviates the PPL
from it’s original direction & it is called as
OPTICAL ROTATION.
 Optical rotation caused by compound changed
with wavelength of light was first noted by Biot in
1817.

19.  Circular Dichroism occurs when the two
circularly polarized components of PPL are
absorbed differentially.
 When the component emerges out there is
an imbalance in their strength & the
resultant two will not be linearly polarized
but elliptically polarized & this
phenomenon is called as CIRCULAR
DICHROISM

20. CIRCULAR DICHROISM

21. COTTON CURVES
COTTON EFFECT:
 French physicist discovered it & he described it as,
The combination of circular dichroism and circular
birefringence is known as cotton effect. Which may
be studied by observing the change of optical
rotation with the wavelength so called ORD & the
curves obtained are called as cotton curves.
Djerassi & Klyne suggested that rotatory dispersion
curves (i.e. plot of optical rotation against
wavelength.) should be classified in to two main
types.
 1. Plain curves
 2. Cotton effect curves.

22.  These are normal or plain curves.
 These curves occurs at absorption maximum.
 The curves obtained do not contain any peak or
inflections and that the curve do not cross the
zero rotation line.
 These curves obtained for compounds which
don’t have any absorption in wavelength where
optically active compounds are examined.
 Ex: Hydrocarbons, Alcohols etc.,

23. • According to compound rotation these curves
are again divided into +ve & -ve curves.

24. ANOMALOUS CURVES
• These curves will show the high peaks & troughs
which depends on the absorbing groups. So they
are called as Anomalous dispersion of optical
rotation.
• These curves will obtain for the compounds which
are having asymmetric carbon & chromophore
which absorbs near UV region.
• These are again divided into 2 types, they are
1. Single cotton effect curves &
2. Multiple cotton effect curves.

25. SINGLE COTTON EFFECT CURVES
• These single cotton curves will show both
maximum & minimum curves at maximum
absorption.
• From the longer wavelength towards the
cotton effect region if crest passes first
through it then the trough it is called as +ve
cotton effect.
• In the same manner if it happens in
opposite way it is called as –ve cotton effect.

26.  The vertical distance between the crest and
trough is called as amplitude “a” and it is
expressed in degrees.
 Molecular amplitude, a = Ф1 – Ф2 / 100
 Ф2 = molar rotation of extreme peak or
trough from large wavelength
 Ф1 = molar rotation of trough or peak from
shorter wavelength

28. MULTIPLE COTTON EFFECT CURVES
• These are a little different from the single
cotton effect curves.
• Here more than two crests & troughs are
obtained .
• Ex: camphor etc.,

29. OCTANT RULE
• It is an empirical rule that establishes an
absolute configuration or stereochemistry
from the sign & intensity of cotton effect.
• It was developed by Djerrasi & coworkers.
• It relates the sign & amplitude of cotton
effect exhibited by an optically active
saturated ketone to the spatial orientation
of atoms about carbonyl functional group.

30. (+) (-)
(-) (+)
• The substituents lying in the co-ordinate
planes make no contribution to the rotatory
dispersion.
• Substituents lying top left & down right
make +ve contribution.
• Substitution lying top right & down left
makes –ve contribution.

31.  Prediction of cotton effect.
 Recognition of optical activity.
 Study of conformational mobility
 Structural elucidation.
 Qualitative analysis.
 Quantitative analysis

32.  Organic spectroscopy by William kemp 3rd edition
page no 279-280.
 Instrumental method of Chemical Analysis by
Chatwal G.R. and Anand S.K.
 Instrumental method of Analysis by
 Willard H.H.,Merritt L.L., Dean J.A., Settle F.A.,
6th edition.
 Instrumental methods of Chemical Analysis by
B.K. Sharma.
 Internet