This document discusses optical rotatory dispersion (ORD), which refers to how the optical rotation of a compound changes with the wavelength of light. ORD curves can provide structural information about compounds, particularly those containing carbonyl groups. ORD is based on how left and right circularly polarized light travels through a sample at different speeds. Related phenomena include circular dichroism and the Cotton effect. ORD and CD measurements can provide information about functional group positions, absolute configurations, and conformational mobility in compounds.
2. Introduction -:
• ORD refers to the change in optical rotation with the change in
wavelength of light source. i.e. applied only in optically active compounds.
• Optical rotation caused by compound changed with wavelength of light
was first noted by Biot in 1817.
• ORD curves in recent years are made use in structural determination by
comparing the curve obtain from compound believed to have related
structures particularly applied to carbonyl compounds.
E.g.. ORD curves have been used to locate the position of carbonyl groups in
steroid molecules.
3.
4. Optical Rotation
RL nn
1-
cmradrotation
n refractive index
wavelength of light
angle of rotation
Usually reported as a specific rotation
[], measured at a particular T,
concentration and (normally 589;
the Na D-line)
Molar rotation [] = []MW10-2
mL100
g
c
decimetersinpathlength
lc
α10
α
2
l
5.
d'c'
α
α
• Concentration of an optically active substance, c’, expressed in
g cm-1 (as density of a pure substance)
• d’ = thickness of the sample in decimeters
''
10α
10α
2
2
dc
M
MM
• M = molecular weight of the optically active component
• the 10-2 factor is subject to convention and is not always included in [M]
9. For a compound to be optically active it must be devoid of the following properties –
1. Plane of symmetry.
2. Center of symmetry.
3. Alternating rotation – reflection axis of symmetry.
• Rotation of plane polarized light (FRESNEL’S EXPLANATION) -:
• 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.
10. The figure below represents how the electric vector of RCPL (ER) & that of
LCPL(EL) combined to give a plane polarized wave (E)
E
El ER
RCPL + LCPL = PPL
Plane of polarization
11. Zero resultant
The two circularly polarized light vibrate in opposite direction with same angular velocity
if refractive index is same.
12. The angle of rotation per unit path length is,
α = π / λ (ηL – ηR )
If RCPL travels faster α is positive & the medium is dextro rotatory,
where as if LCPL travels faster then α is negative & the medium is levo rotatory.
Specific rotation (Rotatory power) -:
It is the rotation produced by a solution in 10 cm length tube having 1 gm of
substance in 100 ml.
[]λ=100 /LC,
where C= gms/100ml, L= Length of polarimeter tube
13. CIRCULAR DICHROISM -:
• Whenever circular dichroism occurs the two circularly polarized components
of PPL are absorbed differentially i.e. one is absorbed more intensively than the
other.
• So when the component emerges out there is imbalance in there strength &
the resulted two will not be linearly polarized but elliptically polarized this
phenomenon is known as circular dichroism.
•The combination of circular birefringence & circular dichroism gives cotton
effect
14. Linear polarized light can be viewed as (a) superposition of opposite circular
polarized light of equal amplitude and phase. (b): different absorption of the left-
and right hand polarized component leads to ellipticity (CD) and optical rotation
(OR).
15.
l
AA RL
4
303.2
cmrad 1-
: ellipticity
l : path length through the sample
A : absorption
16. • Measurement of how an optically
active compound absorbs right-
and left-handed circularly polarized
light
• All optically active compounds ex-
hibit CD in the region of the
appropriate absorption band
• CD is plotted as l-r vs
• For CD, the resulting transmitted
radiation is not plane-polarized but
elliptically polarized
• is therefore the angle between
the initial plane of polarization and
the major axis of the ellipse of the
resultant transmitted light
• ’ approximates the ellipticity
• When expressed in degrees, ’ can
be converted to a specific ellipticity
[] or a molar ellipticity []
• CD is usually plotted as []
kd
o
rl
rl
IIfromk
c
kk
dichroismcircularmolar
10
2
10θyellipticitmolar
dc'
yellipticitspecific
M
•CD plots are Gaussian rather than S -
shaped.
•Positive or negative deflections
depend on the sign of or [] and
corresponds to the sign of the Cotton
effect
•Maximum of the CD occurs at the
absorption max
17. ORD and CD
COTTON EFFECT:
• The combination of circular dichroism and circular
birefringence(circular rotation) is known as cotton effect, which may
be studied by observing the change of optical rotation with the
wavelength so called ORD. It was discovered by a French physicist A.
COTTON.
• Cotton discovered a relation between RP and light absorption in
optically active compounds. As one approaches certain optically
active absorption bands in a compound from long wavelength side the
rotatory at first increases strongly then falls off and changes sign. This
is known as Cotton effect and the curves describing such effect is
called Cotton effect curves.
• The curves obtained by plotting optical rotation v/s wavelength down
to about 350nm using photoelectric spectro-polarimeters, known as
ORD curves or Cotton effects.
18. They are of two types:
1. Plain curves
2. Anamalous curves
(a) Single cotton effect curves
(b) Multiple cotton effect curves
Plain curves (Normal smooth curves or single curves)
Cotton effect is not seen for compounds which absorbs in far UV well below
220nm, because it occurs only near to absorption maximum of the compounds.
The curves obtained do not contain any peak or inflections and that the curve
do not cross the zero rotation line. Such waves are obtained for compounds
which do not have absorption in the wavelength region where optical activity is
being examined so, a plot of against λ is a plane curve. These curves shows
no minimum i.e. they are smooth.
E.g. of the compounds exhibiting such plane curves are alcohols and
hydrocarbons.
19. 2. Anomolous curves:
These curves on the other hand shows a number of extreme peaks and
troughs depending on the number of absorbing groups and therefore known
as Anomalous dispersion of optical rotation.
This type of curves is obtained for the compounds which contain an
asymmetric carbon atom and also contain chromophore, which absorb near
the UV region.
20. APPLICATIONS
• Determination of compounds containing alchols and hydrocarbons
using plain curves
• Multiple cotton effect curves: In this type of ORD curves two or
more peaks and troughs are obtained.
E.g. Functional groups i.e. Ketosteroids, camphor etc. exhibits such
curves
• Most important application of ORD-CD data for organics
• C=O has a weak for the n* transition 280 nm, but it can be easily
observed by ORD-CD on dilute samples (10-2–10-6 M)
21. • Determination of Position of Halogen Substitution
(Constitution)
• Determination of Absolute Configuration
The configuration of the 11-bromo-12-ketosteroid product from the
bromination of the parent 12-ketosteroid was deduced to be (R) from
the observation of a negative Cotton Effect.
CO2CH3
CH3
CH3
Br
O
AcO
11
11--Br (equatorial) gives +CE (as in the parent ketone)
11--Br (axial) gives -CE
22. Demonstation of conformation mobility
On chlorination of (R)-(+)-3-methylcyclohexanone, a crystalline 2-chloro-5-
methyl product is isolated that shows a negative Cotton Effect in octane, but a
positive one in methanol. The negative CE is consistent only with trans
stereochemistry, with independent evidence for axial Cl (in octane).
The change in sign of the CE on changing the solvent to (more polar) methanol
is presumably a reflection of the greater stability of the equatorial conformer in
that solvent.
