UNIT – I
OPTICAL ROTATORY
DISPERSION
POLARIZED LIGHT AND OPTICAL
ROTATION
Dr. G. PRATAP KUMAR
ASST PROF
DEPT. OF BIOTECHNOLOGY
MUTHAYAMMAL ENGINEERING COLLEGE
OPTICAL ROTATORY DISPERSION
Optical rotatory dispersion (ORD) is the
variation in the optical rotation of a
substance with a change in the
wavelength of light
ORD can be used to find the absolute
configuration of metal complexes.
Example, when plane-polarized white
light from an overhead projector is
passed through a cylinder of sucrose
solution, a spiral rainbow is observed
perpendicular to the cylinder.
This leads to optical rotation which is measured by using a ‘polarimeter’.
Measuring optical rotation as a function of wavelength is termed ‘Optical
rotatory dispersion (ORD) spectroscopy’.
Polarization – ‘light’
A light wave is an electromagnetic wave that travels
through the vacuum of outer space.
A light wave that is vibrating in more than one plane
is referred to as unpolarized light.
Polarized light waves are light waves in which the
vibrations occur in a single plane. Thus, the process of
transforming unpolarized light into polarized light is
known as ‘polarization’.
TYPES OF POLARIZED LIGHT
a) Liner or plane polarized light - Vibrating in a single plane
perpendicular to the direction of propagation is called ‘plane
polarised light’.
b) Circular polarized light - When vibration of light are along a
circle lying in a plane perpendicular to the direction of
propagation the light is called ‘circular polarized light’.
c) Elliptical polarized light - When vibration are along a ellipse
lying in a plane perpendicular to the direction of propagation the
light is called ‘elliptically polarized light’.
The Formula
TYPES
of Optical
Rotation
OPTICAL ROTATION
The angle through which the plane of
polarization is rotated when polarized
light passes through a layer of liquid.
The ability to rotate the plane of
Optically active substances are classified
polarization of plane-polarized light by a into two types.
certain substance is called optical a. Dextrorotatory substances –
activity. Substances that rotate the plane of
polarization of the light towards the
Quartz and cinnabar are examples of right are known as right-handed.
optically active crystals while aqueous b. Laevorotatory substances –
Substances which rotate the plane of
solutions of sugar, tartaric acid are polarization of the light toward the
optically active solutions. left are known as left-handed.
INSTRUMENTATION - POLARIMETER
INSTRUMENTATION – ORD Spectroscopy
ORD Curve
Applications of Optical Rotation
Optical rotation is a function of time and it is used to determine kinetic reactions
This is also used to plot optical rotatory dispersion curves for a various range of
wavelengths this helps in analysing molecular structure
The optical rotation is measured on a layer of suitable thickness at the wavelength
specified in the monograph.
If the specific rotation of a sample is known its concentration in the solution can be
estimated.
If the concentration of the material in the sample is known then its specific rotation can
be determined.
The technique may be extended to the determination of optical substances in the
presence of optically inactive species.
CIRCULAR Dichroism (CD)
Measurement of how an optically active compound absorbs circular polarized
light both left and right handed.
Plotted as εl – εr vs λ
Instrument used is CD spectroscopy It is used for study of
biomolecules, their structure and interaction with metals and other molecules.
