optical activity, its measurement and criterias for optical activity is discussed in brief

1. Stereochemistry of
organic compounds
Optical isomerism

2. Optical isomerism
• Isomers differ in their behaviour towards plane
polarized light
• Rotation to right: dextro rotatory (+) or (d)
• Rotation to left: laevo rotatory (-) or (l)
Light
Stereochemistry
of molecule

3. Plane polarized light
rays of different wavelength,
vibrating in all possible planes,
perpendicular to the direction
of propagation of light
Monochromatic light
Filter
Polarizer

4. Optical Rotation

5. Polarimeter
• Magnitude of angle of rotation depends on
• Concentration of solution
• Length of the polarimeter tube

6. Specific rotation
• Used to compare optical rotation of substances
• Intrinsic property
• Different when wavelength is changed
• Specific rotation for an optically active substance at a
given temperature for a given wavelength may be
defined as the rotation (in degrees) produced by a
path of 1dm length in a substance of unit density
(concentration)

7. Cause of optical activity
• Chirality
• Property of non-superimosability of a structure on its
mirror image
Chiral
compounds are
optically active

9. Symmetry and chirality
• Plane of symmetry
imaginary plane that bisects a molecule into halves that are
mirror images of each other
• Centre of symmetry
imaginary point in the molecules such that if a line is drawn
from any group of the molecule to this point and then extended
to an equal distance beyond the point, meets an identical group.
• Axis of symmetry
imaginary axis around which by rotating through a minimum
angle of rotation, the original compound is obtained
• Alternating axis of symmetry
line passing through the molecular structure around which if
rotated through an angle X followed by reflection on a mirror
placed at right angle to the axis produces the same face of the
molecule
Optically active
molecules will not
have a plane of
symmetry and
centre of
symmetry

10. Plane of symmetry
Centre of symmetry
Axis of symmetry
Alternating axis of
symmetry

11. Assymetric molecule: does not have any element of
symmetry
Dissymmetric molecule: does not have plane of
symmetry, centre of symmetry and alternating axis of
symmetry
Dissymmetric
molecules
Assymmetric
molecules
Dissymmetry is
the condition
for optical
activity