Optical isomerism

1
Optical isomerism

• Polarized light

• It is produced when a beam of ordinary light is
  passed through special filter such as a Nicol
  prism or a polaroid lens.

2
Optical Activity

• When a beam of polarized light is passed through
  an optically active substance, the plane of
  polarization is rotated.
• If a substance rotate the plane of polarized
  light, it is optically active / it possesses
  optical activity.

Direction of light
3
Enantiomers

• If two compounds having same structural formula
  (which also implies same molecular formula) but
  it is not superimposable (identical) to each
  other, they are enantiomers.
• Look at the models provided are they the same ?
  How are they related ?
• They are mirror images.
• If a compound is not superimposable to its mirror
  image, they are enantiomers.
• They can rotate the plane of polarized light
  optically active to same extent but different
  direction. (Dextrorotatory and laevorotatory)

4
Optical activity of enantiomers
If they are enantiomers, they have same specific
rotation but to different direction.
5
Criteria to have enantiomers

• A chiral carbon / asymmetric carbon carbon
  attached to 4 different groups of atoms / atoms.
• No plane of symmetry in a molecule.
• Chiral carbon ? no plane of symmetry
• No chiral carbon ? plane of symmetry

6
See examples

• Chiral carbon ? no plane of symmetry
• CWXYZ optically active, possesses enantiomer
• No chiral carbon ? plane of symmetry
• CW2XY optically inactive, does not possess
  enantiomer
• Plane of symmetry has a higher priority in
  considering whether the compound is optically
  active

7
Plane of symmetry in CW2XY

• Plane of symmetry

8
CWXYZ

• It has no plane of symmetry.
• Its mirror image is not superimposable to itself.

9
Why optical activity in CWXYZ but not CW2XY ?
Encounter another molecule which is the mirror
image of the originalone, the effect is cancelled.
Direction of light
One single molecule of CW2XY
10
Why optical activity in CWXYZ but not CW2XY ?

• Polarized light would not encounter another
  molecule which is mirror image to the one it
  encountered before, the effect would not be
  cancelled

Direction of light
One single molecule of CWXYZ
11
Plane of symmetry in CHXCCHX ?
No plane of symmetry
No chiral carbon
But still the mirror image is not superimposible
to itself it is optically active and they are
enantiomers.
12
Butan-2-ol
Any chiral carbon ? Which one is the chiral
carbon ?
13
Lactic acid (2-hydroxypropanoic acid)
Any chiral carbon ? Which one is the chiral
carbon ?

• Racemic mixture equimolar mixture of the d- and
  l- form, optically inactive

14
Distinguishing optical isomers

• Which of them are the same and which of them are
  enantiomers ?

15
Detection of optical activity

• By a polarimeter

Press
16
Detection of optical activity

• Set the analyzer until complete darkness is
  observed. Put the sample into the sample tube.
  Turn the analyzer until complete darkness is
  observed again.
• Turn the analyzer in clockwise direction
  dextrorotatory d-form or form
• Turn the analyzer in anticlockwise direction
  laevorotatory l-form or form
• Specific rotation angle of rotation which the
  plane of polarized light undergoes in traversing
  1 dm of solution of concentration 1 g cm-3.
• ?D 23o