1) Optical isomers, also known as enantiomers, are non-superimposable mirror images of chiral molecules that contain an asymmetric carbon atom bonded to four different groups. 2) Enantiomers rotate polarized light in opposite directions and often react differently with other chiral molecules. 3) Many drugs, amino acids, and other biological molecules exist as enantiomers but only one "handedness" is active in the body, with the other sometimes being harmful. Thalidomide caused birth defects because the inactive enantiomer converted to the active form in the body.

1. OPTICAL ISOMERISM

2. • All molecules have a mirror image –
but in most of the cases image and
object are the same molecule.
H H
C C
H H
H H
F F
fluoromethane

3. • For some molecules the mirror image
is a different molecule (the mirror
image is non-superimposable).
OH OH
C C
COOH HOOC
H H3C H
CH3
(-) lactic acid (+) lactic acid
in sour milk in muscles

4. • Left and right hands are an example
of non-superimposable mirror images.

6. • This usually happens when a molecule
contains a C atom with four different
groups attached (chiral / asymmetric C).
• Such molecules are said to be chiral or
optically active.
a a
C C
d d
b c c b

7. • The optical isomers are called
enantiomers.
• These are distinguished by +/-, D/L or
more correctly Rectus-right/Sinister-left.
• A 50/50 mixture of the two enantiomers
is called a racemic mixture or a
racemate.

8. TASK Which of the following
molecules are optically active?
1) propan-2-ol • butanone
2) 2-chlorobutane • 2-methylbutanoic acid
3) 1-chlorobutane • butan-2-ol
4) 3-methylhexane • 1-chloro-3-methylpentane

9. propan-2-ol CH3 CH CH3
OH
NOT OPTICALLY ACTIVE

10. 2-chlorobutane CH3 CH CH2 CH3
Cl
CH2CH3 CH2CH3
C C
CH3 H3C
H H
Cl Cl
OPTICALLY ACTIVE

11. 1-chlorobutane CH2 CH2 CH2 CH3
Cl
NOT OPTICALLY ACTIVE

12. 3-methylhexane CH3 CH2 CH CH2 CH2 CH3
CH3
CH2CH2CH3 CH2CH 2CH 3
C C
CH3 CH3
H H
CH2CH 3 CH3CH2
OPTICALLY ACTIVE

13. butanone O
CH3 C CH2 CH3
NOT OPTICALLY ACTIVE

14. propan-2-ol CH3 CH CH3
OH
NOT OPTICALLY ACTIVE

15. 2-methylbutanoic acid CH3 O
CH3 CH2 CH C OH
CH2CH3 CH2CH3
C C
H H
CH3 CH3
COOH HOOC
OPTICALLY ACTIVE

16. butan-2-ol OH
CH3 CH2 CH CH3
CH2CH3 CH2CH3
C C
H H
CH3 CH3
OH HO
OPTICALLY ACTIVE

17. 1-chloro-3-methylpentane CH3 Cl
CH3 CH2 CH CH2 CH2
CH2CH3 CH2CH3
C C
H H
CH3 CH3
CH2CH2Cl CH2ClCH2
OPTICALLY ACTIVE

18. • Molecules that are optical isomers are
called enantiomers.
• Enantiomers have identical chemical
and physical properties, except:
• Their effect on plane polarised light;
• Their reaction with other chiral molecules

19. 1-The wave vibrations are perpendicular
to the direction of travel of the wave.
normal light plane-polarised light plane-polarised light after
(waves vibrate in all directions) (vibrates in only one direction) clockwise rotation

20. • Optical isomers rotate the plane of
plane polarised light.
(-)-enantiomer (+)-enantiomer (±)-racemate
(anticlockwise rotation) (clockwise rotation) (no overall effect)

22. 2- Chiral molecules often react
differently with other chiral molecules.
• This is like the idea that a right hand
does not fit a left handed glove – the
molecule must be the correct shape to
fit the molecule it is reacting with.
• Many natural molecules are chiral and
most natural reactions are affected by
optical isomerism.

23. • For example, most amino acids (and
so proteins) are chiral, along with
many other molecules.
• The human body can consume amino
acids that rotate polarised light to the
left.

24. • Many drugs are optically active, with
one enantiomer only having the
beneficial effect.
• In the case of some drugs, the other
enantiomer can even be harmful, e.g.
thalidomide.

25. • In the 1960’s thalidomide was given
to pregnant women to reduce the
effects of morning sickness.
• This led to many disabilities in babies
and early deaths in many cases.

26. O NH O O NH O
O O
H2C C C H2C
CH2 N N CH2
H H
O O
S thalidomide (effective drug) R thalidomide (dangerous drug)
The body racemises each
enantiomer, so even pure S is
dangerous as it converts to R in
the body.

27. CH3 CH3
O O
H C CH2 H2C C H
H3C CH3
S carvone (caraway seed) R carvone (spearmint)

28. CH3 CH3
CH2 C H H C CH2
CH3 H3C
S limonene (lemons) R limonene (oranges)