1. C.I. 3.6
Optical Isomerism

2. Types of isomerism Isomerism Structural isomerism Stereoisomerism
Geometric isomerism
Optical isomerism

3. Different compounds- different properties.
geometric isomerism

4. Optical Isomerism
Arises because of the different ways you can arrange four different groups around a carbon atom.
Hang on!! Aren’t -amino acids a group of compounds that have four different groups around a carbon atom?

5. four groups of electrons around the central atom
four bonding pairs
TETRAHEDRAL shape
bond angle exactly 109

6. alanine
imaginary mirrors

7. The only way to make these two superimpose is to break & reform bonds.
All molecules have mirror images – however they don’t all exist as two isomers.
What makes an isomer is the fact that the mirror image and the original molecule are non-superimposable!
The only way to make these two superimpose is to break & reform bonds.

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

9. Enantiomers ?
Molecules such as alanine that exist in these two forms = optical isomers or enantiomers.
We distinguish between the two enantiomers of a molecule by +/-, D/L or more correctly R/S.
A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.

10. chiral centre of alanine
Some more key words …
Molecules that are not superimposable on their mirror images are called chiral molecules.
A carbon surrounded by 4 different groups is called a chiral centre.
chiral centre of alanine

11. The CORNy rules for naming enantiomers.
Optical isomers exist as L-enantiomers or
D-enantiomers.
L-enantiomer
D-enantiomer R R CO CO N N

12. The CORNy rules for naming enantiomers.
Imagine looking down on the molecule with the single H atom point straight up towards you.
Label the other three groups
COOH = CO
R-group = R
NH2 = N
If CORN is arranged in a CLOCKWISE direction it is the L-amino acid.
If CORN is arranged ANTI-CLOCKWISE it is a D-amino acid.

13. How do enantiomers differ?
Behave identically in ordinary test-tube chemical reactions.
Have same physical properties.
BUT!!
Behave differently in presence of other chiral molecules.
e.g. Taste-buds are ‘chiral’ D-amino acids taste sweet, L-amino acids are tasteless or bitter.