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C.I. 3.6 Optical Isomerism.

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Presentation on theme: "C.I. 3.6 Optical Isomerism."— Presentation transcript:

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.


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