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C.I. 3.6

Optical Isomerism

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Types of isomerism

Isomerism

Structuralisomerism

Stereoisomerism

Geometricisomerism

Opticalisomerism

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Different compounds- different properties.

geometric isomerism

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Optical Isomerism

Arises because of the different ways you canarrange four different groups around a

carbon atom.

Hang on!! Arent E-amino acids a group of

compounds that have four different groups

around a carbon atom?

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four groups of electrons around thecentral atom

four bonding pairs

TETRAHEDRAL shape

bond angle exactly 109r

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O

NH2

CH3 OH

O

NH2

CH3OH

imaginary mirrors

alanine

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All molecules have mirror images howeverthey dont all exist as two isomers.

What makes an isomer is the fact that themirror image and the original molecule are

non-superimposable!

The only way to

make these two

superimpose is to

break & reform

bonds.

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Left and right hands are an example of

non-superimposable mirror images.

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Enantiomers ?

Molecules such as alanine that exist inthese two forms = optical isomers orenantiomers.

We distinguish between the twoenantiomers of a molecule by +/-, D/L ormore correctly R/S.

A 50/50 mixture of the two enantiomers iscalled a racemic mixture or a racemate.

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Some more key words M

olecules that are not superimposable ontheir mirror images are called chiralmolecules.

A carbon surrounded by 4 different groupsis called a chiral centre.

chiral centre

of alanine

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The CORNy rules for naming

enantiomers.Optical isomers exist as L-enantiomers or

D-enantiomers.

O

NH2

CH3 OH

O

NH2

CH3OH

COR

N

COR

N

L-enantiomer D-enantiomer

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1. Imagine looking down on the molecule withthe single H atom point straight up towardsyou.

2. Label the other three groupsa. COOH = CO

b. R-group = R

c. NH2 = N

3. If CORN is arranged in a CLOCKWISEdirection it is the L-amino acid.

4. If CORN is arranged ANTI-CLOCKWISE it is aD-amino acid.

The CORNy rules for naming

enantiomers.

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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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