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5. Chirality

Some more facts about optical activity:

- Specific rotation [α] is the rotation that 100% pure enantiomers produce.
- Opposite enantiomer = opposite rotation.
- Racemic:A perfect 1:1 ratio of enantiomers
- Scalemic:A non-1:1 ratio of enantiomers

1

How to calculate enantiomeric excess.

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2

Calculating Enantiomeric Excess and Observed Rotation

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Calculate the observed rotation when given the following ratio of enantiomers in solution.

3

Calculating Enantiomeric Excess and Observed Rotation

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We can also use these equations to calculate specific and observed rotation when other variables are given to us. Just plug the numbers in! Let's try a few.

4

Problem

When 0.200 g of lactose is dissolved in 10.0 ml of water and placed in a sample cell 10.0 cm in length, the observed rotation is +2°. Calculate the specific rotation of lactose.

5

Problem

Calculate the observed rotation of a chiral mixture that contains 65% (S)-stereoisomer where the [α] of pure (S)-stereoisomer = -118.

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Problem

An optically pure (R)-stereoisomer of a molecule has a specific rotation of – 20°. What specific rotation would be observed for a mixture of the (R) and (S) stereoisomer where there is an enantiomeric excess equal to (S) 60%.