Organic Chemistry

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5. Chirality

Enantiomeric Excess

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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

concept

How to calculate enantiomeric excess.

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example

Calculating Enantiomeric Excess and Observed Rotation

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

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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.

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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%.