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# Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

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
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### concept

How to calculate enantiomeric excess. 4m
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Calculating Enantiomeric Excess and Observed Rotation 50s
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Calculate the observed rotation when given the following ratio of enantiomers in solution. 3

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Calculating Enantiomeric Excess and Observed Rotation 2m
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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.

6
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%.