Ch. 5 - ChiralityWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

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

Concept #1: How to calculate enantiomeric excess.  

Example #1: Calculate the ee and observed rotation for the following chiral mixture where S-enantiomer has [α] = +20.

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

Example #2: Calculate the ee and observed rotation for the following chiral mixture where S-enantiomer has [α] = +20.

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. 

Practice: 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.

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

Practice: 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%.

Additional Problems
You are given a solution containing a pair of enantiomers  (A and B). Careful measurements show that the solution contains 98% A and 2% B. What is the ee of this solution?
The [α] of pure S-normetanephrine is +20°. Calculate the ee of a solution with the following observed values. For each ee calculate the percent of each enantiomer.           +10°  
If the ee of a certain mixture with two enantiomers is 80%. What is the ratio of the two enantiomers in the mixture? a) 90:10 b) 80:20 c) 60:40 d) 85:15 e) none of these
The [α] of a pure chiral compound A is +30°, Calculate the ee of a solution with the value of -15°. What is the percentage of each enantiomer A and B present in the solution?  
The specific rotation of (S)-carvone (at 20°C) is +61. A chemist prepared a mixture of (R)-carvone and its enantiomer, and this mixture had an observed rotation of –55°.(b) Calculate the % ee of this mixture.
The specific rotation of (S)-carvone (at 20°C) is +61. A chemist prepared a mixture of (R)-carvone and its enantiomer, and this mixture had an observed rotation of –55°.(c) What percentage of the mixture is (S)-carvone?
The specific rotation of a pure substance is -5.90°. What is the percentage of this isomer in a mixture with an observed specific rotation of -2.95°?A) 0%B) 75%C) 25%D) 50%E) 80%
Given that (S)-2-butanol has a specific rotation of +13.52 o and (R)-2-butanol has a specific rotation of -13.52o, what is % composition of a mixture whose specific rotation was found to be +6.76o? A) 75%(R) 25%(S)B) 25%(R) 75%(S)C) 50%(R) 50%(S)D) 67%(R) 33%(S)E) 33%(R) 67%(S)
If a sample contains 64.0% of the R enantiomer and 36.0% of the S enantiometer, what is the enantiomeric excess of the mixture? % enantiomeric excess =  
Consider a solution that contains 74.0% R isomer and 26.0% S isomer. If the observed specific rotation of the mixture is -88.0°, what is the specific rotation of the pure R isomer?
If a pure R isomer has a specific rotation of -151.0°, and a sample contains 69.0% of the R isomer and 31.0% of its enantiomer, what is the observed specific rotation of the mixture?