5. Chirality

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

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%.

A solution of an unknown compound (3.0 g of the compound in 200 mL of solution), when placed in a polarimeter tube 2.0 dm long, was found to rotate the plane of polarized light 18° in a counterclockwise direction. What is the specific rotation of the compound?

The observed rotation of 2.0 g of a compound in 50 mL of solution in a polarimeter tube 20 cm long is +138°. What is the specific rotation of the compound?

(+)-Mandelic acid has a specific rotation of +158. What would be the observed specific rotation of each of the following mixtures?

c. 75% (-)-mandelic acid and 25% (+)-mandelic acid

A chiral sample gives a rotation that is close to 180°. How can one tell whether this rotation is +180° or -180°?

(+)-Tartaric acid has a specific rotation Of +12.0°. Calculate the specific rotation of a mixture of 68% (+)-tartaric acid and 32% (–)-tartaric acid.

A solution of 2.0 g of (+)-glyceraldehyde, HOCH₂CHOHCHO, in 10.0 mL of water was placed in a 100-mm cell. Using the sodium D line, a rotation of +1.74° was found at 25 °C. Determine the specific rotation of (+)-glyceraldehyde

A chemist finds that the addition of (+)-epinephrine to the catalytic reduction of butan-2-one (Figure 5-17 ) gives a product that is slightly optically active, with a specific rotation of +0.45°. Calculate the percentages of (+)-butan-2-ol and (−)-butan-2-ol formed in this reaction.

When optically pure (R)-2-bromobutane is heated with water, butan-2-ol is the product. The reaction forms twice as much (S)-butan-2-ol as (R)-butan-2-ol. Calculate the e.e. and the specific rotation expected for the product.

A student wanted to measure the specific rotation of the following propionate derivative (density = 1.12 g/ml).

A sample of the pure compound was placed in a 10.0-cm polarimeter tube, and using the sodium D line, the observed rotation at 20°C was determined to be +46.6° . What is the specific rotation of the propionate derivative?

Using a 1.0-cm polarimeter cell and a solution prepared by dissolving 2.54 g of glucose in 1 L of H₂O, the specific rotation of d-glucose was calculated to be +52.7° at 20 °C using the sodium D line as the source of light. What was the observed rotation of the solution?

Cholesterol (50 mg) was dissolved in 10 mL of chloroform and placed in a 1.0-cm polarimeter cell. This solution produced an observed rotation (using the sodium D line at 20°C ) of - 1.58° . What is the specific rotation of cholesterol?