Optical activity calculations | Stereochemistry | Organic chemistry | Khan Academy

Summary

This video tutorial explores the concepts of optical activity through practical calculations involving cholesterol. It begins by determining the specific rotation of natural cholesterol using observed rotation and concentration, leading to a specific rotation value of -31.5. The video then explains percent enantiomeric excess, calculating it for various mixtures, including optically pure and racemic mixtures. Finally, it assesses a mixture's specific rotation to derive its enantiomeric excess and the percentage of natural cholesterol present, concluding with a verification of the calculations. This engaging approach simplifies complex concepts in stereochemistry.

Takeaways

• 😀 Natural cholesterol is an optically active compound with a specific rotation calculated from its observed rotation.
• 😀 The specific rotation formula is: observed rotation divided by concentration times path length.
• 😀 In the example, the specific rotation of natural cholesterol was calculated to be -31.5 degrees.
• 😀 Percent enantiomeric excess (optical purity) measures the percentage difference between two enantiomers.
• 😀 A solution with 100% of one enantiomer is considered optically pure, with 100% enantiomeric excess.
• 😀 A racemic mixture, containing equal amounts of both enantiomers, has 0% enantiomeric excess and is optically inactive.
• 😀 For a mixture containing 75% of one enantiomer and 25% of the other, the enantiomeric excess is 50%.
• 😀 The percent enantiomeric excess can also be calculated using the observed specific rotation and the specific rotation of the pure enantiomer.
• 😀 In the final example, a mixture with a specific rotation of -27 resulted in a calculated enantiomeric excess of 86%.
• 😀 The total percentage of natural cholesterol in the mixture was found to be 93%, confirming the calculations made.

Q & A

What is the specific rotation of natural cholesterol calculated in the video?

The specific rotation of natural cholesterol is calculated to be -31.5 degrees.

How is specific rotation determined?

Specific rotation is determined by dividing the observed rotation by the concentration of the solution multiplied by the path length.

What is the concentration of cholesterol in the solution?

The concentration of cholesterol is 0.020 g/mL, calculated from 0.300 g in 15.0 mL.

What does a negative specific rotation indicate?

A negative specific rotation indicates that the compound rotates plane-polarized light in the counterclockwise direction.

How do you calculate percent enantiomeric excess (EE)?

Percent enantiomeric excess is calculated by subtracting the percentage of one enantiomer from the percentage of the other enantiomer.

What is the optical purity of a racemic mixture?

A racemic mixture has an optical purity of 0%, as it contains equal amounts of both enantiomers.

What percentage of one enantiomer and the other enantiomer is present in a mixture that is 75% one enantiomer?

In a mixture that is 75% one enantiomer, 25% of the other enantiomer is present, resulting in a percent enantiomeric excess of 50%.

What was the observed specific rotation for the mixture of natural cholesterol and its enantiomer?

The observed specific rotation for the mixture was -27 degrees.

How is the percentage of natural cholesterol in the mixture calculated?

The percentage of natural cholesterol is calculated as 93%, determined by adding 86% (enantiomeric excess) and 7% from the racemic portion.

Why is it important to convert the path length to decimeters for these calculations?

It is important to convert the path length to decimeters because specific rotation calculations require this unit for consistency.