Optical Activity - Specific Rotation & Enantiomeric Excess - Stereochemistry Youtube

This paragraph introduces the concept of optical activity, explaining how molecules can be either optically active or inactive based on their ability to rotate plane polarized light. It delves into the specifics of enantiomers, chiral carbons, and the process of assigning R and S configurations using the Cahn-Ingold-Prelog priority rules. The paragraph clarifies that while enantiomers have identical physical properties, their optical properties differ, with one rotating light clockwise (dextrorotatory) and the other counterclockwise (levorotatory). It also emphasizes that the R/S configuration does not inherently determine the direction of light rotation, which can vary between molecules.

This section explains the process of light polarization and how it is used to determine the optical activity of molecules. It describes how normal light is filtered through a polarizer to create plane polarized light, and how achiral molecules do not affect this light, whereas chiral molecules can rotate it. The paragraph uses the example of a light bulb emitting light in all directions and the subsequent passage of this light through a polarizer and a sample tube containing chiral molecules to illustrate the concept. It concludes by emphasizing the difference between optically active and optically inactive molecules in terms of their effect on plane polarized light.

The paragraph focuses on how to determine whether a molecule is optically active or inactive by examining its structure for chiral centers. It explains that molecules without chiral centers are achiral and optically inactive, while those with one chiral center are optically active. The concept of meso compounds with two chiral centers but a plane of symmetry, which are optically inactive, is introduced. The paragraph provides examples of molecules with different numbers of chiral centers and their optical activity, guiding viewers to identify optically active and inactive molecules through practice.

This section introduces the formulas and concepts related to specific rotation, enantiomeric excess, and optical purity. It explains how specific rotation is calculated using the observed rotation, path length, and concentration, and how it is denoted with subscripts for wavelength and superscripts for temperature. The paragraph also discusses how enantiomeric excess is calculated from the observed rotation of a solution and the specific rotation of a pure enantiomer, or from the known percentages of enantiomers in a mixture. It provides a formula for calculating enantiomeric excess when the amounts of enantiomers are given in grams or moles.

The final paragraph presents a practical problem involving the calculation of specific rotation for a chiral stereoisomer. It provides the observed rotation, mass of the stereoisomer, volume of the solution, temperature, wavelength, and length of the sample tube. The paragraph guides through the process of converting units, calculating concentration, and applying the formula for specific rotation to arrive at the solution. It concludes with the calculated specific rotation value for the given stereoisomer.