More Stereochemical Relationships: Crash Course Organic Chemistry #9

Summary

This episode of Crash Course Organic Chemistry delves into the fascinating world of enantiomers, molecules that are non-superimposable mirror images. The video explains how chiral molecules can interact differently with plane-polarized light, allowing chemists to distinguish between enantiomers using polarimeters. Viewers learn about the concept of enantiomeric excess, racemic mixtures, and the significant role of stereochemistry in various fields like medicine and chemistry. The episode also touches on the historical discovery of enantiomers by Louis Pasteur and reviews different types of isomers, providing a comprehensive understanding of stereochemistry and its applications.

Takeaways

• 😀 Enantiomers are non-superimposable mirror images of molecules with almost identical chemical and physical properties but can have different effects in chiral environments like taste and smell.
• 😀 (R)-carvone smells like spearmint, while (S)-carvone smells like caraway, demonstrating how enantiomers can have distinct sensory properties.
• 😀 Polarimetry allows organic chemists to distinguish between enantiomers by measuring their ability to rotate plane-polarized light.
• 😀 Molecules that rotate plane-polarized light to the left are levorotatory (L or -), and those that rotate it to the right are dextrorotatory (D or +).
• 😀 The specific rotation of a molecule can be calculated using the formula: α observed = [α] = α / (c * L), which depends on concentration and the path length of the polarimeter.
• 😀 Enantiomers do not follow an easy rule for determining which will be levorotatory or dextrorotatory, making polarimetry an experimental process.
• 😀 A racemic mixture, consisting of equal amounts of (S)- and (R)-enantiomers, does not rotate plane-polarized light because their rotations cancel each other out.
• 😀 Enantiomeric excess is calculated to determine the percentage of each enantiomer in a mixture, which can be vital in chemical reactions and pharmaceuticals.
• 😀 The study of stereochemistry, including enantiomers, started with Louis Pasteur’s discovery of tartaric acid’s optical activity in the 19th century.
• 😀 Organic chemists use the formula 2^n to calculate the maximum number of stereoisomers, where n is the number of chiral centers in a compound, like tartaric acid with 2 chiral centers having 4 possible isomers.
• 😀 Meso compounds, like meso-tartaric acid, have an internal plane of symmetry and are not chiral despite having chiral centers, which can lead to fewer stereoisomers than the maximum calculated.

Q & A

What are enantiomers, and how do they differ from one another?

Enantiomers are non-superimposable mirror images of each other. While they share almost identical chemical and physical properties, such as the same melting and boiling points, they interact differently with chiral environments, such as the receptors in our nose or on our tongues, which can result in vastly different smells or tastes.

How can we tell enantiomers apart without directly tasting or smelling them?

Enantiomers can be distinguished through their interaction with plane-polarized light. One enantiomer may rotate the light to the left (levorotatory) or to the right (dextrorotatory), and this rotation can be measured using a polarimeter.

What is plane-polarized light, and how is it used to identify chiral molecules?

Plane-polarized light vibrates in only one direction, and it can be produced by filtering light through a slitted disc or lens. Chiral molecules can rotate this light either to the left or to the right, a property that can be measured using a polarimeter to identify the enantiomer present.

What does a polarimeter do, and how does it measure the rotation of light?

A polarimeter is an instrument used to measure the angle at which a chiral molecule rotates plane-polarized light. By placing a solution of the enantiomer between polarizing filters and adjusting the analyzing filter, the angle at which the light is rotated is measured, indicating the molecule’s specific rotation.

What is the difference between a levorotatory and a dextrorotatory molecule?

Levorotatory molecules rotate plane-polarized light to the left and are labeled as L or minus, while dextrorotatory molecules rotate light to the right and are labeled as D or plus.

What is an optically pure sample, and how does it relate to polarimetry?

An optically pure sample contains only one enantiomer. In polarimetry, the rotation of plane-polarized light by an optically pure sample can be measured to determine its specific rotation. If only one enantiomer is present, it will rotate the light in a consistent direction.

What is a racemic mixture, and what happens when it is placed in a polarimeter?

A racemic mixture consists of equal amounts of both enantiomers. When such a mixture is placed in a polarimeter, the rotations caused by the enantiomers cancel each other out, resulting in no net rotation of the plane-polarized light.

What is enantiomeric excess, and how is it calculated?

Enantiomeric excess refers to the percentage of one enantiomer in a mixture compared to the other. It can be calculated by comparing the observed rotation in a polarimeter with the known rotation of an optically pure sample of one enantiomer.

How did Louis Pasteur contribute to the discovery of enantiomers?

Louis Pasteur discovered enantiomers while studying tartaric acid in the 19th century. He found that tartaric acid crystals could rotate plane-polarized light in opposite directions, and by separating the crystals, he identified the existence of non-superimposable mirror image molecules, which became known as enantiomers.

How can we calculate the maximum number of stereoisomers for a molecule?

The maximum number of stereoisomers for a molecule can be calculated using the formula 2^n, where n is the number of chiral centers in the molecule. For example, tartaric acid has two chiral centers, so it can have up to four stereoisomers. However, some of these may be identical, like meso compounds.