Concepts & Vocabulary
- Every molecule is either chiral (not superimposable on its mirror image) or achiral (superimposable on its mirror image).
- Chiral molecules do not have a plane of symmetry, while achiral molecules have one or more planes of symmetry.
- Stereoisomers vary by spatial arrangement of atoms but have the same atom connectivity.
- Stereoisomers that are mirror images of one another but are not superimposable are called enantiomers.
- A Tetrahedral carbon atom bonded to four different substituents is an asymetric carbon (also called a stereocenter or chiral carbon), which typically leads to a chiral molecule (meso compounds are the exception in section 5.7).
- Enantiomers cause rotation of plane-polarized light in equal amounts in opposite directions. This is called optical activity. Clockwise rotation is called dextrorotatory (+) and counter-clockwise is called levorotatory (-).
- Specific rotation is the amount that a sample of a chemical rotates planne-polarized light. It can be used to calculate the purity of a mixture of enantiomers called the enantiomeric excess.
- Resolution is the separation of a mixture of enatiomers.
- Racemates are defined as a 50:50 mixture of enantiomers, resulting in a sample that is not optically active. The process of forming a racemic mixture is called racemization.
- Use the CIP rules to determine the priority of each substituent attached to a chiral carbon to determine whether configuration is R or S. With the lowest priority group facing away from you, draw an arc connecting groups 1-2-3. If that arc is clockwise, the configuration is R. If counterclockwise, the configuration is S.
- Stereoisomers that are not mirror images of one another are called diastereomers.
- Diastereomers have two or more stereocenters. The configurations of the stereocenters cannot be inverse of each other (example R,R and S,S) because that defines a pair of enantiomers.
- Meso compounds are achiral but have chiral centers. This is caused by having an internal plane of symmetry that allows the two molecules to be superimposable on one another and be optically inactive.
- Each component of a racemic mixture rotates plane polarized light an equal amount in opposite directions, so there is no optical activity.
- Racemic mixtures can be separated into the component enantiomers by reaction with a chiral reagent, which will form diastereomer intermediates of the molecules which can then be separated. Following separation the chiral reagent is removed to yield the two pure enantiomers.
- There are several categories of isomers with the largest distinction between:
- constitutional (structural) isomers that contain the same number of each atom but differ in connectivity
- stereoisomers that have all the same atoms with the same connectivity, but only differ in how the atoms are arranged three dimensionally
- In addition to the diastereomers and enantiomers that have been discussed at length in this chapter, stereoisomers can also be:
- cis/trans or E/Z isomers which differ by spatial arrangement around a double bond
- conformational isomers (conformers) which occur due to free rotation of sigma bonds
- Nitrogen when bonded to three different atoms is chiral, however the lone pair of electrons moves freely between positions on the Nitrogen causing these molecules to become a racemic mixture.
- When bonded to four different atoms in quaternary ammonium salts, nitrogen atoms lead to chiral molecules.
- Organic phosphates with four different groups can also be chiral.
- When a carbon can be converted to a chiral center by changing only one of its attached groups, it is called prochiral.
- If a molecule has two hydrogens on the same atom and replacement of either one with deuterium would lead to enantiomers, the hydrogens are enantiotopic.
- Similarly if this replacement would lead to diastereomer molecules, the hydrogens are diastereotopic.
- If replacement of a hydrogen would not lead to a chiral center being created, they are termed homotopic.
Skills to Master
- Skill 5.1 Identify stereocenters in molecular structures.
- Skill 5.2 Identify whether two structures are identical (not meso), constitutional isomers, enantiomers, diastereomers or meso and identical.
- Skill 5.3 Explain how plane polarized light is used to show optical activity.
- Skill 5.4 Calculate specific rotation from experimental data.
- Skill 5.5 Calculate optical purity and enantiomeric excess from experimental data.
- Skill 5.6 Determine configuration of stereocenters as R or S.
- Skill 5.7 Draw the enantiomer and diastereomers of a given compound with one or more stereocenters.
- Skill 5.8 Identify planes of symmetry in meso compounds.
- Skill 5.9 Describe a process for separating a mixture of enantiomers.
- Skill 5.10 Explain why racemic mixtures are optically inactive.
- Skill 5.11 Explain the difference between constitutional and stereoisomers.
- Skill 5.12 Give an example of a chiral center that is not carbon.
MT 5.1 Memorize the rules for determining R and S configuration.
MT 5.2 Memorize the types of isomers and how to identify them.