Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Stereoisomers are compounds with the same structural formula but different arrangements of atoms in space. They are divided into conformational isomers (rotation about bonds) and configurational isomers.
Configurational isomers are further subdivided into cis-trans (or ) isomers and optical isomers.
Cis-trans isomerism occurs due to restricted rotation, typically around a double bond or within a cyclic structure. (zusammen) refers to high-priority groups on the same side, while (entgegen) refers to high-priority groups on opposite sides.
The Cahn-Ingold-Prelog (CIP) priority rules determine notation based on the atomic number () of the atoms directly attached to the double-bonded carbons.
Optical isomerism occurs in chiral molecules that contain an asymmetric carbon atom (chiral center) bonded to four different groups.
Enantiomers are non-superimposable mirror images of each other. They have identical physical and chemical properties in an achiral environment but rotate plane-polarized light in opposite directions.
A racemic mixture (racemate) contains equimolar amounts of two enantiomers and is optically inactive because the rotations cancel out.
Diastereomers are stereoisomers that are not mirror images of each other. They occur when a molecule has two or more chiral centers and differ in physical and chemical properties.
📐Formulae
(where is the number of chiral centers)
💡Examples
Problem 1:
Determine the configuration for -bromo--chloro-prop--ene.
Solution:
-configuration
Explanation:
On carbon 1, the groups are () and (), so is high priority. On carbon 2, the groups are (carbon ) and (), so is high priority. Since the high priority groups ( and ) are on opposite sides of the double bond, it is the isomer.
Problem 2:
Identify the number of chiral centers and calculate the maximum number of stereoisomers for -dihydroxybutanedioic acid (tartaric acid).
Solution:
2 chiral centers; potential stereoisomers (though one is a meso compound).
Explanation:
The molecule has chiral centers at and . Using the formula , where , we expect 4 stereoisomers. However, because of internal symmetry in one form (the meso form), there are actually only 3 distinct stereoisomers: (+), (-), and meso.
Problem 3:
A sample of pure -limonene has a specific rotation of . A mixture of enantiomers shows a rotation of . Calculate the percentage of the -enantiomer in the mixture.
Solution:
Explanation:
Optical purity (enantiomeric excess) = . An of means of the mixture is the pure isomer and the remaining is a racemic mixture ( and ). Total enantiomer = .