Overview of Stereochemistry and Nomenclature
Class Structure: The class focuses on stereochemistry, with particular attention to nomenclature and distinguishing characteristics of stereoisomers. The wrap-up of stereochemistry is planned for next Monday.
Key Learning Objectives
Understand naming conventions for stereoisomers using IUPAC rules.
Differentiate between enantiomers and diastereomers and understand their properties.
Prepare for an exam covering stereochemistry at the end of a break period.
Break Schedule
Break Duration: Wednesday through Friday, with an exam at the beginning following the break.
Study Aids: A study guide and a practice exam will be provided to assist in preparation during the break.
Wedge-Dash Representation
Wedge-Dash Combinations: The student is tasked to draw wedge-dash combinations of a compound with two chiral centers.
Number of Combinations: For a compound with two chiral centers, the number of wedge-dash combinations is calculated as follows:
Formula: 2^n where n is the number of chiral centers.
In this case: 2^2 = 4
Chiral vs. Meso Compounds
Definition of Meso: Meso compounds possess chiral centers but are achiral overall due to an internal plane of symmetry.
Determining Meso: The identification of achiral compounds occurs through analyzing the symmetry of the wedge-dash representations:
Chiral Centers: All drawn combinations contain at least one chiral center.
Achirality: Combinations that exhibit a plane of symmetry are labeled as achiral and therefore meso.
Chiral Definitions and Concepts
Chiral Centers: Molecules can be chiral if they cannot be superimposed on their mirror images. All combinations are analyzed based on the presence of such centers.
Mirror Images: If the mirror images are the same, the compounds are considered achiral.
Superimposable: When one molecule can be manipulated through rotation or flipping to match another, they are considered identical.
Nomenclature of Stereoisomers
Naming Enantiomers: Enantiomers are stereoisomers that are non-superimposable mirror images of each other. For example, naming the compound is based on designating either R or S for the configuration at chiral centers:
R and S designations are based on groups attached to the chiral carbon using atomic number priorities.
Prioritizing R and S
Rule for Prioritization: Assign priority based on the atomic number of atoms directly bonded to the chiral center:
Highest atomic number receives the highest priority (1).
If atomic numbers are the same, continue prioritizing until point of difference is found.
In example: Hydrogen (atomic number 1) receives the lowest priority (4).
Orienting the Molecule: The groups are arranged so that the lowest priority is represented by a dash (going back).
Configuration Determination: Draw a circle from priority 1 to 2 to 3:
Clockwise to designate R.
Counterclockwise designates S.
Applying the Method: For the compound 2-bromobutane:
One enantiomer is determined as R-2-bromobutane, and the other as S-2-bromobutane through assigned priorities and circular orientation.
Alternative Method for R and S Assignment
Hand Method: Using the left and right hands to remember whether configurations are R or S without needing to reorient molecules:
Thumb points to the lowest priority (4). Using the formed fingers to wrap from 1 to 2 to 3 dictates whether it's R (right hand) or S (left hand).
Example Structures and Analyzing Stereochemistry
Given structures should be analyzed to identify chiral centers, draw wedge-dash combinations, and finishing nomenclatures.
Discusses two chiral centers and emphasizes understanding that:
Each center must be evaluated separately, using R or S designations based on atomic priorities and orientations.
Combining Configurations
Understanding how to recognize the arrangements of compound structures leads to the determination of their function and behavior in chemical reactions.
Changing configurations provides immediate knowledge of different stereoisomers, further understanding of isomers, and their implications in chemical reactivity.
Assignment Reminders
Students will have homework assigning compounds to practice R and S designations. Couples of stereogenic centers will be analyzed, and relationships will be established based on previously covered material.
Closing Thoughts
Last points stress on reviewing concepts thoroughly for evaluation to ensure a complete understanding of how stereochemistry processes impact molecular configuration, design, and their chemical implications.