lecture recording on 03 March 2025 at 13.51.38 PM

Chiral Centers

• Definition of Chiral Center: A chiral center has four different attachments, which gives it the potential to exhibit R or S stereochemistry.

• Identification: Even in the absence of wedges (indicating spatial orientation) or dashes, a carbon can still be a chiral center as long as it has four different substituents.

• Example of Non-Chiral Centers: A carbon with three hydrogens and one wedge or dash does not constitute a chiral center, as it does not have four different attachments.

Assigning Stereochemistry

• Group Comparisons: When examining chiral centers, compare the entire group of attachments to determine differences. For instance, if one carbon has a bromine and a methyl group while another has three hydrogens, these are considered different groups.

• Priority Rules: When assigning priorities (for R/S configuration), if two groups are tied as carbons, consider substituents attached to those carbons to break the tie.

• Full Sequence: If two groups are identical, evaluate the next substituents outward to assign priorities.

Visualizing Stereochemistry

• 3D Shapes: The three-dimensional tetrahedral shape must be defined to determine whether the chiral center is R or S.

• Newman Projections: Transitioning from bond-line structures to Newman projections helps visualize the configuration:

  • The staggered form displays different orientations, while the eclipsed form is necessary for Fischer projections.

  • A staggered configuration involves ones that don't overlap and an eclipsed configuration consists of overlapping groups.

Fischer Projections

• Conversion to Fischer: To convert a Newman projection to a Fischer projection, you must rearrange the groups correctly to maintain their spatial relationships, typically positioning the front group upwards.

• Understanding Projection Rules: In Fischer projections, horizontal bonds represent wedges (coming towards you) and vertical bonds represent dashes (going away from you).

• Determining Configuration: Assign priority groups in the Fischer projection to establish the configuration (R or S). The lowest priority should be oriented away from you for proper counting.

Stereoisomers Relationships

• Enantiomers: These are pairs of molecules that are non-superimposable mirror images (like left and right hands). They exhibit identical properties in achiral environments but behave differently in chiral environments.

• Diastereomers: These molecules are stereoisomers that are not enantiomers. They have parts that are mirror images but are not completely superimposable. Their physical properties differ, which allows for their separation (e.g., by boiling point).

• Conformational Isomers: Rotated versions of the same molecule (staggered vs. eclipsed) fall into this category. Despite differences in orientation, configurations remain the same.

Identifying Relationships for Molecules

• Same Connectivity: To assess if two molecules are conformational, diastereomeric, or enantiomers, first ensure they share the same attachments and connectivity.

• R/S Configuration Matching: If two molecules exhibit the same R/S configuration, they are identical; if opposite, they are enantiomers. If some configurations match and others differ, they are diastereomers.

• Constituent Comparison: If upon comparing constituents and configurations you notice differences only in certain groups, this indicates whether they are diastereomers or not.

• Meso Compounds: Meso compounds have chiral centers but possess a plane of symmetry, making them achiral even though they contain chirality centers.

Summary of Key Points

• Chirality: Key for understanding stereochemistry in organic compounds.

• Visualizations: Fischer projections and Newman projections provide different ways to visualize molecular structures and stereochemistry, each with its rules for conversion and analysis.

• Understanding Relationships: Recognizing between conformers, diastereomers, and enantiomers is essential for determining properties and interactions in chemistry.