organic chem end of chapter 5

Course Overview

• Problems discussed are supplementary to current knowledge; availability of additional help.

• Conclusion of Chapter 5 scheduled for today.

• Next week’s exam to cover Chapters 3, 4, and 5, tentatively planned for Friday.

Isomerism

• Cis-trans isomers are classified as diastereomers.

  • Diastereomers:

  • Nonsuperimposable and non-mirror images.

  • Different physical properties compared to enantiomers.

  • Can typically be separated.

  • Enantiomers are more challenging to separate, requiring methods like chiral resolution or chiral chromatography.

Stereoisomer Types

• Cis-trans diastereomers: Specific type of stereoisomers with one or more chiral centers.

• Diastereomers criteria:

  • At least one chiral center remains constant while others change.

• Example provided: Two chiral centers with configurations from (RR) to (SR) are diastereomers.

Calculating Stereoisomers

• Formula for calculating possible stereoisomers: $2^n$, where $n$ = number of chiral centers.

  • Example:

  • 1 chiral center: $2^1 = 2$ stereoisomers.

  • 2 chiral centers: $2^2 = 4$ stereoisomers.

  • 3 chiral centers: $2^3 = 8$ stereoisomers.

Molecule Analysis

• Example with three chiral centers: 8 possible stereoisomers.

• Configurations analyzed based on chiral center positions and mirrored counterparts.

• Determination of relationships:

  • Enantiomers: Opposite configurations.

  • Diastereomers: At least one configuration unchanged between two compared molecules.

Chirality and Optical Activity

• Molecules with one chiral center are chiral and exhibit optical activity.

• Molecules with multiple chiral centers can be achiral if they possess a plane of symmetry.

  • Example: Meso compounds.

  • Criteria for meso compounds:

    • Contains an even number of chiral centers.

    • A plane of symmetry divides the molecule into mirror images.

    • Meso compounds do not rotate plane polarized light.

Fischer Projections

• Define Fischer projections:

  • Horizontal lines represent groups projecting out towards the viewer.

  • Vertical lines represent groups receding into the plane of the paper.

• Assigning configurations:

  • Convert Fischer projections for clarity; ensure lowest priority group points away.

  • Assignment example walks through prioritization (C, O, etc.) to determine (S) or (R).

Biological Relevance in Fischer Projections

• Utilized frequently to depict configurations of carbohydrates and amino acids.

• D/L Sugars:

  • Configuration indicated by the hydroxyl group position on the penultimate carbon.

  • Left = L sugar, Right = D sugar.

Model Construction Exercise

• Two, three dibromobutane model construction to visualize correct spatial arrangements of atoms.

• Exact assignments should match objective geometric arrangements necessary for stereochemical accuracy.

Relationships and Stereoisomers in Cyclic Compounds

• Analyzes chiral and achiral configurations in cyclic organic molecules:

  • 1,2-dimethylcyclopentane and isomer analysis (superimposable mirror images).

  • Identifying meso compounds through symmetry analysis.

Examples Include:

• Symmetric arrangements affecting optical properties:

  • One, four dimethylcyclohexane: Meso compound properties noted.

  • Comparison of configurations (cis-trans) to determine stereochemical behavior.

Beyond Carbon Chirality

• Chirality can also occur in non-carbon atoms (e.g., silicon, germanium, nitrogen, sulfur).

  • Example given: Esomeprazole, a chiral sulfoxide used in pharmaceuticals.

Non-chiral Centers with Chiral Shapes

• Molecular structures can exhibit chirality without explicit chiral centers (e.g., bulky naphthalene rings).

• Discusses significance and applications of complex chiral structures without traditional chiral centers.

Conclusion

• Review emphasized due to complexity.

• Additional practice encouraged for clarity on stereoisomer relationships and Fischer projections.

• Reminder to consult instructor with questions during next class session.