Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

5. Chirality

R and S Configuration

Organic Chemistry

5. Chirality

R and S Configuration

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Problem 50a

Textbook Question

Assign the absolute stereochemistry for each of the following molecules drawn in their Fischer projection.
(a)

Verified step by step guidance

Step 1: Identify the chiral center in the molecule. In the given Fischer projection, the central carbon atom is the chiral center as it is bonded to four different groups: Br, Cl, H, and CH₃.

Step 2: Assign priorities to the substituents based on the Cahn-Ingold-Prelog priority rules. The priority is determined by atomic number: Br (atomic number 35) > Cl (atomic number 17) > CH₃ (carbon, atomic number 6) > H (atomic number 1).

Step 3: Orient the molecule so that the lowest priority group (H) is pointing away from you. In a Fischer projection, horizontal bonds represent groups coming out of the plane, and vertical bonds represent groups going into the plane. Here, H is on the horizontal line, so it is coming out of the plane.

Step 4: Determine the order of the remaining substituents (Br, Cl, CH₃) in terms of priority. Trace the path from the highest priority (Br) to the second (Cl) to the third (CH₃).

Step 5: Assign the stereochemistry based on the direction of the path traced. If the path is clockwise, the configuration is R. If the path is counterclockwise, the configuration is S. Since H is coming out of the plane, the configuration determined must be reversed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional organic molecules, particularly useful for depicting stereochemistry in carbohydrates and amino acids. In these projections, vertical lines represent bonds that extend away from the viewer, while horizontal lines represent bonds that come towards the viewer. Understanding how to interpret Fischer projections is crucial for assigning stereochemistry accurately.

Absolute Stereochemistry

Absolute stereochemistry refers to the spatial arrangement of atoms in a molecule and is designated using the R/S system. This system is based on the Cahn-Ingold-Prelog priority rules, which assign priorities to substituents attached to a chiral center. By determining the configuration of these substituents, one can classify the stereocenter as either R (rectus) or S (sinister), which is essential for understanding the molecule's 3D structure.

Chirality

Chirality is a property of a molecule that has non-superimposable mirror images, often due to the presence of chiral centers, typically carbon atoms bonded to four different substituents. This characteristic is fundamental in organic chemistry as it influences the behavior of molecules in biological systems, including their interactions and reactivity. Recognizing chirality is essential for accurately assigning absolute stereochemistry in molecules.

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