Textbook Question
Indicate whether each of the following structures is (R)-2-chlorobutane or (S)-2-chlorobutane:
c.
d.
781
views
5. Chirality
R and S Configuration
Problem 50a
Textbook Question
Assign the absolute stereochemistry for each of the following molecules drawn in their Fischer projection.
(a) 
Verified step by step guidance1
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.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
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.
Recommended video:
Guided course
09:56
Monosaccharides - Drawing Fischer Projections
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.
Recommended video:
1:38Polymer Stereochemistry Concept 1
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.
Recommended video:
Guided course
05:10What is chirality?
Related Videos
Related Practice