8.2. Question 8.4.E.1 PASS - stereocenter configuration R or S
- Page ID
- 452334
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For the following compounds, assign R or S configurations for each stereocenter:
a) 
b) 
c) 
- Answers
-
a) S
b) R
c) S
- Strategy Map
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Step Hint 1. Assign priorities to substituents and label them one through three. 2. Count in the direction of 1 to 3 and identify if this is clockwise or counterclockwise. Clockwise = R
Counterclockwise = S
- Solution
-
a) Goes counterclockwise around the stereocenter.
answer S
b) Goes clockwise around the stereocenter.
answer R
c) Goes counterclockwise around the stereocenter.
answer S
- Guided Solution
-
Download Guided Solution as a pdf
Guided Solution Hint This is a theory type problem that requires you to assign the correct configuration to a carbon stereocenter (R or S).
See LibreText 8.4 Absolute Configuration R-S Sequence Rules For the following compounds, assign R or S configurations for each stereocenter:
a)
b)
c)
The rules to assign priority to substituents:
1. First, examine the atoms directly attached to the stereocenter of the compound. A atom with a higher atomic number takes precedence over a atom with a lower atomic number. Hydrogen is the lowest possible priority atom, because it has the lowest atomic number.
2. If there are two or more substituents which have the same element directly attached to chiral carbon, proceed along the substituent chains until a point of difference is found. Determine which of the chains has the first connection to an atom with the highest priority (the highest atomic number). That chain has the higher priority.
3. For assigning priority, multiple bonds are treated as if each bond of the multiple bond is bonded to a unique atom. For example, an alkene substituent (CH2=CH-) has higher priority than an ethyl substituent (CH3CH2-).
R vs S configuration
- To identify the configuration of a stereocenter, you must identify if your order of priority goes in the clockwise or counterclockwise direction.
- Clockwise = R
- Counterclockwise = S
Complete Solution:
a)
Bromine is first Priority (atomic number 35)
Oxygen is second priority (atomic number 8)
Carbon is third priority (atomic number 6).
Goes counterclockwise around the stereocenter.
answer S
b)
Iodine is first Priority (atomic number 53)
Fluorine is second priority (atomic number 9)
Carbon is third priority (atomic number 6).
Goes clockwise around the stereocenter.
answer R
c)
Iodine is the first priority (atomic number 53),
Oxygen is the second priority (atomic number 8),
Carbon is the third priority (atomic number 6).
Goes counterclockwise around the stereocenter.
answer S
Check your work!
Check to make sure you are following the guidelines of labeling priority. Ensure you go atom by atom.
Why does this answer make chemical sense?
The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound, and stereocenters are labeled as (R) or (S). The naming rules used provide an unambiguous name that indicates the stereochemistry of the molecule.
(question source from page titled 6.14: Additional Exercises https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_Chem_420_-_Organic_Chemistry_I/Text/06%3A_Stereochemistry_at_Tetrahedral_Centers/6.14%3A_Additional_Exercises, shared under a CC BY-NC-SA 4.0 license, authored, remixed, and/or curated by Libretexts)