Absolute Configuration
Absolute configuration in chemistry refers to the spatial arrangement of atoms within a chiral molecular entity. It is typically relevant in organic molecules where carbon is bonded to four different groups, creating two possible enantiomers. The most common labeling method uses the descriptors R (rectus) or S (sinister) and is based on the Cahn–Ingold–Prelog (CIP) priority rules[4].
The CIP rules are a standard process to completely and unequivocally name a stereoisomer of a molecule. The purpose of the CIP system is to assign an R or S descriptor to each stereocenter in a molecule[6].
To determine the absolute configuration of a molecule, the following steps are typically followed:
- Assign priorities to the groups attached to a stereocenter based on atomic number (1 = highest priority, 4 = lowest priority).
- Rotate the molecule so that the lowest priority group faces away from you.
- Draw a circle from the highest priority group to the next highest priority to the next highest.
- If your rotation is clockwise, it is an R stereocenter. If it is counterclockwise, it is an S stereocenter[1].
It's important to note that the R/S system only applies to chiral molecules. If a molecule is achiral, the R/S system doesn't apply[2].
In biochemistry, there is an additional nomenclature system that you may come across that is a bit antiquated. However, for non-laboratory purposes, it is beneficial to focus on the R/S system[1].
The absolute configuration is different from relative configuration, which are things like cis and trans. Both of the following compounds can have a trans relative configuration, but they can have the exact opposite absolute configurations. These are enantiomers[1].
The absolute configuration of a molecule can be experimentally determined using X-ray diffraction analysis or chemical correlation with a molecule whose structure has already been determined via X-ray diffraction[7]. However, these methods have limitations and are typically used in laboratory settings.
Citations:
[1] https://chem.libretexts.org/Courses/Providence_College/Organic_Chemistry_I/07:_Stereochemistry/7.03:_Absolute_Configuration
[2] https://www.khanacademy.org/science/organic-chemistry/stereochemistry-topic/optical-activity/v/rs-system-new
[3] https://chem.libretexts.org/Courses/Purdue/Purdue:_Chem_26505:_Organic_Chemistry_I_(Lipton)/Chapter_3._Stereochemistry/3.6_Cahn-Ingold_Prelog_Rules
[4] https://en.wikipedia.org/wiki/Absolute_configuration
[5] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_I_(Liu)/05:_Stereochemistry/5.03:_Chirality_and_R_S_Naming_System
[6] https://en.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rules
[7] https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Chirality/Absolute_Configuration_R-S_Sequence_Rules
[8] https://youtube.com/watch?v=kFD6hzLseVs
[9] https://www.khanacademy.org/science/organic-chemistry/stereochemistry-topic/optical-activity/v/cahn-ingold-prelog-system-for-naming-enantiomers
[10] https://www.chem.ucalgary.ca/courses/350/Carey5th/Ch07/ch7-5.html
[11] https://en.wikibooks.org/wiki/Organic_Chemistry/Chirality/R-S_notational_system
[12] https://youtube.com/watch?v=rMsEuzT5t0s
[13] https://youtube.com/watch?v=5Zg9FYKFAlQ
[14] https://www.vanderbilt.edu/AnS/Chemistry/Rizzo/chem220a/priority.pdf
[15] https://www.sciencedirect.com/topics/chemistry/absolute-configuration
[16] https://study.com/academy/lesson/cahn-infold-prelog-priority-rules-in-chemistry.html
[17] https://www.chemistrysteps.com/how-to-determine-r-and-s-configuration/
[18] https://www.masterorganicchemistry.com/2016/10/20/introduction-to-assigning-r-and-s-the-cahn-ingold-prelog-rules/
[19] https://youtube.com/watch?v=yzfcrwJ37kI
[20] https://www.masterorganicchemistry.com/2017/01/17/determining-rs-2-the-method-of-dots/