What is meant by absolute configuration?

Answer 1

According to the Internatioinal Union of Pure and Applied Chemistry (IUPAC), every molecule (compound/chemical structure) has to have it's own specific name. Noted: some molecules have a chemical name and a common name, for example, isopropyl alcohol also known as rubbing alcohol, respectively, the chemical name and the common name. Yet, every molecule has to have a specific name, otherwise confusion occurs when two different molecules are named with the same name! In the case of a pair of Enantiomers, of which, is the only case in which Absolute Configuration (RS Configuration) is determined to name two elements (the pair of Enantiomers). Remembering that a pair of Enantioimers only vary in their 3-dimensional spatial arrangement. In order to do this, the Absolute Configuration (RS Configuration) is determined.

To determine the Absolute Configuration (RS Configuration), the Stereogenic Carbon (also called a Stereocenter) has to have 4 different elements single bonded to the carbon element (stereogenic carbon). Prioritize the elements using the Cahn, InGold and Prelog (CIP) rules, once that is done you will need to, according to the CIP rules, draw a circle (arrow) from the #1 priority element (group) to the 2nd to the 3rd and take note of which direction the arrow is going, that is, to the right or the left. If the arrow goes to the right it is denoted as R, if it goes to the left it is denoated with an S. One Enantiomer will be R and the other will be S, and assigned to the name of the molecule. Therefore, each Enantiomer of the pair of Enantiomers, has a distinct name using an R or an S.

The McLaurin Method describes a method using your hand, arm and three digits to determine the correct configuration of each stereocenter (stereogenic carbon). It is simple to learn and covers more tha 5 different molecular representations with several examples, so the professor or the molecule(s) in question will never fool you on a quiz, exam or final exam, DAT or MCAT. Yet, you will have to learn the CIP rules on your own, I did not discuss how to use the CIP rules on my website, yet how to determine the configuration of a stereocenter.

NOTE: The pair of Enantiomers (2 molecules differing by 3-dimensional spatial arrangement), discussed in the chapter on Stereochemistry, only differ by their rotation of plane polarized light. Just because the configuratin is, for example, to the right, denoted by an R, does Not mean it rotates plane polarized light to the right, it may and it may not! Those rotations of light have been pre-determined and are on a chart that you should and would have at your side. Also, if the configuration is to the left, denoated by an S, does Not mean it rotates plane polarized light to the left, it may or it may not! Once again, those rotations of light have been pre-determined.

Good Luck with the method I put together, you can do a search for McLaurin Method and you will find it on the first page of search results, and it is straight forward and will be EASY to remember and use throughout your years of learning Stereochemistry, Isomers, Streeoisomers and a pair of Enantiomers

PS: Absolute Configuration references the 3-dimensional spatial arrangement of elements in space. The pair of Enantiomers are assymeteric, non-superimposable on it's mirror image and have Chirality and Handedness.