However, this is very important, and it is a requirement when assigning the R and S configuration, that The lowest priority does not affect the direction of the arrow. Is it part of the game and how do you use it? So, we discussed the roles of priorities 1, 2, and 3 but what about the lowest priority? We did not mention anything about the arrow going to it. If the arrow goes counterclockwise, the absolute configuration is S.Īnd this is another important thing to remember:Īll the chirality centers in enantiomers are inverted (every R is S, every S is R in the enantiomer).
Starting from the bromine and going to the oxygen and then the carbon, we can see that this time the arrow goes counterclockwise. The priorities are still the same since all the groups around the carbon are the same. Now, let’s see what would be the absolute configuration of the enantiomer: So, remember: Clockwise – R, Counterclockwise – S. If the arrow goes clockwise, like in this case, the absolute configuration is R.Īs opposed to this, if the arrow goes counterclockwise then the absolute configuration is S.Īs an example, in the following molecule, the priorities go Cl > N > C > H and the counterclockwise direction of the arrow indicates an S absolute configuration: So, based on this, bromine gets priority one, the oxygen gets priority two, the methyl carbon is the third and the hydrogen is the lowest priority-four:ĭraw an arrow starting from priority one and going to priority two and then to priority 3: The higher the atomic number, the higher the priority. Give each atom connected to the chiral center a priority based on its atomic number. In our molecule, we only have one carbon with four different groups and that is the one with the bromine and we are going to assign the absolute configuration of this chiral center.įor this, you need to follow the steps and rules of the Cahn-Ingold-Prelog system. These are called chirality centers (chiral center, stereogenic center).
To assign the absolute configuration, we need to first locate the carbon(s) with four different groups (atoms) connected to it. Let’s see how it works by looking first at the following molecule and we will get back to the 2-chlorobutane after that:Īssigning R and S Configuration: Steps and Rules
So, we need an extra piece of information to distinguish enantiomers (and other stereoisomers) by their names properly addressing the stereochemistry as well.Ĭahn, Ingold, and Prelog developed a system that, regardless of the direction we are looking at the molecule, will always give the same name ( unlike the wedge and dash notation).Īnd that is why this is also known as the absolute Configuration or most commonly referred to as the R and Ssystem. The problem with the wedge and dash notation is that it is not a universal approach and quickly loses validity when we simply look at the molecule from the opposite direction: These molecules are not the same compound – they are non-superimposable mirror images which are known as enantiomers: However, they don’t look exactly the same as the Cl atom points in different directions – wedge and dash. If we name these two alkyl halides based on the IUPAC nomenclature rules, we get the name as 2-chlorobutanbe for both: What is the R and S Configuration and why we need it?
0 kommentar(er)
