Charles Collingwood teaches the Cahn-Ingold-Prelog rules for assigning R and S configurations to chiral centers. Rank substituents by atomic number of attached atoms, then trace from highest to lowest priority: clockwise is R, counterclockwise is S. He works through examples like NH2 > COOH > CH3 to show how to handle ties and double bonds.
Assigning absolute configuration to enantiomers. The R and S configuration of enantiomers. What we're looking at is we're looking at putting an absolute configuration on a chiral center and assigning it an R or an S value, which stands for rectus and sinister. Rectus means right or clockwise, and sinister means anti clockwise. So what we take is we take our enantiomer, enantiomer here with the chiral center label with a star, and we have to assign a priority to each of these groups. Now, you can see the lowest priority group here is a hydrogen. Priorities depend upon the atomic number of the atom that's attached to the chiral center. So hydrogen's got atomic number one, so it's the lowest priority. So we mentally rotate this molecule so the hydrogen is hidden by the carbon. We get diagram two. And then what we'll see is, we'll see we have three groups which we can effectively arrange in a circle. Now, the highest priority group here is the amine group, because nitrogen has got an atomic number of seven. This is followed by the carbon of the carboxylate group, because carbon is followed by two oxygens. So carbon is effectively attached by a double bond to one oxygen, which counts twice and and a single bond to another oxygen, whereas the other carbon is only attached to hydrogens. So the priority moves from NH2 to COOH to CH3. That gives us a clockwise rotation. And so this would be designated rectus for clockwise. Priorities are assigned according to the Carnengel prelog priority system, where we just use the atomic number. So CH3 is a carbon, which is an atomic number of 6, and attached to 3 hydrogens, which would be 1, 1, 1. The CH2Cl group is 6 attached to atomic number 17 and 2 ones. So the CH2Cl group has a higher priority than the CH3 group. If we have double bonds involved, then we can count the atomic number twice. So aldehyde group cho is effectively C, which is 6, attached to O, twice, which is 8. 8. So the priorities of the aldehyde group are 6881. The carboxylate group are 6881. So carboxylate group takes priority over the CHO group here. If we have a look at two examples. The first example you can see we look at the priorities. You can see the hydrogen is the lowest priority. So we mentally rotate so that we're looking down towards the carbon with the hydrogen behind it, and we find that we have on the top an ethyl group to the right OH group and to the left a CH3 group. So the priorities, well the OH group takes the highest priority because that's an 8. The ethyl group is 6. With the 6 attached we're going to. Whereas the methyl group is a 6 attached to a 1. So the priorities move in an anti clockwise direction. So this would be S for sinister. You can see the picture we have here and we rotate them round priorities anticlockwise. S for sinister. The same thing we do here. This time if we have a look at these, we've got the priority. The lowest priority to the highest priority is the ester group. Ester. Ester group, sorry, is the coch three group, the acyl group, effectively that takes the highest priority because it's carbonyls688. Then we have the carbon attached to an oxygen 6, 8 and then we have the methyl group. So once again this is anti clockwise or sinister.
