Now that we're pretty familiar with Fisher projections it turns out that there's going to be a new way to determine R and S for them, the reason is because converting these structures to bond line and then figuring out our R and S would be pardon my French a bitch, alright? So what we want to do is want to figure out an easy way to figure out R and S and there actually is there's a shortcut that we can use so I want to go ahead and show you guys that right now, alright? So this is basically the way it works we want to determine the location of the lowest priority group, OK? what I mean by location is you want to figure out is it vertical or is horizontal, OK? If and the lowest priority group is always going to be 4 so basically I just want to know where is 4, OK? If 4 is vertical, OK? if 4 is faced vertical then the Chirality is exactly as it looks and I'm just going to go from 1 to 3 I'm going to draw that arrow and that's going to be it, OK? Whereas if 4 is horizontal, OK? Then the Chirality is just going to be flipped so wherever you draw you're just going to take the opposite sign, right? So here I have two examples this would be an example where H is vertical so notice that my fourth priority group is vertical so that means that it's going to be as it looks so I'm just going to say 1 to 2 to 3 it's going a S direction so that's actually going to be the final answer, the final answer for this Chiral's center would actually be S, does that make sense so far? Now let's look at this next one here's my Chiral center, 4 as you can see is now horizontal so that means it's going to be flipped so once again I'm going to do 1 to 2, 2 to 3, 3 to 1 it looks like S but it's actually going to be R because of the fact that it's horizontal, now keep in mind for Fisher projection I'm never swapping groups so I'm not swapping out 1 and 4 or anything like that all I'm doing is I'm just taking the chirality and I'm flipping it if it happens to be horizontal or I'm keeping it if it happens you vertical so in some ways this is actually easier than we learned for the other compounds and when you have a very big Fisher projection that has a lot of Chiral centers you're going to be thankful that you have this method that you can just go really really fast and figure it out, OK? So what I want you guys to do now is go ahead and determine the absolute configurations meaning the R and the S for this Fisher projection using this formula and using this format and then when you're done I'll go and show you guys how to do it.
Determine location of lowest priority group:
If vertical, chirality is as it looks
If horizontal, chirality is flipped.
Determine the absolute configurations for all chiral centers.
Example #1: Determine the absolute configurations for all chiral centers.