[Music] all right in this video we're going to go over absolute configuration this is a difficult topic for a lot of students so hopefully this video is going to be able to help you out so first of all absolute configuration is a way for us to describe the spatial position of atoms or groups in a chiral center as you'll recall from one of our previous videos chiral centers are atoms that are bound to four different substituents an absolute configuration is referring to designating each chiral center as an r or s configuration now one thing i do want to mention is that absolute configuration does not depend on other atoms or groups in the molecule this is different from another topic we're going to discuss which is relative configuration in relative configuration you assign the configuration based on the position of one group relative to other groups in the same molecule that's not how absolute configuration works okay so now how do we assign r or s well the first thing you want to do is you want to take a look at the four different substituents on your chiral center and you want to assign priorities using the con angled prelog rules now you want to be very careful here because these steps are very important to follow in order to get the correct absolute configuration so make sure you pay attention the first step is you're going to take a look at your four substituents and you're going to rank by atomic number as you recall atomic number is the number of protons in your atom so let's take a look at an example here we've got a molecule on the top left and right in the center we have our chiral center this atom is bound to four different substituents a hydrogen a hydroxyl and amine in a methyl group so if we're assigning priority by atomic number we can see between oxygen nitrogen carbon and hydrogen on the periodic table oxygen has the greatest atomic number so that means oxygen is going to be priority number one nitrogen has the second greatest atomic number so let's do carbon is three and hydrogen is four because it has the smallest atomic number of just one now there are some situations where ranking by atomic numbers is not going to be enough for instance if we take a look at this molecule over here our chiral center is this atom and if you want to rank the priorities by atomic number you've got an oxygen you've got a carbon you've got a hydrogen and a deuterium the oxygen and the carbon you can do right oxygen certainly has the greatest atomic number carbon has the second greatest atomic number but hydrogen and deuterium are isotopes and isotopes have the same number of protons for different numbers of neutrons and they both then have one proton so we can't rank them by just atomic number so in these situations where you have a situation where the atomic number is going to be the same the next step of the con angle free log plurals is to rank by atomic mass so we want to look at the atomic mass of deuterium and hydrogen and we know that deuterium has a greater atomic mass than hydrogen because deuterium has an extra neutron than hydrogen alone so that would allow us to rank deuterium as priority number three in the hydrogen as priority number four all right but of course we can still have even more complicated situations what is the atomic number and the atomic masses the same and we do have a situation like that over here our central atom is our chiral center and we've got the hydrogen we've got a hydroxyl group then we have two carbons right the carbon atoms they're both carbon they have the same number of protons they have the same atomic mass so that's where we have to follow the next step of the con angle prelog rules which is you have to find the first point of difference and when i say find the first point of difference that means you have to look at what atoms are bound to those two substituents with the same atomic number and mass number and you're going to compare those substituents and when you compare those substituents you're going to repeat a and b which means you're going to compare the atomic number and the mass number again now one other thing that's going to be important is very very often when you're finding the first point of difference you might encounter a double bond or a triple bond there's a specific way you treat these according to the con angled prelog rules and that is multiple bonds count as multiple atoms so for instance if you have a substituent that is double bonded to carbon it doesn't count as one carbon that counts as two carbons all right so let's apply that additional rule to this molecule over here starting by atomic number we can say that the hydroxyl group the oxygen is number one the hydrogen is number four it's got the lowest atomic number these two carbons as we said again or said earlier same atomic number same atomic mass so to look at the first point of difference this carbon you can see that it's bound to an oxygen and two hydrogens this carbon on the top it's double bonded to an oxygen but that double bound to an oxygen remember we count that as two oxygens so you've got two oxygens in a hydrogen so now in comparing these two you want to find the first point of difference and again we're going by a and b so you're going to start with atomic number again so they both have an oxygen but the first point of difference is that on the top you have an oxygen whereas on the bottom you have a hydrogen and since oxygen has greater atomic number than hydrogen that means that this substituent on the top is priority number two and this is priority number three all right i've got one more example here on the bottom right to get more practice for the signing priorities our chiral center is going to be this atom right here you have to be a little careful because it looks like it only has three substituents but you know there's a hydrogen that's not drawn in so we'll go ahead and draw in that hydrogen and now we can see we have four different substituents if we want to assign priorities to them we start by atomic number so among the four atoms you've got a hydrogen two carbons and a nitrogen nitrogen has the greatest atomic number so that's number one hydrogen has the smallest atomic number that's going to be number four these two carbons it's the same situation again same atomic number same atomic mass we have to find the first point of difference so this carbon on the top we can see that it's bound to a sulfur but it's also bound to two hydrogens that aren't drawn in so we have a sulfur and two hydrogens here we have a carbon and it's bound to an oxygen and it's double bonded to another oxygen again multiple ones can has multiple atoms so this counts as two oxygens so in total we have a carbon down to three oxygens on the bottom so to compare these two again we go back to atomic number and it's true you've got three oxygens versus a sulfur and two hydrogens but sulfur has greater atomic number than oxygen so because of that the substituent on the top is priority number two and this is priority number 5. all right so you want to make sure you're following these rules carefully you don't want to just you know look at which group has greater mass because that's going to get you the wrong absolute configuration there are these specific rules to follow all right so now that we know how to assign priorities to the different substituents on the chiral center we now need to follow the next step now the next step a lot of you will recall from your organic chemistry classes where you want to put the lowest priority substituent in the bath and then you want to look at your substituents one two and three if they're clockwise it's r if they're counterclockwise it's s that works and it's perfectly fine but you have to remember one of the problems with that approach is if your substituent is not facing the back all right if your substituent is not facing away from you you know in this case it's in the plane of the page you have to redraw that molecule all right so you have to redraw the molecule with the hydrogen facing the back or you have to be able to visualize the molecule in three dimensions pretend you're looking at the molecule from the bottom and that's very complicated right so the approach that i'm going to show in this video it's a way for you to assign absolute configuration without ever having to redraw the molecule and that's the right hand rule so we have over here step two step two is the right hand rule you're used to hearing about this for physics but now we're going to have a right hand rule for organic chemistry as well there are three steps here and the first step your thumb is going to point to priority number four the second step your fingers are going to point towards priority number one and finally you're going to curl your fingers and when you curl your fingers and your fingers curl from 1 to 2 to 3 then we're going to say that your absolute configuration is r however if you curl your fingers and you curl from one to three to two then that's going to be s all right so let's go ahead and try to apply this to a few examples for our first molecule we want to orient our thumbs and fingers properly but before i i just want to review real quick what these lines dashes and wedges mean so remember if you have a line it means that you're in the plane of the page so that means hydrogen is in the plane of the page it points straight up the hydroxyl group is in the plane of the page and points down into a left wedges mean they're coming out of the page so this is coming out of the page towards you and towards the bottom right dashes mean they're going into the page so that means that's going into the page away from you and also facing the right so keeping those in mind as we said your thumb points towards priority number four so here parenting number four is in the plane of the page and points straight up your fingers point towards priority number one which is also in the plane of the page and it's pointing down into the left so you should have your thumb and fingers oriented like this you're then going to curl your fingers and you're going to look at what you hit next from one and you can see when you curl your fingers you're going to hit two and then three so this time when you curl your fingers it's one two three so this is going to give you r for the absolute configuration let's take a look at another example here on the top right again thumb points towards priority number four this time it's going into the plane of the page away from you and also to the top right so your thumb should be pointing away from you towards that hydrogen your fingers should be in the plane of the page and towards the bottom left so your hand should be oriented like this you're then going to curl your fingers and when you curl your fingers you're going to see that you curl up and when you curl up you're going to hit priority number three and then eventually prior to number two so because you're curling one three two this is going to be s for its absolute configuration let's take a look at another one here on the bottom left this one's a little bit trickier because the way it's drawn is not what the molecule actually looks like right that plus side doesn't mean all those substituents are in the plane of the page you have to remember this is a fischer projection and in a fischer projection you need to remember the horizontal substituents are coming out of the page and the vertical substituents are going into the plane of the page so one of the common ways to memorize this is to just pretend that someone is coming and giving you a hug so that means the horizontal substituents are coming towards you and the vertical substituents are going away from you all right so keeping that in mind our thumb is going to be priority number four that means it should be facing the right and coming out of the page towards us our fingers should be pointing to party number one so they should be facing the left and also towards us so this is what your hands should look like when you curl your fingers you're clearly crawling down so the next substituent you're going to hit is priority number three and eventually you'll curl up the priority number two so in this case we're curling one three two so this is also going to be s for its absolute configuration you