[Music] all right in this video we're going to go over relative configuration relative configuration is a way for us to describe the spatial position of atoms or groups relative to other atoms or groups in the molecule and the best way to understand this is just by taking a look at a few examples now the rules for assigning relative configuration is different depending on the number of substituents that you have so we'll look at the simpler case where we only have two substituents when we're referring to substituents we're looking at substituents generally about a double bond or a ring so here we can consider an example where we have a double bond and we have two isomers so we can see is you have two very similar looking molecules the only difference is that in one case the bromines are on the opposite side of the double bond in other case the bromines are on the same side of the double bond remember double bonds do not exhibit free rotation so these are different molecules you cannot just rotate a bromine to the bottom and say they're the same so because they are different molecules we can assign a relative configuration to them and the way it's assigned is if your substituents are facing the opposite side then we see that you have the trans configuration whereas if your two groups are facing the same side then you have the cis configuration and this applies not only to double bonds but it can apply also to rings so let's take a look at a couple examples so here i've drawn two molecules and again this molecule is trans because you can see in the ring the two bromine atoms are on opposite sides of the ring in this molecule this is cis because the two bromine atoms are on the same side of the ring now another good thing for you to keep in mind for the mcat in terms of trans and cysts is knowing which one is more stable and generally the trans isomer is more stable than the cis isomer and that's because the trans isomer is less likely to experience steric interactions so let's write this down first the trans isomer is more stable than the cis isomer so if you recall steric interactions refers to groups that come in close proximity to each other when that happens their electron clouds may overlap which is an unfavorable interaction because all electrons are negative so they repel each other when you have the groups that are cis are on the same side they're closer to each other and more likely to interact whereas if they're trans they're on opposite sides and less likely to interact so if they don't have those steric interactions they're going to be more stable all right so that's how it works if you only have two substituents let's now talk about the more complicated case if you have more than two substituents so what we can draw is a double bond and what we're going to have is a hydrogen a methyl group we're also going to have a deuterium and we're going to have a fluorine so now we can see that things are more complicated so the way that we assign the relative configuration here is actually an application of something we've discussed before and that is the first step you're going to do is you're going to assign priorities using the con angle prelog rules and you'll recall in the con angle prelog rolls we rank by atomic number then by atomic mass then by finding the first point of difference if necessary and remembering that if you have multiple bonds that counts as multiple atoms so here if we're looking at atomic number fluorine is going to have the greatest atomic number than carbon deuterium and hydrogen have the same atomic number but deuterium has a greater atomic mass so this is three and this is four so now the assignment here is not cis or trans what you assign is e or z and the nice way to think about it is just think of z like cis and e is like trans so what you can say is if your highest priorities are on the same side then that is z however if your highest priorities are on the opposite side then that will be e and we can see here in terms of our highest priorities on both sides of the double bond which is the two and the one you can see that they are on opposite sides so in this case this molecule is e like trans we can take a look at another molecule which is going to give us a z and that could be a molecule like this all right so if we take a look at this molecule we can rank by atomic number fluorine is number one hydrogen is number four we've got two carbons same atomic number same atomic mass we've got to find the first point of difference this carbon is bound to three hydrogens this carbon is bound to an oxygen and double bonded to an oxygen that double bond counts as two oxygens so this is technically bound to three oxygens oxygen of course has greater atomic number than hydrogen so that means this is going to be priority number two this is priority number three and now we can see the highest priority substituents on each side are on the same side both facing down so this is going to be z all right so that's how you assign relative configuration [Music]