Non-Carbon Chiral Centers

Concept: Concept: Understanding Other Chiral Atoms

6m
Video Transcript

Alright guys in this video I kind of want to have the final word on Chirality because it turns out that there's one assumption we've been making that isn't totally correct which is that a Chiral center is always going to be a carbon, it turns out guys that there is a such thing as non-carbon chiral centers and that's what we're going to focus this page on so Chirality can exist on atoms other than carbon, technically any atom that is capable forming four different bonds to four different types of atoms could be a Chiral center, now carbon is definitely the most common situation and you can probably get through all of Orgo just looking at carbon but some professors some textbooks may try to you know quiz you a little bit harder on these other atoms that can also form 4 bonds and become Chiral, OK? So, what are these atoms? Well silicon, silicon is right under carbon, right? So, silicon is like a no brainer that we definitely chiral but also nitrogen which in some cases makes 4 bonds, phosphorous which in some cases makes 4 bonds and Sulphur which in some cases makes 4 Bonds, OK? So, these are all atoms that we want to potentially be looking at as Chiral centers but again rarely but it can happen, OK? Now I have all these different types of Chiral centers drawn out for you but one of these is not chiral and the one that's not chiral is any neutral nitrogen with a lone pair, now I just told you guys that nitrogen can be chiral so why am I saying that? Well let's look at what an Amine looks like when it has a lone pair, well you've got the nitrogen you've got your 3 groups for sure 1, 2, 3 now does your lone pair count as a fourth group? That's basically the question is the lone pair a fourth group? Well it turns out that for a neutral Amine the answer is no and the reason is because for it to count as a fourth group it should stay in one place if it moves or it sort of switching places it can't be a fourth group and a Amines have this amazing ability to do something called Amine inversion, what Amine inversion means is that the lone pair isn't always stuck to the top, it can also flip to the bottom so that means that this isn't going to be a Chiral molecule because I never know exactly where that lone pair is, it could be at the top it could be at the bottom the way I like to think of it is it's almost like an umbrella, right? So imagine that I'm holding this umbrella above my head and I've got the R groups facing down and then after it inverts my umbrella goes up and I break my umbrella so this is like the bottom one is kind of like your broken umbrella situation, OK? Well Amine inversion is very easy to do because it has very low energy, OK? 24 Kilojoules per mole is all you need to invert that lone pair which in most cases at ambient temperature there's plenty of energy around to do that so it turns out that a typical Amine is not chiral so don't freak out you're pretty much never going to have a chiral Amine so don't worry too much about that but Sulphur is in a similar situation notice that like a Sulphonium salt, OK? This would be a Sulphur that has a positive charge because it's missing some electrons but notice that if you put sulfur in the same position where it's got those three groups and it's got a lone pair the energy for that lone pair to invert is much much higher it's way higher so in room temperature this is not going to invert So that means that this is chiral because you've got your group 1 group 2 group 3 and I know that my lone pair is going to stay stuck right there, why? Because I don't have enough energy in the environment to invert that lone pair, OK? Now if you're really interested in thermodynamics and stuff there is a temperature at which this loses its Chirality, right? If you increase the temperature enough then the ambient energy will be sufficient to make that lone pair invert but again that's a very very high temperature so in most conditions this is a chiral molecule now let's look into the other ones really quick so what kind of amine or what kind of nitrogen is chiral? A Quaternary Amine so that means any nitrogen that has 4 bonds to different things so in this case 1, 2, 3, 4 that definitely could be Chiral again it's only going to be nitrogen with a plus charge because any time nitrogen has four bonds it has a plus it's not happy, right? So also Sulfoxides so Sulfur that has a sulfoxide as a functional group we have 2 R groups, you have a double bond O and you have a lone pair this can be chiral as well because it's difficult to invert 1, 2, 3 and then this counts as group 4, Phosphines so in Orgo 2 we're going to deal with some Phosphines and what you're going to see is that 1, 2, 3 these are chiral because phosphines are much more difficult to invert than nitrogen, so phosphines kind of like our Sulfonium salt this is going to be a very high energy of inversion so this is going to stay exactly the way it is and then finally the no brainer silicon, silicon is an analog of carbon meaning that it's an atom that behaves very similar to carbon so this would be a Chiral center if it was a carbon it's also a chiral center if it's a silicon so I'm going to go ahead and check that off, making sense? Awesome.

Concept: Concept: Determining R and S with Lone Pairs

3m
Video Transcript

This might be complete overkill but I'm just adding this just to be comprehensive what if your professor were to ask for the R and S naming of a non-carbon Chiral center? Well it turns out that it's very easy to do that's why I'm teaching you there's just one extra rule, you're going to use the same R and S naming that we used before but the extra rule is that the lone pair is always group number four so whereas with our other molecules when we saw hydrogen we always assumed OK that's the last priority group that's group number four now there's something even worse than hydrogen and that is a lone pair because a lone parent doesn't even have an atom in it, right? It's just electrons, OK? So I'm going to show you guys how to do R and S with a phosphine but I do have to make one correction go ahead and take this Ethel group and change it to a hydrogen so for all of these change it to a hydrogen the reason being is that there's a typo here for some reason I put Methyl as 2 and Ethel as 3 should have been the other way around so I'm just going to change it to hydrogen and that will actually be the third group so let's do that everywhere cool. So notice that you would use your prioritization just like before according to atomic number, right? And what I have here is phenol vs methyl vs hydrogen, phenol is going to beat methyl, methyl is going to beat hydrogen but hydrogen actually beats the lone pair, OK? Now according to our R and S rules remember that your fourth group always has to be where? On the dash, right? So we're going to swap we're going to take one which is on the dash or we're going to take four and we're going to swap those numbers, right? So that means that what it becomes is that now the four is in the back so I don't care about it anymore and now I just go ahead and I do my rotation so this is my new 1, my new 2, my 3 I go around that looks like what direction? It looks like an S, right? So I'm going to write S here but since I had to swap that means I'm also going to swap signs at the meaning that this is actually an R Chiral center and then done, alright? So now you guys are just experts on all chiral centers they don't even have to be carbon you even know how to Sulphur, silicon, phosphorus and nitrogen. Awesome so let's move on to the next video.