Alright, guys. In this video, I 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: a chiral center is always a carbon. It turns out that there is such a thing as non-carbon chiral centers. And that's what we're going to focus on this page. So chirality can exist on atoms other than carbon. Technically, any atom capable of forming 4 different bonds to 4 different types of atoms could be a chiral center. Now, carbon is definitely the most common situation, and you could probably get through all of Organic Chemistry just looking at carbon. But some professors, some textbooks may try to quiz you a little bit harder on these other atoms that can also form 4 bonds and become chiral.
So, what are these atoms? Well, silicon, for instance, is right under carbon, so silicon is like a no-brainer. It could definitely be chiral. But also nitrogen, which in some cases makes 4 bonds, phosphorus, which in some cases makes 4 bonds, and sulfur, which in some cases makes 4 bonds. These are all atoms that we want to potentially be looking at as chiral centers. But again, it's rare, but it can happen.
Now I have all these different types of chiral centers drawn out for you. But one of these is not chiral. 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. You've got the nitrogen, and you've got your 3 groups for sure, 1, 2, 3. Now, does your lone pair count as a 4th group? That's basically the question. Is the lone pair a 4th 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 4th group, it should stay in one place. If it moves, if it starts switching places, it can't be a 4th group. And 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, 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 almost like an umbrella. 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 it's like a broken umbrella situation.
Well, amine inversion is very easy to do because it has very low energy. Okay? 24 kilojoules per mole is all you need to invert that lone pair, which in most cases, in 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 sulfur is in a similar situation. Notice that like a sulphonium salt, this would be a sulfur 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 3 groups and it's got a lone pair, the energy for that lone pair to invert is much 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.
Now, if you're really interested in thermodynamics and stuff, there is a temperature at which this loses its chirality. 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 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 happen with the nitrogen with a plus charge because anytime that nitrogen has 4 bonds, it has a plus. It's not happy, right? So also sulfoxide. So sulfur that has, it's called a sulfoxide. There's a functional group where you have 2 R groups, you have a double bond to 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 Organic Chemistry 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 sulphonium salt, are going to have a very high energy of inversion. So this is going to stay exactly the way it is. And 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.