Non-Carbon Chiral Centers: Videos & Practice Problems

Chirality can occur in non-carbon atoms capable of forming four different bonds, such as silicon, nitrogen, phosphorus, and sulfur. While nitrogen typically does not form chiral centers due to amine inversion, quaternary amines and certain sulfoxides can be chiral. For R and S naming, the lone pair is treated as group four. Understanding these concepts is crucial for recognizing chiral centers beyond carbon, enhancing comprehension of stereochemistry and molecular interactions.

Understanding Other Chiral Atoms

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.

Determining R and S with Lone Pairs

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 4. So whereas with our other molecules, if we saw a hydrogen, we always assumed, okay, that's the last priority group, that's group number 4. Now there's something even worse than hydrogen, and that is a lone pair because a lone pair doesn't even have an atom in it, right? It's just electrons, Okay? So we're going to 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 ethyl group and change it to a hydrogen. So for all of these, change it to a hydrogen. The reason being that there's a typo here. I, for some reason, I put methyl as 2 and ethyl as 3. It should have been the other way around. So I'm just going to change it to hydrogen and then that will So before according to atomic according to atomic number, right? And what I have here is phenyl versus methyl versus hydrogen. Phenyl is going to beat methyl. Methyl is going to beat hydrogen. But hydrogen actually beats the lone pair. Okay. Now according to our R and S rules, remember that your 4th group always has to be where? On the dash, right? So we're going to swap. We're going to take 1, which is on the dash, and we're going to take 4 and we're going to swap those numbers, right? So that means that what it becomes is that now the 4 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 one, my new 2, my 3. I go around. That looks like what direction? It looks like an S. So I'm going to write S here. But since I had to swap, that means I'm also going to have to swap signs at the end, meaning that this is actually an R chiral center and I'm done. All right. Cool guys. So now you guys are just experts on all chiral centers. They don't even have to be carbon. You even know how to do sulfur, silicon, phosphorus, and nitrogen. Awesome. So let's move on to the next video.

Here’s what students ask on this topic:

What are non-carbon chiral centers in organic chemistry?

Non-carbon chiral centers are atoms other than carbon that can form four different bonds to four different types of atoms, making them chiral. Common examples include silicon, nitrogen, phosphorus, and sulfur. While carbon is the most common chiral center, these other atoms can also exhibit chirality under certain conditions. For instance, silicon, being an analog of carbon, can easily form chiral centers. Nitrogen can be chiral in quaternary amines, and sulfur can be chiral in sulfoxides and sulfonium salts. Phosphorus can also form chiral centers in phosphines.

Why is a neutral nitrogen with a lone pair not considered chiral?

A neutral nitrogen with a lone pair is not considered chiral due to a phenomenon called amine inversion. In amine inversion, the lone pair on the nitrogen can flip between different positions, making it impossible to determine a fixed spatial arrangement of the groups attached to the nitrogen. This inversion occurs with very low energy (around 24 kJ/mol), making it common at room temperature. As a result, the nitrogen does not maintain a consistent chiral center, and thus, it is not considered chiral.

How do you assign R and S configurations to non-carbon chiral centers?

Assigning R and S configurations to non-carbon chiral centers follows the same rules as for carbon chiral centers, with one additional rule: the lone pair is always considered the lowest priority group (group 4). First, assign priorities to the groups attached to the chiral center based on atomic number. The lone pair is always group 4. Then, orient the molecule so that the group with the lowest priority (the lone pair) is in the back. Determine the sequence of the remaining groups (1, 2, 3) and assign R (clockwise) or S (counterclockwise) configuration based on their arrangement.

Can sulfur form chiral centers, and under what conditions?

Yes, sulfur can form chiral centers under certain conditions. Sulfur can be chiral in compounds like sulfoxides and sulfonium salts. In sulfoxides, sulfur is bonded to two different R groups, a double-bonded oxygen, and a lone pair. The lone pair does not easily invert, making the sulfur chiral. In sulfonium salts, sulfur has three different groups and a positive charge, which also prevents inversion of the lone pair, maintaining chirality. The high energy required for inversion in these cases ensures that the sulfur remains chiral at room temperature.

What is amine inversion, and how does it affect chirality?

Amine inversion is a process where the lone pair on a nitrogen atom flips between different positions, causing the spatial arrangement of the attached groups to change. This inversion occurs with very low energy (around 24 kJ/mol), making it common at room temperature. Because the lone pair does not stay in a fixed position, the nitrogen cannot maintain a consistent chiral center. As a result, neutral amines with a lone pair are not considered chiral. However, quaternary amines, where nitrogen forms four bonds and has a positive charge, do not undergo inversion and can be chiral.