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Aldehyde introduction

Aldehyde introduction. Created by Sal Khan.

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Video transcript

We've done several videos now on amines, which I forgot to mention, and this is probably the most fun notion about amines, is that they tend to be smelly. Let me write this down. Amines tend to be smelly. And I can make the smelly smell a little bit. And in general, in their gaseous state, they'll often smell like ammonia, so it's a very strong smell, or in their liquid state, they usually smell like fish, or even dead fish. They smell like fish. And, in particular, if you look at trimethylamine, so this is-- let me draw it like this. H3C, you have your nitrogen. You have CH3 and then you have your CH3. And you can even draw the lone pair. This is trimethylamine. This is the fish smell. This right here is the fish smell. Now, the whole reason why I brought this up is because in this video, we're going to introduce a new type of molecule, and that's an aldehyde. And I want to contrast their smells. Now, some of the smaller aldehydes still have a pretty strong smell, in particular, formaldehyde. And we'll talk a little bit more about why this is an aldehyde. So this right here is formaldehyde, and that's the common name. And you've probably seen it used as a kind of preservative. Maybe even in your biology class, if there's like a dead frog in a solution, it's probably inside of formaldehyde. Let me write this down. Formaldehyde. Once again, this is the common name. I'm going to teach you in a second how to systematically name these things. But when you have larger aldehydes, they actually can have a pretty sweet or even a rosy smell. So you have something like benzaldehyde, which you actually saw when we started the benzene derivatives. So this is benzaldehyde right over here. Let me write this down. This is benzaldehyde. And then this molecule here, just when I tell you what it's called, you'll probably guess what it smells like. So this has a benzene ring like this. And then you have one, two, three carbons. The last one's double bonded to an oxygen and then a hydrogen like there, and they have a double bond right over here. And this is called cinnamaldehyde. And as you could guess, this smells like cinnamon. And, in fact, this is the molecule in cinnamon that gives it its smell, although you don't want to have large quantities of it or it might be poisonous. But in cinnamon, it's a very pleasant thing. So larger aldehydes tend to have kind of a nice, rosy, sweet, flowery smell. The smaller ones tend to be kind of pungent. If you open up that jar in your biology class and smell that frog, it will not be a pleasant smell. So now that I've talked about smell enough, I think, let's talk about what makes an aldehyde an aldehyde. And you might even see a pattern here. In all of these aldehydes that I've drawn, we have a carbon double bonded to an oxygen. And actually, this part of it right here, and we'll see this over and over again, this is called a carbonyl group. So that is a carbonyl group. This right here is a carbonyl group. And you even see it over here. But that by itself is not the distinctive feature of an aldehyde. You'll see that in other types of molecules. Let me write it here. Carbonyl. What's distinctive about an aldehyde is, attached to the carbon in the carbonyl, you have a hydrogen. And sorry, I forgot to draw this hydrogen here on benzaldehyde. You have a hydrogen here. So, in general, an aldehyde is something that looks like this. You have a carbonyl group. You a carbonyl group. You have a hydrogen, and then you just have some other type of carbon chain. This right here is the simplest possible aldehyde, and actually, this chain ends up being just another hydrogen. Now, these three that I've shown right here, these are their common names, and these are the way that most people will talk about formaldehyde, benzaldehyde, or cinnamaldehyde, so they're just kind of good to know. And I'll show you one more. And this is a pretty important one. Let me do it down here so we have some space. So another one looks like this, so bonded to a CH3 over here and then a hydrogen. This right here is called acetaldehyde. And I just wanted to expose you to these common names because this is what people normally use for these molecules. Now, there is an IAPUC-- I always forget the acronym. There is a systematic way to name them. It's actually pretty straightforward. You just look at the longest carbon chain, and you always start numbering at the carbon that's in the carbonyl group. And it's always going to be at the end of the chain because that's always going to be bonded to a hydrogen. Actually, let me just make clear that this is also an aldehyde. You have your carbonyl group, and on one end, you have a hydrogen, just like that. Now, the systematic way of it, you just look at the longest carbon chain. Over here, there's just one carbon. So here, you would call this, the systematic name, you would call this methanal, not methanol. Methanol would mean you would have an OH group. But since you have this double bond and you have a hydrogen, or you could say this double bond is at the end of the carbon chain, this is an aldehyde. So the systematic name is methanal, not methanol. Let me make that clear. This is methanal right there. This one, I won't do cinnamaldehyde or benzaldehyde, because this is really the only way that people name them. Actually, frankly, formaldehyde, people never call it methanal. And acetaldehyde, they'll never call it what I'm about to name it, but the systematic name is, the longest carbon chain, you have one, two carbons, so it is ethanal. Ethanal, I don't want to pronounce it incorrectly. So let's do a couple more of these, just to make sure we have a decent understanding of the systematic names here. And in general, if you have a really long chain, the systematic names are what is used. So this thing right here, what would you call it? It's clearly an aldehyde. You have a carbonyl group, a hydrogen, and you have one, two, three carbons. So three carbons, the prefix is prop-, so it's propanal. It's an aldehyde. If we want to do something slightly more complicated, let's do something like this. What would this be called? Let's see, we have one, two, three, four, five. We have five carbons. So it will be pent-. Penta-, and then it's obviously an aldehyde, so it's pentanal. And you always assume that you start numbering at the carbon in the carbonyl group, so one, two, three, four, five. And on the two carbon, you have a methyl group. So this is 2-methyl. So you have a methyl group right here. So this is going to be 2-methylpentanal.