Hey guys! Let me teach you how to name aldehydes. Like many other functional groups, aldehydes are going to be modifying the root name of the carbon chain. Take for example alcohol. We know that alcohol is you take out the e and you replace the e of the alkane with the suffix -ol. Then ethane would become ethanol. It’s the same thing with aldehydes. We’re just going to change one letter. It's going to be the suffix -al.
That actually could be a little confusing because those suffixes sound really similar. Ethanol would be an alcohol. Ethanol would be a two-carbon aldehyde. You have to be careful about how you say those vowels because they are pretty similar to each other.
It turns out that there is one major difference between aldehyde and other functional groups. It has to do with the definition of what an aldehyde is. If you think about it, aldehydes are by definition terminal carbonyls. They’re always at the end of the chain. What that means is that we don't need to number them the same way we would number other functional groups. In fact, it does not receive a location. You would never say that something is 1-ethanal. Why? Because the carbonyl has an H on it. Remember, aldehydes have an H on them. That’s means that by definition on the end of the chain so you don’t need to name the location.
Instead of saying 1-hexanal, you just say hexanal because there’s no possibility of having 2 or 3 hexanal. Why? If it was a 2 or 3, that would be inside of the chain and that’s called a ketone. It becomes a different functional group if it’s internal. But if it’s terminal, then it’s an aldehyde and we don't need to name the location. We just assume it’s at one of the ends.
There are going to be some times where you're not able to put the aldehyde inside of the root and it has to stick off. A good example of this is ring. We’re going to do a naming example in a second. Whereas you notice the aldehyde is not part of the root chain because the root chain is going to be this five-membered ring. When that happens, then aldehyde gets a special name because it’s a substituent. As a substituent, it receives the name carbaldehyde. It does receive a location. If you're able to put it into the root chain, then the root is going to be -al and there’s going to be no location because you assume it’s at one of the ends.
However, if you're not able to put the carbonyl, the aldehyde in your root chain, let’s say that it’s a substituent. It’s a branch off of that root chain. Then you do have it give it a location because they need to know where on that chain is it and you have to call it a carbaldehyde instead of an aldehyde to indicate that it is now a substituent.
If you’re a little bit lost, don't worry about it. We’re going to do two examples. By the end of these two examples, you’re going to have a pretty good idea of what's going on. Go ahead and try the first one. Name it according to the rules that I told you and then I’ll give you the answer
Alright so the first thing we would do is we would start off with just the alkane nomenclature rules which say that you have to find the longest carbon chain and ideally we want the functional group to be on that chain so I'm going to pick the six carbons. So that would be one, two, three, four, five, six. This would typically be a hexane but I'm going to switch out that the E ending for hexanal. Also notice that my aldehyde is part of that chain so am I going to number it as a one hexanal? No guys, I'm just going to leave it as hexanal because I'm assuming that it's at one of the ends of the chain because it's an aldehyde.
Now we're not done, we have a substituent and that's just going to be you know we would definitely get our first carbon name or number to the aldehyde that's going to get priority. It's a modifier so it gets priority and then we go from there so it will be a 5-methylhexanal. Once again just to prove my point a little bit more guys the reason that I don't have to put 5 methyl 1 hexanal is because this is the only place it could be. It's either going to be here or it's going to be here because it has an H on it. If I were to let's say put the carbonyl in the middle like this that's not an aldehyde anymore, it's a ketone. Ketones you have to name the location but aldehydes you don't. Cool guys, so that's that one. Move on to the next.