Nomenclature and properties of acyl (acid) halides and acid anhydrides

Lecturer: The next few videos we're going to look at the nomenclature and properties of carboxylic acid derivatives. Let's start with an acyl halide. Here's our general structure of an acyl halide. On the left side we have an acyl group, on the right side we have a halogen. You could also call this acid halides. They're derived from carboxylic acids. If we look at this carboxylic acid on the left here, a two carbon carboxylic acid, we could convert that to a two carbon acyl halide over here on the right. If we want to name our acyl halide we have to think about the name of the carboxylic acid. This, of course, is acetic acid. Let's go ahead and write out acetic acid here. If we want to name the corresponding acyl halide we need to think about dropping the -ic ending, and then the acid. We drop -ic and acid, and we add -yl and then the halide. Let's go ahead and write that out, so we drop the -ic and we add the yl and then we add the halide. We have a chlorine here so we're going to write chloride. We would call this acetyl chloride. Let's go ahead and show that right here. We add the -yl and then we add the halide portion. We could have also called this ethanoic acid. Ethanoic acid would be the IUPAC name but everyone says acetic acid. If we were to call this ethanoic acid, once again think about drop your -ic and then the acid part, drop this portion, then add -yl and then chloride. Let's go ahead and write that in here. We go ahead and add the -yl in and then the chloride like that. That would be ethanoyl chloride as our name. Let's go ahead and show this portion, once again, the -yl portion and then our halide. In terms of physical properties of acyl halides we need to think about the interaction of two molecules here. Let me go ahead and draw in another molecule of acetyl chloride. Acetyl chloride has a boiling point of approximately 51 degrees Celsius. Let me go ahead and write that in, so approximately 51 degrees Celsius. We know that acetyl chloride is a polar molecule. The oxygen here is more electronegative than this carbon, so we have a partial negative and we have a partial positive. This chlorine is also withdrawing electron density from our partially positive carbon, so we have a polar molecule. Acetyl chloride is polar right here. This is polar. Same molecule so this is polar. We have a partial negative, partial positive. Once again this chloride is also withdrawing electron density this way. We have two polar molecules interacting which we know is a dipole dipole intermolecular force. There's an attractive force between these molecules which is dipole dipole. Let me go ahead and write that. It's a dipole dipole interaction with molecules of actyl chloride. We know that dipole dipole interactions are stronger than London dispersion forces, so acetyl chloride has a higher boiling point than say a two carbon alkane, like ethane. It's a little bit harder to pull these molecules apart than it is to pull molecules of ethane apart, therefore this boiling point is higher than that for a two carbon alkane. However this boiling point is lower than that of acetic acid. To think about that we'll need to draw in another molecule of acetic acid. Let me go ahead and do that. Drawing in another molecule of acetic acid. We can see that there's opportunities for hydrogen bonding. There's a hydrogen bond here, and a hydrogen bond here. Hydrogen bonding, go ahead and write that. Hydrogen bonding is the strongest type of intermolecular force. Therefore the boiling point of acetic acid is going to be higher, it's somewhere around 118 degrees Celsius. It's harder to pull these two molecules apart because hydrogen bonding is a stronger intermolecular force than dipole dipole. That gives you some idea of the boiling point of acyl halides. In terms of solubility in water you can't really say that something like acetyl chloride is soluble in water because it reacts so violently with it. Acetyl chloride is extremely reactive and it reacts very quickly and often violently with water, so we can't really say that it dissolves in water. Let's move on to acid anhydrides. Let's look at how to name an acid anhydride. Acid anhydrides can be thought of as being derived from carboxylic acids too. If we look over here in the left once again we have acetic acid here, this is acetic acid. If we take two molecules of acetic acid and combine them we can form an acid anhydride. Let's think about what happens. We're going to lose water here and the word anhydride means without water. If we take off the water and take this portion, take this acyl group and this over here and stick them together, then we form our anhydride over here on the right. Because our anhydride was formed from acetic acid we call this acetic anhydride. These are pretty simple names. You keep the acetic part and drop the acid, and just add anhydride. This is acetic anhydride. If you thought of acetic acid as ethanoic acid, if you prefer to use the IUPAC name, ethanoic acid. Let me write ethanoic acid here. Once again just drop the acid part and add anhydride. You could call this ethanoic anhydride. Ethanoic anhydride. Once again anhydride meaning without water. Let's look at how to name another anhydride. Let me go down here and get some more room. We're trying to name this anhydride over here on the right. To do that we need to think about the carboxylic acid, from which it can be thought of as being derived. Here we have two molecules of benzoic acid. Let's go ahead and write benzoic acid here. I'm not talking about exact chemical reactions, I'm just thinking about the acid anhydride and how to put it into the different carboxylic acids. If we do the same thing we did before, we think about the term anhydride being loss of water, we take out water here and stick those together, once again you can see we form the anhydride on the right. This portion plus this portion gives us our acid anhydride. Once again we're not doing exact chemical reactions here. Just for the sake of nomenclature we can just drop the acid and add anhydride. This would be benzoic anhydride. This would be benzoic anhydride, like that. Let's look at another example. This time we don't have symmetry. When I'm thinking about some carboxylic acids for this one, over here on the left I recognize benzoic acid. Let me go ahead and write that down. Benzoic acid is being present. If I think about over on the right side, this portion, if I think about a carboxylic acid this way I see that's acetic acid. I have benzoic acid and acetic acid. To name our anhydride we drop the acid part and we're going to add anhydride. We have to think about using the alphabet here. A comes before B, so to write the name of our anhydride we would write acetic benzoic anhydride. Acetic benzoic anhydride. Once again when you see an anhydride and you're trying to name it just think about the carboxylic acids and that will help you figure out the name. In terms of physical properties of acid anhydrides let's look at an example here. Over here on the left we have acetic anhydride, which is a polar molecule. It's moderately polar because we have these carbonyls here. The oxygen is partially negative, this carbon down here is partially positive, the same thing for all these carbonyls. It's a moderately polar molecule. That's a negative sign right there. There's going to be some attraction between these molecules. There's going to be some attraction between the negative and the positive charges. We have a fairly polar molecule and a fairly polar molecule, so we can say that there's some dipole dipole interaction present. Between molecules of acetic anhydride there's some dipole dipole interaction. There's also of course London dispersion forces as well. The boiling point for acetic anhydride turns out to be approximately 140 degrees Celsius. Let's go ahead and write that in here, so approximately 140 degrees Celsius. We can compare that to a carboxylic acid that's similar in terms of number of carbons and oxygens. For acetic anhydride we had one, two, three, four carbons. Over here on the right this is butanoic acid. We have one, two, three, four carbons. Then we have two oxygens for butanoic acid and we have three oxygens for acetic anhydride. They're similar in terms of sizes, but when we think about comparing their boiling points, over here on the right butanoic acid has a boiling point of approximately 164 degrees Celsius, it has a higher boiling point because once again there's some hydrogen bonding present. There's some hydrogen bonding present because we're talking about a carboxylic acid here. And once again hydrogen bonding is a stronger intermolecular force than dipole dipole so it's harder to pull apart molecules of butanoic acid, therefore it takes more energy, it takes a higher temperature to pull these molecules apart to turn them into a gas. Once again, H-bonding is a stronger intermolecular force than dipole dipole. When we think about the solubility of an acid anhydride in water, once again it's kind of difficult. Something like acetic anhydride is going to react with the water. Acetic anhydride is also fairly reactive. Not quite as reactive as an acyl halide but it does react with water, so we can't really say that it dissolves very well in it. We'll talk much more about the reactivity of these carboxylic acid derivatives in a later video.