On this page, we're going to focus on those specific Z-type reactions that turn carboxylic acid into other types of derivatives. We're going to go a little bit more in-depth on specific reagents that can transform certain types of derivatives to each other. The first one is the synthesis of acid chloride. This should be difficult to do because if you remember, acid chloride is the most reactive acyl compound. It's all the way over here. To get carboxylic acid all the way over here, I'm going to need a strong reagent. That's exactly what we're going to use. You guys should have probably seen this reaction already at some point in organic chemistry. But the most common reagent to do this is SOCl2. That's a very common reagent to add chlorine to all kinds of things, to alcohols and to carboxylic acids. But there are other reagents we can use. We can also use that's supposed to be an or. We can also use PCl3 or PCl5. These reagents are full of chlorines and they're particularly good at turning a carboxylic acid into an acid chloride. This is helpful for us synthetically because once you have an acid chloride, you can turn that into any other derivative because of how reactive it is in very high yields. Awesome. Let's move on to the synthesis of amines.
Carboxylic Acid to Acid Chloride - Online Tutor, Practice Problems & Exam Prep
Carboxylic acids can be converted into several carboxylic acid derivatives using specific reagents. These include acid chlorides, amides, and (indirectly) nitriles.
Synthesis of Acid Chlorides
Video transcript
Synthesis of Amides
Video transcript
According to my three rules, would it be favorable to turn a carboxylic acid into an amide? Does that go in the right direction in terms of favorability? Remember that carboxylic acid was in the middle of the page and amide was all the way on the left side. Yes, it would be because carboxylic acid is more reactive than an amide. That's exactly what we see. When you react carboxylic acid with ammonia, you are going to get some amide. But there is a problem.
It turns out that the energy difference between these two acyl compounds isn't high enough to give us good yields of amides. Actually, what winds up forming predominantly is an ammonium salt. The way around that is to use a lot of heat when you're doing this reaction. If you use a lot of heat, you can dehydrate the salt back to an amide. This reaction actually does follow the three rules.
You're thinking, "Why are you teaching me this if we already learned it in the three rules?" Because it turns out that your yield is just a little bit too low to make it a great way to synthesize an amide. You have to use heat in order to force it to make the amide. It turns out in order to avoid those harsh heat conditions, chemists have found another molecule that's a dehydration agent. This dehydration agent is called DCC. Here I've shown you what the structure is. You might not need to know what the structure is, but you should know that DCC and this are the same thing. DCC, when coupled with NH3, dehydrates the amide by itself. We don't need heat. It greatly increases our yield. Instead of having to use a lot of heat to get amide, we can combine NH3 with DCC and we're going to get a huge yield of our amide.
Many times, you're going to see this agent, DCC, used to boost these reactions that are already favorable but to make them happen at higher yields. The whole point of this area is that you'll understand what the role of DCC is. Let's move on to the next one.
Dehydration of Amides
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We've been talking this whole time about how nitriles are carboxylic acid derivatives. But not once have I mentioned how to actually make a nitrile. It turns out that they're not that easy to make. There's actually only one reagent that we're really going to learn in this course that helps us or one type of reaction. That's a dehydration reaction. It turns out that really the major way to make nitriles in this course is going to be to dehydrate amides. This is a mechanism that I'm not going to teach you and that you're most likely not responsible for. I've never seen it on an exam. You should just know the reagents for it. We're going to use either P2O5 which is also seen sometimes as P4O10. It should be a dimer of that compound. I'm just saying this is the same thing. Or you can use SOCl2. Both of these reagents are going to dehydrate the amide and turn it into a nitrile. Why is that important? Because this is the only way to make a nitrile up until this point. We've never learned another way to do it and this helps to bring it into the family of carboxylic acid derivatives. If I want to make a nitrile, I know I have to make the amide first, and then I dehydrate the amide. Let's move on to our last reaction.
Hydrolysis of Nitriles
Video transcript
Finally, we know that by definition, nitriles can be hydrolyzed to carboxylic acid. This happens both in base and in acid. But typically, it's an acid-catalyzed reaction. As you can imagine, you wind up getting water attacking the carbonyl. You wind up kicking electrons up to nitrogen. I'm not going to show you guys the whole mechanism here, and it's not a mechanism that is highly emphasized in this section. But you can imagine that what you wind up getting is something that has like an imine derivative. You end up with something that looks something like this. From there, we have an acid workup. Since we're already in an acidic environment, it's not hard to imagine how through an acid workup, this imine derivative could be turned into a carboxylic acid. That's really all I want to mention. I'm much more interested for you to just memorize these reagents and not specifically know their mechanisms since these are not very important mechanisms for this section of the course.
Let's move on to the next page.
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