Concept: Concept: Synthesis of Acid Chlorides2m
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’ 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 PCl¬3 or PCl¬5. 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 and very high yields. Awesome. Let’s move on to the synthesis of amides.
Concept: Concept: Synthesis of Amides3m
Alright guys. So, 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 and that's exactly we see guys, when you react the carboxylic acid with ammonia you are going to get some amide, okay? 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 and actually, what winds up forming predominantly is an ammonium salt, okay? Now, 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, okay? So, this reaction actually does follow the three rules, you know, you're thinking, well, why are you teaching you this, if we already learned it on the three rules? Well, because it turns out that your yield is just a little bit too low to make it a great way to synthesize an amide, okay? You have to use heat in order to force it to make the amide. Now, it turns out in order to avoid those harsh heat conditions, chemists have found another molecule that's a dehydration agent, okay? So, it's a dehydration agent and this molecule is called DCC, okay? So, 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, okay? DCC, when coupled with NH3 dehydrates the amide by itself, we don't need heat, so it greatly increases our yield. So, 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 over amide, so many times you're going to see this agent DCC used to boost these reactions that are already favorable but to make them happen in higher yields, okay? So, the whole point of this area is that you understand what the role of DCC is. Alright, let's move on to the next one.
Concept: Concept: Dehydration of Amides2m
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 and 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, and that's a dehydration reaction, so it turns out that really the major way to make nitriles in this course is going to be to dehydrate amides, okay? Now, 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, okay? You should just know the reagents for it, we're going to use either P2O5, which is also seen sometimes as P4O10, should be a dimer of that compound. So, 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. Now, if I want to make a nitrile I know I have to make the amide first and then I dehydrate the amide. Alright, let's move on to our last reaction.
Concept: Concept: Hydrolysis of Nitriles2m
Finally, we know that by definition nitriles can be hydrolyzed to carboxylic acid and this happens both in base and an acid but typically it's an acid catalyzed reaction, okay? So, as you can imagine you wind up getting water attacking the carbonyl, you wind up kicking electrons up to the M, I'm not going to show you guys the whole mechanism here and it's not the mechanism that is highly emphasized in this section but you can imagine that what you wind up getting is something that has like an egg and it's like an amine derivative, okay? You're going to end up with something that looks something like this, right? And from there we have an acid workup, right? So, since we're already in an acidic environment it's not hard to imagine how through an acid workup this amine derivative could be turned into a carboxylic acid, okay? So, 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, okay? So, let's move on to the next.
Show the products of the reactions of these carboxylic acids with PBr3/Br2 before and after hydrolysis.
(a) pentanoic acid (b) phenylacetic acid (c) succinic acid (d) oxalic acid
Propose a mechanism for the reaction of benzyl acetate with methylamine. Label the attacking nucleophile and the leaving group, and draw the transition state in which the leaving group leaves.
Show how you would use appropriate acyl chlorides and amines to synthesize the following amides.
(b) acetanilide (PhNHCOCH3)
Give the products expected from the following reactions.
(a) acetyl chloride + ethylamine
Provide a mechanism for the following reaction.
Predict the product for the following reaction