Organometallics on Nitriles

Concept: Concept: General Reaction

2m
Video Transcript

In this video, we're going to talk about how you can make ketones from nitriles. Like carbonyls, nitriles have a very strong dipole along the carbon nitrogen bond which is going to make the carbon highly electrophilic. This is very similar to carbonyls. That means that when you react a nitrile with a nucleophile, you can expect to get nucleophilic addition. Very popular nucleophiles that we've used in the section are organometallics like Grignard, RMgBr. and RLi. Remember that these reagents have full negative charges on them. What’s also advantageous about them is that they have alkyl group so that they can add Rs. It turns out that if you use one of these reagents on a nitrile in acidic environment, you’re going to get a ketone. The R group here is actually coming from the Grignard or the organolithium. That R gets added to the carbon and winds up turning into a carbonyl. You might be a little bit confused because you're thinking, “Johnny, what happened to the nitrogen?” We had a nitrogen. It really looks nothing like the original compound. For this, we’re going to have to go into the mechanism which yes, I'll show you the whole mechanism. But it turns out that this mechanism is going to make a lot more sense you than you think because it's really just a variation of stuff we’ve learned before. In this next video, I’m going to show you the full mechanism for how you can turn a nitrile into a ketone. 

Concept: Concept: Mechanism

8m
Video Transcript

So guys, for this mechanism I'm just going to bring down the exact compound that we had, the exact nitrile. We just have to choose what R group we want to add to it. Let's just make it a methyl group to make it really easy. So I'm going to use C H 3 M G B R and keep in mind that we kind of separate these reagents. We have, we reacted the grignard first and then we have our H 3 O plus acid work up. So guys the mechanism for the grignard is so easy that everyone here should know it regardless of whether your professor likes mechanisms or not you should know the first part, the second part the acid work up I'm going to show you anyway because I'm just that kind of guy but it's a mechanism that you probably don't need to be able to draw but we'll just do it in case. So the negative attacks the carbon, the electrons go up to the nitrogen. So what we make is a compound that looks like this, N R R. Now this looks a lot like a ketone but it's got a nitrogen with a negative charge on it. So how do we fix that? Well this is where the acid work up comes in. So I'm going to take my, oops I'm trying to use a different colour, I'm going to take my water in acid and I'm going to use that to protonate and that's going to give me a molecule that looks like this. I've got my cyclohexane on one side, I've got my methyl on another.

Guys, does this molecule look familiar to you in any way? Do you know what the name of this functional group is? Guys this is just an imine. This is an imine. If you don't remember what an imine is or have no idea, go back to the imine videos you can type in in our search bar because I showed you this mechanism already and it turns out do we know how to make a ketone into an imine? Think about it. Do we know how to make a carbonyl into an imine? Sure, that's just going to be the acidic addition of an ammonia derivative or an ammonia into the carbonyl. Do we know how to go from an imine to a carbonyl? Essentially the reverse reaction. Of course we do because that's just the reverse reaction remember that all of these additions to carbonyls are reversible so by definition do we know how to go from an imine back to a ketone? Absolutely. All we're going to do is we're going to do a reverse imine mechanism. So if you wanted to you could go back right now to our imine videos and you could just reverse everything I did and you could come up with a mechanism on your own but I know that if I tell you to do that a lot of guys are going to complain and say Johnny just draw the mechanism for us so I'm going to do it. So let's draw the mechanism but I want you guys to be more mentally engaged now because you know that this is just a reversed reaction of a mechanism you're already supposed to know. So we've got nitrogen, we got our imine and the first thing we're going to do is we're going to protonate. We're doing the exact reverse reaction of making an imine. So we're going to protonate the nitrogen. Remember this all happens in equilibrium so this is going to give me a molecule that looks like this, N H 2, and that has a resonant structure because this is a positive charge so I'm going to draw my resonant structure up and this is going to give me N H 2 with a positive. What happens there? Guys well this is a really perfect candidate for nucleophilic addition of water. Why water? Well because first of all I have H 3 O plus present. Second of all I'm trying to go back to the carbonyl so this is going to attack. Now guys if you don't mind I'm going to flip the location of the water and the nitrogen so that I can form the water at the top. So don't hate me I'm going to do this. I'm going to take my water and add it up here now and then I'm going to put my nitrogen at the bottom. I literally just flipped the locations. Do you guys remember what happens now? We're just going backwards from the imine mechanism so we're going to do a proton transfer because we're trying to get rid of the N H 2, so the N H 2 is going to grab a proton, give the electrons to the O, I'm going to get O H single bond, methyl, N H 3 positive. Wonder what I'm going to do with that? And my cyclohexane. Alright guys, any ideas on what I can do now? Get excited guys you should know this alright? So we can do an elimination to kick out the good leaving group N H 3. So I can take my lone pair, make a double bond, kick out my N H 3 and this is going to give me a molecule that looks like this, positive methyl plus I have my N H 3 leaving group and what's the last step? The last step guys is to regenerate my catalytic acid. So I'm going to take my water and I'm going to take away an H and lo and behold bam I have a ketone from a nitrile. Isn't that sick? Isn't that awesome guys? Don't you love it when organic chemistry starts making sense because you actually have learned these things before. I love it.

So anyway guys, just so you know this entire process that I just went through right now is also called the acid work up. So if you ever see acid work up, if you ever see those words, acid work up usually relates to the hydrolysis of a nitrogen group into an oxygen group like a ketone so an acid work up will take an imine derivative and turn it back to a carbonyl. This is the reverse reaction of an imine reaction. So as you guys can see now we go back up here this reaction makes a lot more sense. We added the grignard and then we did an acid work up to get back to the ketone. Now just one more disclaimer here guys, or word of advice, like I said most professors want to know this reaction, want you to know the mechanism, want you to know the reagents but don't necessarily need you to draw a whole acid work up. In fact most professors are fine if you just write acid work up. So that's going to be up to you and your professor again this semester I want you guys to be very involved, ask your professor questions, saying you don't have to go to their office hours just ask after class like hey do I need to be able to draw the whole acid work up for this mechanism but regardless hopefully it's not that hard because you already know the imine mechanism. Alright guys, so that's it for this video. Let's move on to the next one.