Preparation of Organometallics - Online Tutor, Practice Problems & Exam Prep

Organometallics are alkylating agents that consist of a group 1A or 2A metal bonded to a carbon structure, making the carbon a strong nucleophile. Key types include sodium alkanides, Grignard reagents, organolithium compounds, and Gilman reagents, each formed through specific reactions with alkyl halides. Organometallics can also act as strong bases, capable of deprotonating acidic hydrogens in alcohols, which can lead to unwanted reactions. Understanding their preparation and reactivity is crucial for effective organic synthesis.

General Reaction

Hey guys. Now we're going to talk about a topic that scares a lot of students, but it's really not that bad. Honestly, I think it's just the name that scares people. And the topic is organometallics. All right. So now you're thinking this is going to be terrible. Organometallics sound very confusing, but they're not. They're very easy actually. All they are is they're alkylating agents. What is an alkylating agent? It means that I'm going to be able to use this reagent to put an alkyl group on something else. So it's the first thing. What it's going to consist of is usually a group 1A or 2A metal. So I'm talking about the first and the second column of the periodic table. Those metals are going to be directly bonded to a carbon structure. Now if you think about it, that's going to give this a very interesting molecular property because typically, we usually see electronegative atoms attached to carbon.

There have been several instances this semester where you've been exposed to an electronegative atom bonded to carbon. For example, this functional group right here, C with an X on it. Remember, X stands for halogen. So this would be an alkyl halide. And what would be the typical dipole of an alkyl halide? Do you guys remember? Where would I draw the dipole towards? Towards the X, right? So I would get a dipole towards the X. That's going to give me a partial negative here, a partial positive here. What kind of charge do I have on that carbon? Well, we have a positive charge, meaning that this carbon is going to be a great electrophile. Okay. And that's typically the way that carbon acts. A lot of reagents that we've seen this semester, carbon is going to react as an electrophile. But organometallics are different because the dipole goes in the opposite direction. So that means that instead of this carbon being a great electrophile, organometallics are examples of where your carbon is an amazing nucleophile. That's going to change everything.

Well, the carbon is an amazing nucleophile because it has a negative charge now and that means that now I'm going to be able to use this in a unique set of reactions because normally we don't have carbon with a negative a negative charge, but now we do. So there are 4 types of organometallics that you guys will be responsible for. There's responsible for. There are 4 types that you need to know and they're commonly talked about in organic chemistry. The first one is sodium alkanides. These are the easiest type because most likely if you've been doing your homework, you've already seen one of these at least in another chapter. The way that organometallics work is we usually use a terminal alkyne to react with a strong base.

Now that strong base, there are a lot of different strong bases we could use, but typically it's NaNH₂ or NaH. These bases have the ability to pull off, let's just say I'm using NH₂^- they have ability to pull off the most acidic hydrogen on that molecule. When we learned in the acids and base chapter how to predict acidity, we would have found that the H at the very end of that terminal alkyne is very acidic compared to the others. So I would grab that H with my base and I would give a negative charge to my carbon. What does that mean? Well, that means that my final product is now going to have a negatively charged carbon.

And what is there a spectator ion? Yes. Remember that there was also an Na⁺ present. We just dissociated it. The Na positive can wind up making a bond to my negatively charged carbon. This is an ionic bond. Do you guys remember what the definition of ionic means? That's really easy. This is going back to chapter 1. But an ionic bond was simply a bond that had such a great difference in electronegativity that there's essentially no sharing. Basically, you can draw a bond, but really there's very little sharing going on of electrons. Since this bond is ionic, you can draw it 2 different ways. You can either draw it as a bond like we've done here or you could also choose to draw it as ions with a negative charge on carbon and Na⁺ separately. Both of these representations are perfectly accurate because one of them shows that there is a bond, ionic, but the other one shows how the bond is very weak, meaning that and weak in terms of that it can be easily dissociated because I have almost full charges on both. Does that make sense guys? This is obviously a carbon here. That is a carbon. I just didn't draw it but it is carbon. The reason we call this an organometallic is because sodium is my metal. Remember that I said the group 1 or 2 metals are typically your metals. So I could actually just boil this down to being Rm. And is going to be the structure that we're going to use for all organometallics. So r simply means I have some kind of carbon group with a negative and m means I have some kind of group 1 or 2 metal with a positive. Does that make sense? This one is one that hopefully you've already seen by this point in the course. This is your first organometal. Let's go on to the next one.

How do we want to make a Grignard reagent? Now, I know I might have just lost a few of you guys because that does not look like it says Grignard. It looks like it says Grignard. But it's pronounced Grignard. Just got to live with that one. Just take it from me. I've been doing this for a while.

Ruining Organometallics

Really quickly, I just want to talk about this blank space that we didn't fill out. I just want to let you guys know that because organometallics are strong nucleophiles, that means they're also going to be able to react as strong bases. When you're a strong nucleophile, you can usually be a strong base and that means you're good at pulling off what? Protons. Remember that nucleophile is according to the Lewis definition, which means you're a good electron pair donor. But when I say base, that's according to Brønsted-Lowry. What I'm saying is that you're going to be a great proton acceptor. So, how does that play into things? Why is that important? Because you have to be aware of cross reactions with acidic hydrogens.

So let's say that I have an organometallic, RM. Right? Remember that we've been using RM all day to represent an organometallic. And I introduce it to a molecule with an alcohol. And I introduce these to each other. Let's say that I want the organometallic to react at some other part of the molecule. Well, this would be a problem for me. The reason is because, as you guys might remember, the hydrogen on an alcohol is acidic. That's going to be a problem because my R is basic; it has a negative charge. So what you're going to wind up getting is a mechanism where your negative winds up pulling off the H and attaching a negative charge to the O and what you wind up getting is something that we consider a ruined organometallic, where what you're going to get is an O-negative. Okay. And then that's going to be attached to basically, it's just going to be R-H. So all we did here is we just protonated the R group. Usually, we don't want to just protonate the R group. We want it to react with something else, like an electrophile. So this would be a really bad idea. Bad sad face because I don't want just to get a negative charge and a protonated R. What I prefer is to react with an electrophile. Okay. E⁺.

All I'm trying to say here is that you have to be very careful about alcohols, water in particular, and obviously carboxylic acids, stuff like that. All of these things, you have to be very careful about because they have acidic hydrogens that can mess up your organometallic, and this is what we consider ruined. Even in your book, it's going to talk about how ruining an organometallic by introducing it to one of those three things, or just some kind of acidic proton, some kind of proton that has a positive charge.

Okay guys, so I just wanted to go over that really quick. Let's move on to the next topic.

Here’s what students ask on this topic:

What are organometallic compounds and why are they important in organic chemistry?

Organometallic compounds are molecules that contain a bond between a carbon atom and a metal, typically from group 1A or 2A of the periodic table. These compounds are crucial in organic chemistry because they act as strong nucleophiles, allowing for the formation of carbon-carbon bonds. This property makes them valuable in various synthetic reactions, enabling the construction of complex organic molecules. Key types include sodium alkanides, Grignard reagents, organolithium compounds, and Gilman reagents, each with specific preparation methods and reactivity profiles.

How are Grignard reagents prepared and what are their uses?

Grignard reagents are prepared by reacting an alkyl halide with elemental magnesium in the presence of an ether solvent, such as diethyl ether. The general reaction is:

$R_X+{\mathrm{Mg}}_{\mathrm{Et}}\to R_{\mathrm{MgX}}$

Grignard reagents are used to form carbon-carbon bonds by reacting with electrophiles, such as carbonyl compounds, to produce alcohols. They are essential in organic synthesis for building complex molecules.

What are the differences between organolithium compounds and Grignard reagents?

Both organolithium compounds and Grignard reagents are organometallics used to form carbon-carbon bonds, but they differ in their preparation and reactivity. Organolithium compounds are prepared by reacting an alkyl halide with elemental lithium, while Grignard reagents are made with magnesium. Organolithium compounds are generally more reactive than Grignard reagents, making them suitable for reactions requiring stronger nucleophiles. The general preparation for organolithium is:

$R_X+{\mathrm{2Li}}_{\mathrm{Et}}\to {\mathrm{RLi}}_{\mathrm{LiX}}$

What are Gilman reagents and how are they prepared?

Gilman reagents, also known as lithium dialkylcuprates, are prepared by reacting an organolithium compound with copper(I) iodide. The general preparation involves two steps: first, forming the organolithium compound, and then reacting it with copper(I) iodide:

${\mathrm{2RLi}}_{\mathrm{CuI}}\to R_{\mathrm{2CuLi}}$

Gilman reagents are used in organic synthesis for coupling reactions, where they can form carbon-carbon bonds by reacting with alkyl halides.

Why must organometallic compounds be handled carefully in the presence of water or alcohols?

Organometallic compounds must be handled carefully in the presence of water or alcohols because they are strong nucleophiles and bases. They can react with acidic hydrogens in water or alcohols, leading to the deactivation of the organometallic compound. This reaction results in the formation of a protonated R group and a negatively charged oxygen, effectively ruining the organometallic reagent and preventing it from participating in the desired synthetic reactions.