In the last post we introduced the concept of organometallic compounds – molecules where carbon is bound to a less electronegative atom such as Li, Mg, Cu, and many other metals. We said that carbon in these molecules tends to be electron-rich and thus have nucleophilic character, in contrast to functional groups such as alkyl halides, aldehydes, ketones, and epoxides where carbon has electrophilic character. If you need a refresher on what I meant by nucleophilic and electrophilic, read that post first.
In this post we’ll talk about how certain types of organometallic compounds are made – specifically organolithium and Grignard reagents.
Let’s start with organolithium reagents because they’re the simplest.
Organolithium Species Are Carbanions
If you look closely, you can approximate the structure of an organolithium reagent (R-Li) as “R(–)” , with lithium as the positive counter-ion: in other words, a carbon bearing a negative charge (we call these species “carbanions”). If you think back to earlier lessons on acids and bases, this structure might look familiar – it’s the conjugate base of a species R-H.
Using butane to stand in for “R-H” here, we get:
Since conjugate bases are made through deprotonation, we might naively think that we could make organolithium species by taking an organic molecule and just adding a super-strong base to rip off the proton.
If you’ve covered alkynes, you’ve seen that that process actually works pretty well in the case of terminal alkynes. They are quite acidic species, having a pKa of 25 or so. Their unusually high acidity is due to the considerable s-character on the carbon (meaning that the lone pair is held closely to the nucleus).
Trouble is, as we move towards alkenes and alkanes, the direct deprotonation approach doesn’t work so well. That’s because… well, it’s hard to find any species more basic than a deprotonated alkane! Just as the only thing sharp enough to cut a diamond is another diamond, about the only thing basic enough to deprotonate an alkane is… another deprotonated alkane.
This isn’t a practical approach to make organolithiums for several reasons – primarily, the fact that there are often many C-H bonds and it’s hard to selectively remove just one. [For instance, if you tried to make 1-pentyllithium by deprotonating pentane, you could potentially end up with multiple different isomers].
Thankfully, another approach has been devised.
Making Anions Through Reduction, Not Deprotonation
If you look at the reaction below, and count the electrons carefully, you might note that the product has two more electrons than the starting material. In other words, particularly if you remember the OIL RIG mnemonic, reduction has occurred.
This means that if we were to add some species which was particularly likely to give up its electrons, we might thus be able to effect this transformation.
Can you think of any members of the periodic table which hold onto their electrons particularly loosely? If you said “the far left part of the periodic table” (particularly the alkali and alkaline earths), ding ding ding! you would be correct.
Lithium, in short, would be a great choice as a reductant for this reaction. [We’ll get to the other metals in a minute].
What might be a good choice for X? One factor which would make this reaction easier is if X(–) was a fairly stable species – a good leaving group, in other words. Good candidates for X are halides such as Cl, Br, and I. A bad candidate would be H(–) or some other strongly basic version of R(–) , since we’d be generating another unstable anionic species.
Now let’s get to specifics.
Making Organolithium Reagents From Alkyl Halides
Lithium, having a very low ionization energy (i.e. it loses its electron easily) is a powerful reducing agent. Since lithium only has a single valence electron, however, we must add two equivalents if we are to complete the reduction reaction.
To make organolithium reagents, we start with alkyl halides, and add powdered lithium metal (Li or sometimes written as Li0 to distinguish it from the ion Li(+) ).
Occasionally the solvent for this reaction is written below the arrow. A common solvent is pentane. Some instructors like to include it. Some don’t. Regardless of whether it’s written there or not, it doesn’t participate in the reaction.
This reaction works for alkyl chlorides, bromides, and iodides, as well as alkenyl halides (fluorides excepted).
If you recall that alkyl halides can be made from halogenation of alkanes, this method thus gives us a 2-step method for formation of highly basic alkyl lithium species from alkanes.
We’ll cover the many useful applications of organolithium reagents in a future blog post.
An aside: What about Organosodium Reagents?
So if lithium works, why not go further down the column of the periodic table? Why not use sodium?
This is an excellent idea – sodium is a great reducing agent, after all. The trouble is, when we try to make organosodium reagents from alkyl halides, what tends to happen is that the carbanions that form then go on to react with our starting alkyl halide (in an SN2 process). The result is a pretty useless reaction you likely don’t need to care about that we call Wurtz coupling.
That’s OK, however. Organolithium reagents are plenty reactive enough for almost every purpose that we’d otherwise want organosodium reagents to do. I mean, who needs Chuck Norris when you’ve got Jackie Chan?
Likewise for organopotassium reagents. The one application of organopotassium reagents that sees common use is a reagent called Schlösser’s Überbase , which is strong enough to deprotonate allylic C-H bonds, something not easily done by organolithium reagents.
Organomagnesium Reagents (Grignard Reagents)
I’m going to skip organoberyllium reagents here (beryllium is highly toxic, and rarely sees use) and move straight across from sodium over to magnesium, which comprises a second very important family of organometallic reagents.
The process for making Grignard reagents is very similar to making organolithium reagents: start with an appropriate alkyl halide and add magnesium. Since magnesium has two valence electrons, only one equivalent of Mg is required to balance the reaction. Here’s two examples, showing formation of alkyl and alkenyl Grignard reagents.
Note that Grignards can be made from alkyl or alkenyl chlorides, bromides, and iodides – but not fluorides.
What, you might ask, are Et2O and THF? These are solvents, which are often written in the reaction scheme, but don’t actually participate in the reaction itself. Et2O (diethyl ether, or, sometimes, “ether”) and THF (tetrahydrofuran) are popular choices. If you’re an introductory student, you probably don’t want to know a the deeper reasons why, nor do you need to, so don’t click this link to find out.
In contrast with most of the reagents and reactions we talk about at MOC, you’ll likely have personal experience doing this reaction!
Sitting around for a few minutes staring at your flask containing Mg, ether, and organohalide do absolutely nothing is a rite of passage for every student of organic chemistry.
One key contributor to the “finicky” nature of forming Grignard reagents is that the reaction occurs on the surface of the magnesium metal. For this reason the reaction is highly surface area dependent. Breaking the Mg up into very small chunks will accelerate the reaction. Furthermore, Mg that has been sitting out in the open for awhile often has a surface coating of magnesium oxide (MgO) which is unreactive with alkyl halides. Breaking up the surface helps to expose fresh, unoxidized Mg to the reactants. A pinch of iodine (I2) or 1,2-dibromoethane can also help to kick-start things.
What’s the mechanism of Grignard formation? Usually not covered – it involves free radicals – but if you’re curious, click here for an image.
From a practical perspective, one key thing to make sure of when preparing organolithium or Grignard reagents is that the solvent and glassware are completely dry. Water (pKa 16) is death to Grignard and organolithium reagents, which as we said above, act as the equivalent of highly basic alkyl and alkenyl anions.
In the next post, we’ll talk about this and also some other complications of making Grignard reagents.
Next post: Organometallics Are Strong Bases