In this leah4sci video we're going to look at the 3 common organometallics including
the grignard, organolithium, and gilman reagents.
Organometallics are great for new carbon to carbon bonds including chain elongation, opening
rings, attacking carbonyls and more.
But let's start at the beginning, the name organometallic comes from the word organo
meaning carbon and metallic meaning metal.
When carbon is bound to a lower electronegativity metal carbon is going to have a partial negative
charge and the metal a partial positive charge.
Wait wait, let's back this up all the way to polar bonds in general chemistry which
I cover in more detail at the link below.
To remind you that when you have a bond between two atoms, we look at the difference in electronegativity
between atom A and atom B and if that difference is significant you'll have an unequal sharing
of electrons for example, if atom A has an electronegativity of 2.0 and B is 0.7, the
difference is 1.3, telling us that the more electronegative atom A has a partial negative
charge and the less electronegative atom B has a partial positive charge due to the unequal
sharing of electrons.
We've already seen carbon with a partial charge for example, the partially positive carbon
in your substitution elimination reactions when bound to an electronegative halogen or
the partially positive carbon on a carbonyl when double bound to an oxygen due to resonance.
In both of these cases, carbon being partially positive makes it susceptible to a nucleophilic
attack but that's not the case when we're looking at organometallics.
Carbon has an electronegativity of 2.5, magnesium is 1.2 for a difference of 1.3, lithium is
1.0 for a difference of 1.5, and copper is 1.9 for a difference of 0.6.
Notice that the difference is greatest between magnesium and lithium making these 2 more
reactive which will be important for later reactions and copper at 0.6 is a lot less
reactive but what's most important is that in each case, carbon is more electronegative
making carbon the reactive species in the organometallic reactions.
Why is this important?
Carbon is not very electronegative, it does not like to hold a negative charge and when
it's forced to hold that partial charge, it acts almost like a carbanion almost like carbon
has a lone pair of electrons is so unstable and therefore wants to attack.
Organometallics can act as both a nucleophile and a base, the difference of which I discuss
in the video link below.
Now that we understand the basics, let's look at the individual organometallics starting
with organomagnesium or the grignard reagent.
The grignard reagent is the most common organometallic that you're going to see in organic chemistry
with the following structure, an R group bound to magnesium metal, bound to a halogen most
commonly bromine but you'll also see chlorine and iodine.
The grignard reagent is formed when you react a molecule that has a halogen on it for example,
an alkyl or vinyl or aryl halide in this case I have bromobenzene with magnesium in an ether
solvent such as diethyl ether or tetrahydrofuran to give you a product where the magnesium
inserts itself in between the carbon and the halogen.
Fortunately, you're not responsible for the radical mechanism, you only need to understand
the product.
We have a partially positive magnesium bound to a partially negative halogen and a very
partially negative and very reactive carbon.
This carbon here will act like the carbanion in the grignard reagent.
A solvent like ether is very important because the negative oxygens will donate their electronegativity
and complex with the magnesium metal as follows.
If oxygen is a good solvent atom, why not just use something like water?
Very simple, remember we said organometallics can act as a nucleophile and a base?
The expected reaction of a grignard with epoxide is for the grignard to attack the epoxide
and open the ring but in the presence of a polar protic solvent like water, grignard
will go for the easier proton in an acid base reaction to grab that hydrogen, give oxygen
back its electrons, completely destroying the reagent.
CH3 is now bound to water's hydrogen, giving you methane gas that bubbles out of solution
along with an epoxide that did not undergo a reaction because there’s no grignard left
to attack it.
And so you want to make sure your solvent, while contributing the electronegative oxygen,
doesn't inadvertently destroy the grignard because it also has a polar protic component.
Why does it attack the proton rather than the epoxide?
Simple, molecules are lazy, they look for the easiest reaction and if you look at the
reactivity between the carbon on epoxide or the proton, the proton is a lot more reactive
and therefore the atom that gets attacked.
Grignards will undergo many different reactions from opening epoxide rings, attacking carbonyls,
and even a double attack on carboxylic acid derivatives like this acid chloride, in fact
because there are so many I have an entire tutorial and grignard cheat sheet on my website
link below at leah4sci.com/grignard.
In each situation, after the grignard attacks we're left with an O- that we need to add
a source of protons for a neutral alcohol product.
Only add the water after the grignard completed the reaction, meaning in step 2 so that you
don't inadvertently destroy the grignard by adding the water too soon.
The next organometallic we'll look at is the organolithium which as the name implies has
organo, a source of carbon bound to a lithium atom.
If you look back at the periodic table you'll remember that lithium in group 1 only has
1 valence electron and is very reactive and so when lithium gives off that 1 electron
to give us Li+ we have a redox reaction in this case oxidation where carbon, the recipient,
gets reduced as follows.
If I have an alkyl halide and react it with 2 atoms of lithium I get an organolithium
+ LiBr.
We need 2 lithium atoms because carbon will require 2 electrons, 1 from each lithium to
become carbanion-like bound to that spectator or partially positive lithium, the 2nd positive
lithium will hang out with bromine somewhere in solution.
If you remember our polarity, lithium gives us the most electronegative carbon and is
therefore most reactive even at cold temperatures which is great for coupling reactions where
other reagents typically don't work.
For example, say I have this vinyl alkyl halide and I want to extend the carbon chain without
losing that pi bond, simply take that carbon component that you want to add to the molecule
and introduce it as an organolithium.
Are you with me so far?
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Last but not least, let's take a look at organocuprate also called the Gilman reagent or the diorganocopper
reagent.
This is a more reactive form of just a carbon bound to a copper because if you'll remember
the electronegativity difference between carbon and copper is very low.
In fact, copper is so slow to react that to even make this, we need to start with a stronger
organometallic, you guessed it, organolithium.
If I take my organolithium and I react it with copper iodide, I get what is called a
transmetallation reaction where the metals swap places so that the carbon chain is now
bound to copper and lithium iodide wanders somewhere in solution.
And so I take my organocopper reagent and I attempt to react it with a carbonyl like
this ketone and I get nothing, no reaction.
It is not strong enough to attack where the other organometallics would and so we give
this reagent the power of a 2nd carbon to make it slightly more reactive so we can get
something out of it.
When I react organocopper with organolithium, I get a copper atom that is now bound to both
R groups with a negative charge countered by a positive lithium.
You don't have to show all these steps, instead you're more likely to come across something
like this, 2 alkyl halides reacting with 2 lithium like we saw before to give us 2 organolithium
which you then react with copper in an ether solvent to give you your Gilman reagent which
is 2 R groups bound to 1 copper and 1 lithium.
The 2nd positive lithium hangs out with the halogen somewhere in solution.
Remember, copper is still not as reactive but it's reactive enough to give us some very
key reactive situations.
Remember how grignard attacks an acid halide twice because the Gilman reagent is less reactive,
it’ll only attack the acid halide once, stopping at the ketone.
Another place you'll see a clear distinction is the Michael addition which I discuss in
detail at the link below where the grignard will do a 1,2 attack directly at the carbonyl
where the Gilman will give us a 1,4 attack at the pi bond position for two very similar
yet very different products.
For even more on organometallics visit my website link below at leah4sci.com/metallic,
the link again is leah4sci.com/metallic.