Leah here from leah4sci.com, and in this video we're going to look at Grignard reagents from
their formation, reactivity and mechanism.
The grignard reagent pronounced Grignard, not Grig-nard after Victor Grignard, who won
a Nobel Prize for this is a reagent that has an R group, meaning some carbon chain bound
to a magnesium metal bound to a halogen, typically chlorine bromine or iodine.
This is very useful in chain elongation because it's the strongest of the organometallics
that you're going to see.
Where are organo tells us we have a carbon chain metallic bound to a metal.
In this case, the carbon bound to the magnesium metal has such a difference in electronegativity
that the magnesium becomes very partially positive, making that carbon very partially
negative almost to the point of being a carbanion.
A carbanion is a carbon with a negative charge, a very unhappy group, and therefore very,
very reactive.
When you think of the grignard mechanism, I want you to imagine that the electrons that
sit as a covalent bond between carbon and magnesium are just sitting on the carbon like
a negative charge ready to attack.
And so you show your arrow going from the electrons to that positive thing.
This is going to create a new bond between the carbon group on the grignard and whatever
we just attacked.
Before we go into mechanisms.
Let's start with the simple Grignard formation.
To form a grignard, I need to have an Alkyl halide, for example, methyl bromide.
We take this alkyl halide and react it with magnesium.
Typically with something like ether.
The solvent you use is very important because you are not allowed to use a polar protic
solvent.
More on this in a minute.
The Grignard that forms has our initial carbon chain.
The methyl group with the magnesium inserted between the carbon and the halogen.
This right here is a methyl grignard with the methyl carbon being the carbon ion like
ready to attack group.
I showed this with a bromine which is most common, but you're also going to see it with
iodine and chlorine where bromine and iodine react quickly and chlorine a little bit slower.
So why do we use an ether solvent?
First, make sure you review my polar protic and aprotic solvent video link below and then
take a look at ether compared to something like alcohol.
The common ethers you'll see for Grignard formation are Et2O which is diethyl ether
or THF tetra hydro florine which is a hetero cyclic ring with an oxygen in it.
Compare this to other common solvents that you're used to seeing like water or alcohol.
Remember, the Grignard is so reactive it wants to attack anything and everything.
So let's say I have a reaction and I managed to form a grignard with a CH3 bound to the
MGBR and it's dissolved in water.
The problem is water has a partially negative oxygen atom and partially positive hydrogen.
So if that Grignard somehow manages to form the now the new grignard range and immediately
takes advantage of its carbon ion like state, attacks the hydrogen atom from the protic
solvent, giving oxygen back its electrons and giving me something that looks like this
purple CH3 from the grignard is now bound to the green hydrogen from water.
In solution we also have an OH minus and the useless MGBr plus just floating around in
solution.
So sure, we created the grignard, but we immediately destroyed it and got nothing out of it.
Think of this almost like sending a little kid to the ice cream store.
Go get me an ice cream.
Kid's gonna go, he's very happy, he gets the ice cream, but he eats it on the way.
Did you do what you ask?
Yes, He got the ice cream.
Did you get anything?
No.
Because he ate it.
Your ice cream is gone.
Same thing for something like alcohol.
The grignard will attack the hydrogen, give oxygen back it’s electrons.
Once again, I have CH 3 bound to an H and solution.
Grignard is destroyed and I'm left with and alkoxide and an mgbr plus.
Now that we know what not to do, let's take a look at the grignard reaction mechanism,
starting with an ethyl grignard reacting with acetone.
Since grignard can attack many compounds, I created a mini cheat sheet that you can
find on my website leah4sci.com/grignard.
The most common is the carbonyl that gives you an alcohol and the mechanism looks something
like this.
First, remember, the bond between carbon and magnesium is the bond that's going to attack
because this is the carbon that's very carbon ion like recall that in a carbonyl we have
resonance between the carbon and the oxygen, making the oxygen very partially negative
and the carbonyl carbon very partially positive.
The grignard reaction begins when the carbon ion like electrons are going to reach out
and attack the partially positive carbonyl carbon.
This would put a fifth bond on the carbon, which can't happen.
And so the PI bond is going to get kicked up onto the oxygen atom.
What we get is a carbon chain as before with only one bond to oxygen.
Oxygen had two pairs of blue electrons and now gets a third pair of black electrons with
a formal charge of minus one attached to the carbonyl carbon we now have a new bond to
the ethyl group, which is from the ethyl grignard.
Off in solution, we also have the MGI plus.
And many professors like to see it drawn next to the oxygen to show the counter Ion pair.
But, we're not done,because oxygen still has a negative charge pair.
So in the next step we'll add in some water or acid.
But we're not done because oxygen still has a negative charge so let's add in acid, which
you can just write as H3O plus and I'll draw one of the H pluses here.
The negative oxygen reaches for the hydrogen to give me an alcohol as the final product.
This looks a little messy so if you want to redraw it or along this carbon chain has 4
carbons, 1, 2, ,3, 4 with an alcohol on carbon 2 and a methyl group also on carbon 2.
Notice that as we did this mechanism, we have the Grignard effect happen first and only
when it was done when we already had the irreversible carbon to carbon bond formation is when we
added the water.
If I were to write out this reaction with the steps instead of the mechanisms and I
drew it like this with an MGI and then added H3O plus, this would be completely incorrect.
Remember, grignard are going to attack acetic or polar protic groups and that will destroy
your grignard.
To make sure you get full credit, add the one to show that the grignard attack is the
first step and then 2 to show that the acid workup is your final step after the reaction
is done to give the final alcohol product.
Now that you've seen the mechanism and overview of the reaction, here's a shortcut for both.
Coming up with a grignard product and recognizing what happens if you're given the product and
have to come up with a reactants.
If you see this question on the exam, very simple, identify your carbonyl carbon, break
the PI bond and on the carbonyl carbon, add the grignard carbons 1, 2.
Then make sure you add a hydrogen onto the oxygen to form your alcohol.
And there you have it.
It's a little bit messy, but if you count one, two, three, four, a methyl group and
then alcohol on carbon two, one, two, three, four, alcohol and methyl on carbon two.
So you're given a product and told to show how you would form this from a grenade.
What you do is identify the alcohol because the alcohol is the product of a grignard attacking
a carbonyl and recognize the carbon holding the alcohol was a carbonyl.
So add your carbonyl back, cross out the hydrogen and then break right next to it.
So we can break here or here.
I'm gonna choose to break here.
This portion is the carbonyl that got attacked and for the other portion just add an MGx.
For this reaction, our reactants would include an aromatic grignard and a 2 carbon carbonyl,
in this case an aldehyde.
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For even more organometallics including the grignard reactions mini cheat sheets visit
my website leah4sci.com/grignard.