In this Leah4sci video, we're going to look at the reaction and mechanism between a Nitrile and
Grignard to form a ketone along with a shortcut to quickly identify the products.
The mechanism can look overwhelming so here is what I want you to recognize,
a Nitrile, has a carbon triple bound to a nitrogen and the ketone does not have a nitrogen
therefore, in this mechanism we have to break the bonds between carbon and nitrogen one at a time
and replace the bonds on nitrogen with hydrogen. Grignard wants to attack the Nitrile because the
bond between carbon and nitrogen is polar, carbon is partially positive and nitrogen
is partially negative and as we know about Grignard reagents which I teach on my website
linked below or at Leah4sci.com/Grignard . The Grignard carbon is very nucleophilic and it's
looking for positive charge and so the electrons between carbon and magnesium will reach for that
partially positive carbon atom forming too many bonds and so kicking out the first bond between
carbon and nitrogen. This gives us a carbon that now has 2 bonds to nitrogen, the initial lone pair
as well as the new pair that came from that triple bond with a negative charge and the single bond
between carbon and the incoming Grignard methyl group. Now that the Grignard is destroyed with an
MgBr + Cation in solution now we can bring an acid which earlier would have destroyed the Grignard.
Nitrogen will use this negative electrons to reach for that hydrogen, collapsing those electrons back
on to oxygen for a molecule that looks like this, nitrogen now has just one lone pair and
that hydrogen atom, giving me this neutral Imine. The next phase of this reaction is
to hydrolyze the Imine to give us a ketone by slowly adding hydrogen on to the nitrogen.
Given that we're in an acidic solution, nitrogen's lone pair of electrons is most nucleophilic
and will reach out for the proton on another hydronium, collapsing these electrons onto oxygen,
giving us an Iminium which is a carbon double bound to nitrogen, two hydrogen atoms,
no lone pairs and a positive charge. Even though the positive charge sits on the nitrogen, nitrogen
is still an electronegative atom and will start pulling on the electron density between itself
and carbon making this carbon very partially positive. This will attract the electronegative
oxygen on water to attack the carbon and collapse the electrons between carbon and nitrogen up onto
the nitrogen to give us a molecule that now has a single bond between carbon and nitrogen,
two hydrogen atoms and one lone pair for no charge. And let's not forget the
oxonium which is oxygen bound to carbon with only one lone pair and a positive charge.
For the next step we have a proton transfer which can be shown in two different ways.
Some professors will allow you to show an internal proton transfer
where the neutral nitrogen will grab a proton, collapsing the electrons back
onto oxygen. Other professors want you to show it in two distinct steps as follows:
Step 1, a water molecule in solution grabs that proton, giving oxygen back its electrons. And Step
2 which I'll show in the same drawing nitrogen reaches for a hydrogen off another hydronium
molecule giving oxygen back its electrons for what ultimately looks like a proton transfer
because nitrogen now has three protons, one from the first proton transfer,
second proton transfer and the one it just grabbed in solution since it has four bonds and no
electrons it gets a positive charge meanwhile the oxygen has its initial lone pair along with the
electrons that came from the bond between itself and hydrogen making it neutral. But this molecule
is still unstable because of the positive nitrogen which again makes carbon partially positive,
attracting this oxygen to attack as follows. Oxygen will use its lone electrons to collapse
towards the carbon forming a pi bond, forming too many bonds to carbon which will kick out the
best leaving group, a nitrogen with three bonds which gives us a molecule that looks like this.
We have our initial skeleton. I'm going to move the oxygen up here with the initial bond from
when the oxygen attacked and the second bond from the electrons that collapsed towards the carbon.
Oxygen has one hydrogen atom and it still has one lone pair of electrons for a positive charge.
Off in solution we have the nitrogen with its three hydrogen atoms from the three subsequent
attacks along with the lone pair that it got from being kicked off the carbon atom.
Nitrogen, in this case, ammonia is happy to float away somewhere in solution but we still have to
get rid of that positive charge which happens when yet another water molecule in solution
uses its electrons to grab the hydrogen and give oxygen back the electrons to finally,
finally give us our final product which is our carbon skeleton with a double bond to oxygen,
the initial lone pair and the lone pair it just got from the proton removal.
Wow, that's a lot. I challenge you to go back to this reaction and ask yourself,
what is the pattern? I notice that we simply had a series of steps for protonating nitrogen and
breaking the bonds between carbon and nitrogen. Now that we have the mechanism, let's look at a
shortcut to quickly identify the product without having to draw out all the steps but first make
sure you give this video a thumbs up, subscribe to my channel and hit that bell notification so
you don't miss out on any new videos. As soon as you identify a Nitrile reacting with a Grignard,
know that your product is going to be a ketone as follows, break the bond completely between carbon
and nitrogen to redraw your skeleton with just a carbon, put a carbonyl on to that carbon and
then add a single bond to the R group that came from the Grignard, in this case a methyl.
For even more on Grignard reactions along with the Grignard cheat sheet, visit my website link
below or go to Leah4sci.com/Grignard, the link again Leah4sci.com/Grignard.