hey it's professor Dave I want to show you the Beckmann rearrangement.
so we're gonna take a look at this reaction called the Beckmann rearrangement
developed in 1886 by a german fella named Ernst Beckman so it's pretty old
chemistry here and and the purpose just to give you the broad overview the
purpose of this reaction is to transform a ketone into an amat so you can see
we've got two our group's here R 1 and R 2 on this ketone and then we're going to
the amide which essentially means that all we've done is we've taken a nitrogen
atom and we have inserted it into one of those carbonyl carbon alkyl group bonds
so that's right in there so the R 2 is shifted one position over and we've got
the nitrogen in there instead so let's take a look at the mechanism here we've
got our ketone here and we're gonna we're gonna react with with this
compound here NH2OH and this nitrogen atom is able to attack here okay so and
and we're gonna we're gonna skip a little bit here but we know that this
oxygen there's gonna be some proton transfer we're gonna lose that oxygen in
the form of water that's going to leave completely and then this group is now on
there so the nitrogen atom is going to attack it's actually gonna attack twice
it's kind of similar to some of the reactions we've looked at about imine
formation- and enamine formation that kind of stuff it's just going to
attack kick that water off and and we've got actually what's called an oxime so
this functional group is called an oxime where we've got C double bond N with
an with a hydroxyl on the nitrogen that's what that functional group is
called is an oxime and now what can happen is we can there's a couple of
ways to do this a very simple way is to just protonate it we can be doing this
in acidic conditions we can also turn this hydroxyl into some kind of leaving
group derivative like a sulfonate or something like that but we'll just keep
it simple we're just gonna protonate so we've got a water leaving group right
there and now this is where the rearrangement takes place so what can
happen is this carbon-carbon bond can can go here kicking off water and then
nitrogen can go let's put that lone pair can go and pick up the alkyl group so
you can see that this carbon-carbon bond has now become a third carbon nitrogen
bond thus kicking off water while nitrogen
goes in and coordinates to that alkyl group in its place
so it's we've we've rearranged things essentially based on on thermodynamics
right we're talking about bond energies but if we cook it up this is what's
gonna happen a little bit of heat we can get rid of water and we can totally
rearrange this thing so it's a little tricky looking but see if you can follow
the bonds what we're left with has two resonance structures we'll look at this
one first we've got our one connected to this carbon and now this carbon has that
third bond right here right this arrow means there's now three bonds to
nitrogen we've kicked that off but this nitrogen is now coordinated to R two
right so this is the key step this is the way that nitrogen gets stuck in
there it's not that nitrogen was forced into a carbon-carbon bond it's due to
this rearrangement where things sort of swap places so nitrogen is now bound to
r2 this is one of the resonance structures of what we're left with but
we can also this has a formal positive charge here we know that carbon is more
electrical more favorable for carbon to be positively charged than nitrogen and
so we can take one of these pi bonds put it onto nitrogen there by putting the
positive charge over onto carbon these are two resonance structures this one
may be slightly more contributing and this is also the one that will make
sense water is now going to react with that carbon and there will be some
rearrangement and we're going to end up with our amide product okay so this is
the very basic most basic mechanism for the Beckmann rearrangement there is a
little bit more to talk about because the thing is if we have an asymmetric
ketone meaning two different R groups we can get a discrepancy where we then have
to decide which R group is going to be the one that migrates in the
rearrangement and so we could go over many many examples but just to summarize
basically the R to group in this case the one that is migrating because this
sigma bond is is is sort of is sort of dissipating
as this one is forming it does have to sustain a very slight partial positivity
in the migration therefore the more substituted our group or the carbon that
is more substituted connected to this carbon is going to be the one that's a
little bit more favorable to be transferred now there are many examples
and we can look at product distributions but to just say that in a very basic way
that the group let's say that this was a more branched group than this one that
is what would make that more favorable to be transferred but we can have a
product mixture with each group migrating in different circumstances but
this is the Beckmann rearrangement we've got a ketone we're forming the ox I'm
and then this is the key rearrangement step where things sort of swap order and
then water attacks and we get our amat so that's the basic mechanism for the
Beckmann rearrangement
so let's just look at two more things real quick with
the Beckmann rearrangement let's just take a look at a cyclic a cyclic
substrate it's basically the same thing but it's just a little bit interesting
we've got cyclohexanone again we're gonna do the same thing react with the
same material and we're gonna get our ox I'm just like before we're gonna get our
oxime but then what's gonna happen after the rearrangement remember that
we're inserting that nitrogen right this is going to pop out and the nitrogen is
going to end up in the ring and so what we end up with is a cyclic amide and
just an interesting bit of information a cyclic amide is called a lactam just
like a cyclic ester is called a lactone the similar terminology is for a cyclic
amide we're gonna call that a lactam and this is actually very interesting
chemistry because you'll notice we've created a seven membered ring right and
so six membered rings are very common seven membered rings are a little bit
more difficult to form and so if you happen to want a seven membered lactam
this is actually the a very very good technique to use because in you're
getting both the seven membered ring and the nitrogen insert in the ring in one
reaction here so we've got our seven membered lactam so that's what
happens when we use a cyclic substrate so
one limitation that we also want to talk about with the Beckmann rearrangement
remember that we said that the the our group that migrates tour that forms the
new bond to the nitrogen is typically going to be the one that is more
substituted because it has to sustain some partial positivity in the
transition state as it migrates as it coordinates to the nitrogen and so we do
want to be aware of something called a Beckman fragmentation so this is
actually Beckman fragmentation and so what happens is we've got this group
here right so we've got this is a tertiary carbon and so what we expect we
know that this carbon sorry this bond is going to go here and we're gonna kick
off water but what we expect is for the nitrogen to then go and and finish off
the rearrangement and coordinate to this carbon but because this is a because
this is a tertiary carbon and tertiary carbo cadion is quite stable sometimes
what we can get is just a fragmentation whereby we do not achieve this nitrogen
carbon bond and so if you follow we've got our group and then this carbon and
then the new bond we've got a triple bond to nitrogen and we're just going to
be left with a nitrile and then the tertiary carbocation will be left on
its own will lose one of these protons will will right will get will get the
alkene product and so this is just something to be aware of if you have a
situation like this where you have a lot of branching right this carbon has is
connected to three other carbons so it's it's energetically feasible for this to
remain on its own without having to gain another bond to nitrogen so this is the
Beckman fragmentation this is the one main limitation we need to be aware of
when looking at Beckmann rearrangement but that's a little overview of the Beckmann rearrangement.