professor Dave here let's talk about carbonic acid derivatives
so let's take a look at carbonic acid so that's this here we can think of it almost like a
double carboxylic acid we've got carbonyl and then we've got OH on either
side so this is actually fairly unstable in water we this is found in any
carbonated beverage this is in your sodas and stuff because essentially we
can get what we would consider decarboxylation CO2 will evolve and
we'll get a water molecule so this is what's making your drinks fizzy and so
but some of its derivatives are very important substrates or even
intermediates in certain synthetic techniques so we want to look at some of
the derivatives of carbonic acid so let's look at this one here we've got
this dichloride this is called phosgene so just the way this is sort of a double
carboxylic acid we could think of this as sort of a double acid chloride this
is a highly toxic gas this was actually used in World War two as a chemical
weapon for a little while and so from phosgene we can actually do a lot of
things we can react phosgene with an alcohol like say methanol and that can
give us a carbonate so this is a dimethyl carbonate this is essentially
like a double ester we could think of it as an ester over here in an ester over
there so we've got oo r and o are on either side of a carbonyl and then we
can react phosgene with an amine like ammonia to
generate a urea and so this is just this is just urea but there are other urea so
we can replace hydrogen with alkyl and those would also be considered ureas
but this is just called urea this is a natural product in your body and so
we've got this is essentially like a double amide right we've got we've got
amide on either side of the of the carbonyl so that is urea so these are
all symmetrical derivatives right we've got double acid chloride double ester
double amide but we can also have asymmetric
we can have asymmetric molecules this is called a urethane or it can also be
called a carbamate and so we've got OR and then we've
got amine so it's sort of like an ester and an amide combined is essentially
what that is and then we've got a chloroformate so
chloroformate we've got OR and we've got Cl so this is
essentially like an ester and an acid chloride combined so these are your
derivatives of carbonic acid so these are useful these have a lot of different
utilities let's quickly look at urea synthesis so we want to know how to make
ureas as we said we can go from phosgene to some urea and so let's quickly get a
closer look at that so we can understand how that's done so let's say we've got
phosgene we're gonna react with some a me and maybe not ammonia but maybe a
primary amine of some kind we know that that's going to be able to attack here
and so we've got a situation where the a mean has displaced one of those
chlorines and now so let's take another equivalent of this amine and what we can
do is this going this is going to grab this proton here and then this will form
a second bond between nitrogen and carbon and that'll kick off this
chlorine and so that gets us to this here so we've got a double bond between
nitrogen and carbon and we've got we've still got that carbonyl there and so
this is called an isocyanate so this is an isocyanate here that refers to this
functional group here and now what we can do with the isocyanate is we can
have another equivalent of that amine attack and we know that this is going to
go ahead and attack here and then this pi bond can just go ahead and get
protonated so let's get that proton and so that's going to get us to to a urea
we've got this particular urea so we added one a mean there and then we
kicked off that other chlorine and then we added another equivalent of that a
mean here and so that got us this urea now one thing we want to understand is
this isocyanate this is this intermediate here under certain
conditions that can actually be isolated so if that could be so sometimes if we
can't do it we're just going to get this symmetrical urea here but we can also
get an asymmetric urea if we are able to isolate this isocyanate and if we were
able to do that that would mean that we could react with a different amine so
let's say we've got some other alkyl group on there that would mean that we'd
be able to get an asymmetric product so this would have been from the first
reaction and this would have been from the second reaction so there's a lot we
can do here in terms of synthesizing urea whether they're symmetrical or
asymmetrical so that is one way of synthesizing urea so we just looked at
one way to synthesize urea so now let's look at some ways we can get other
carbonic acid derivatives one of these ways is going to involve generating
something called an acyl nitrene so let's look at how we're gonna do that let's
start with this acid chloride and we can react with the azide ion so you may not
have seen the species before anytime you see N3 minus you will eventually see
this that's what this is we've got n double bond N double bond N and so of
course these outer nitrogens bear a formal negative charge because they're
contributing six electrons they've each got two lone pairs and then the central
one has a formal positive charge it's only contributing four and so this
obviously is going to be able to attack the carbonyl carbon and we're going to
get this acyl azide so this acyl azide is quite unstable and what can
happen is it's very it's very energetically favorable to lose N2 this
is just nitrogen gas so if this if this bond leaves with this nitrogen right
here that's going to neutralize that nitrogen and we're just going to get N2
but the other thing we're gonna do and especially if we heat this up a little
bit this is what's gonna happen so what we're left with is this acyl nitrene
now this is a bit of a strange species because if you look at this nitrogen
this does not have a full octet right we've got this bond and we've got two
lone pairs so it is it is neutral it does not have a formal charge but it
does not have a full octet so this is an extremely unstable compound it's a
highly reactive compound so now what can we do with an acyl nitrene so
if we have an acyl nitrene this wants to react with something really bad and if
it doesn't have anything to react with it'll actually react with itself and
this is called a Curtius rearrangement so what can happen is this lone pair
that's gonna go ahead and form that double bond there and then this R
group is actually going to migrate over to the nitrogen and so now this this
alkyl group that was connected to the carbonyl carbon is now connected to the
nitrogen instead and we formed that CN double bond as well so we've got we've
got our isocyanate and this is a very powerful synthetic technique because
we've completely rearranged the molecule the R group and now instead of being
attached to the carbonyl is on the other side of that nitrogen so that's quite an
interesting transformation and now from the isocyanate we just saw how from the
isocyanate we can get to the urea but we could react with other things too we
know that we're gonna be able to do some reactions right there's a lot of react
there there's this carbonyl carbon is reactive we can have nucleophiles attack
there so we could we could use an alcohol and we could get a carbamate
right if we add this row R prime right there we're gonna get a carbamate we if
we use water instead if water attacks we could get carbamic acid and so this is
carbamic acid we've got the amine over here and then OH over there the only
thing is that this is quite unstable this will rapidly decarboxylate so we're
gonna lose all that and we're gonna end up with the amine over there so this is
very interesting chemistry with the Curtius rearrangement just to give you
one other method of forming the acyl nitrene that is a little safer we can
start over here we've got this amid and we can react with bromine under basic
conditions so we need very strong basic conditions and so this hydroxyl is going
to go ahead and grab one of these protons over here so here's our
intermediate we've got two resonance structures so we've got the negative
charge there or we can put the negative charge up here let's work from this
resonance structure this negative charge can go ahead and grab one of the bromine
atoms in the bromine molecule and so now we're here right bromine is
attached to nitrogen right there and now again we're in strongly basic conditions
so actually another another hydroxide can grab this proton and then this will
stay here and this will leave so what we've done here is we've formed a
nitrene again so that bromine is gone this nitrogen already had a lone pair it
now has an additional lone pair from these electrons in the nitrogen hydrogen
bond and we have formed a nitrene in another way so these are very
interesting intermediates that can be used to do some interesting chemistry be
at the Curtius rearrangement or otherwise so that's a little bit about
how to synthesize carbonic acid derivatives