hey it's Professor Dae, let's talk about epoxides
so we know a little bit about the functional groups that are generated
when oxygen is attached to hydrogens or alkyl, we know H2O is water
that's an oxygen attached to two hydrogens, if we replace one of those
hydrogens with alkyl we get an alcohol, and then if we replace the other
hydrogen with another alkyl group that's gonna be ROR these are two alkyl groups
were gonna get an ether, so we may or may not know a little bit about ethers but at
any rate a cyclic 3-membered ring that is either is called
an epoxide, so the simplest form of an epoxide is oxacyclopropane, it's just
literally just a three-membered ring with the oxygen as one of those members and
what we need to understand about an epoxide is that it is
susceptible to nucleophilic attack in a way that we might not expect for simple
carbon oxygen bonds. so see the thing is that we we might look at this carbon
oxygen bond and go, yes that's a polar bond but it's not any more polar than
your typical carbon-oxygen bond you might see from a hydroxyl
group but the thing is that there's a lot of ring strain going on here
these are 60 degree angles but these are sp3 carbons so we know that sp3
carbons like to exhibit tetrahedral geometry, that's 109.5
degree bond angle, so we're taking a 109.5 degree bond angle, and we're
compressing that into 60 degree angles so it's kinda like compressing a spring
it's really eager to to release and so that makes this irregularly
susceptible to nucleophilic attack so here's some SN2 nucleophile
this is going to pop open like that and so this what we're
gonna get, it's an SN2 product, the only difference is that the leaving group
which is this oxygen, once it picks up a proton is going to remain tethered to
the molecule so it's not a typical reaction but so it's the ring strain
that's driving this. now the other thing that we want to understand is that
there is a high degree of
regioselectivity in terms of which carbon nucleophiles go to attack and the
reason for this is sterics. so let's say we have another SN2 nucleophile
that wants to do something here, now it's got a choice of which carbon it's going
to attack. the thing is this has two methyl groups on it, this doesn't have
any alkyl on it so from a steric standpoint it's a very easy
choice, this is gonna go all the way over here, it's gonna attack that carbon
because it's much more sterically available
gonna pop this epoxide open and there we go we've got our methoxy group that
attacked and then here's our two methyl groups and then there's the hydroxyl
once it picks up a proton it'll turn into a hydroxyl group and so once again
it really is the ring strain that's driving these these nucleophilic
substitutions and so this is one thing that epoxides will do
so we've seen what happens when an epoxide participates in a substitution reaction now let's
look at what happens to an epoxide under acidic conditions. so here's our epoxide
now we know that this just like any oxygen, just like a hydroxyl oxygen
oxygen can go ahead and protonate, and then what happens is that, before we saw how a
strong nucleophile was going to be able to do substitution on an epoxide but
once this is protonated this behaves similarly to a hydronium, or
an OH2+ on a linear molecule and so this is gonna be susceptible to attack from
even just something like methanol so this is gonna go ahead and pop open and
then once after the fact, that'll deprotonate, a solvent
molecule will pick it up we're going to get our product right here. so once again
in acidic conditions this can protonate and then even something weak
like methanol is gonna go ahead and pop open the epoxide because once again the
ring strain as well as wanting to neutralize that oxyanion. now here's the
thing, we looked at the regioselectivity before where a strong
nucleophile was going to want to substitute at the less
sterically hindering position, now here's the thing, let's say we're looking
at acidic conditions here, so now we protonate again, now you know how, let's
say, to do like an SN1 we might protonate a hydroxyl then we've got a water
leaving group, the water's gonna leave, its gonna leave a carbocation, so this is a
little bit like that in the sense that once we protoonate this is gonna be very
happy to open up because a carbon atom can better accommodate a positive charge
than an oxygen atom because the carbon is more electropositive so it's
gonna wanna get that positive charge, so this ring is really ready to open
up and so that means that one of these two bonds is going to begin to weaken
its gonna begin to weaken a little bit and so what that means is that because
this position is more substituted than this position, this bond is going to end
up being much more likely to begin to weaken than this one because as this
bond weakens we begin to generate some partial positive character right there
and because this is a quaternary carbon it's going to be
very capable of sustaining that partial positivity because it is more
substituted, so the thing is here under acidic conditions this is actually gonna
go against sterics and go to the more hindered side and pop it open that way
once again solvent will after the fact take that additional proton and
we're gonna get this. so this is a regioselectivity that is different from the
one that we saw, because of these acidic conditions we're going to see this bond
weakening because this carbon is more able to sustain this partial
positive charge than this one is, and that partial positivity, that
electrostatic interaction outweighs any of the steric influence so that's what an
epoxide will do under acidic conditions.
since we are talking about exposes we might as well talk a little bit about how they might form and one strategy we have for
generating epoxides involves reacting alkenes with certain peroxybenzoic
acids like MCPBA. MCPBA is meta-chloroperoxybenzoic acid so meta-chloro
means from this group there's a chloro in the meta position, so meta-chloro, and
then peroxybenzoic acid, we know benzoic acid is a benzene ring and a carboxylic
acid so that would be the carbonyl oxygen and then 0H but the
peroxy, so like hydrogen peroxide, we're talking about an oxygen-oxygen covalent
bond. so peroxy is why we have the OOH so it's a little bit different than a
typical carboxylic acid, so alkene + MCPBA, one of these oxygen atoms is getting
transferred into that pi bond and we have an epoxide so this is a
synthetic strategy we have for generating epoxides to be used in
synthesis so let's really quickly look at the mechanism. now an oxygen-oxygen
covalent bond is actually pretty weak so it's susceptible to attack by this pi bond
let's say this flips over like that, not literally, but let's just pretend it
goes right here like this then we're gonna get this oxygen-oxygen bond
snapping over
onto that carbonyl carbon, this pi bond is gonna go ahead and grab that hydrogen
and then this bond will be the other bond forming the epoxide. so really if we
want to make sure we see which bonds are in the epoxide it's
going to be that one and that one so these are the two bonds that have formed
the carbon-oxygen bonds in the epoxide and so this blue oxygen which is the one
that's further away in MCPBA that's the one that ends up in the epoxide so we've
got these two bonds right here and then the rest of this is a leaving group as
a carboxylic acid, we basically just snagged that one oxygen and shoved it
into that pi bond and so this is a synthetic strategy we have to generate epoxides.
thanks for watching guys subscribe to my channel for more
tutorials and as always feel free to email me