hey guys, its professor Dave. I want to tell you about Witting reactions.
so the Wittigreaction is another reaction that generates new
carbon-carbon bonds
specifically carbon-carbon double bonds so the Wittig reaction makes
alkenes so take a look here, the substrate in a Wittig reaction will always
be some carbonyl containing compound like a ketone or aldehyde
and here these alkyl fragments could be anything, i've drawn them as methyl groups
but they
could be absolutely anything and then that is going to react with a Wittig
reagent so this is a very specific kind of molecule here we see
a phosphorus atom bound to three phenyl groups, a phenyl group is simply a
benzene ring
as a substituent on a larger molecule and then also bound to some alkyl
fragments, again these alkyl fragments can be absolutely anything
so the phosphorus atom there bears a formal positive charge
and the carbon atom that it is attached to has a negative charge will see why in a
minute
and so those two will react to generate some alkene
these alkyl fragments being whichever ones these were
and then this as a byproduct so what is this Wittig
reagent what is that, how do we form it, well here we see the triphenylphosphine
so here's a phosphorus atom
the three benzene rings there and then a lone pair
phosphorus has five valence electrons so we have three covalent bonds and
then a lone pair
and then this triphenylphosphine can just do regular SN2
on some alkyl halide like an alkyl bromide
let's say, so the lone pair is gonna go attack that carbon, this is a
there's a polarity to that bond and just regular SN2 so now we will have a
phosphorus-carbon bond
and furthermore the phosphorus will have a formal positive charge
right because it has given one of its electrons in that lone pair
to the carbon atom so now we have that bond then a little bit of base
can extract a proton off from whenever
there could be a hydrogen
attach here, R is a general alkyl, could be isopropyl or anything like that
so a base would extract a proton and now we have a formal negative charge
on this carbon so this is the Wittig reagent is what results
is an example of a ylide
a ylide is a kind of zwitterion
we know that a zwitterion is a molecule that has both a positive and negative
formal charge
on the same molecule like an amino acid would be an example
but a ylide is specifically a zwitterion
that bears the formal positive and negative charges on
adjacent heteroatoms so heteroatoms being atoms of different elements
and then adjacent because they are next to each other so here's a phosphorus
and here is a carbon they are adjacent they are different elements
so that qualifies this Wittig reagent as a ylide
and this Wittig reagent is going to react with some carbonyl containing
compound so let's take a look at what this mechanism looks like
so I 've just taken what's up here and redrawn it to show you what's going on
and we know that the carbon
connected to the phosphorus in the Wittig reagent is gonna have a formal negative
charge
and so just like with all the other reactions we've been looking at throughout
this entire series
we know that reactions are simply examples of electron access
interacting with electron deficiency so some negativity some positivity
they react so it shouldn't be much of a surprise that a formally
negatively charged
carbon atom will attack a partially positive
carbon atom in this carbonyl alright because of the dipole on the carbonyl
so that will attack here and then this pi bond will go up there the only way
this carbon atom can accept the new bond is by losing another one
to go up there and so these two methyl groups are here
these two methyl groups are here and now there is a new bond
between these carbons so there that is, that's one of the two new
carbon-carbon bonds
they're being formed by the reaction so there is that and now we have this
intermediate
and it shouldn't be too surprising once again that a negatively charged oxygen
would interact with the formally charged phosphorus
in an intramolecular fashion
so phosphorus can accommodate five bonds that violates the octet rule
as can almost any atom apart from
oxygen carbon nitrogen these ones so once were down a row
phosphorous, five valence electrons, five covalent bonds, works perfectly fine
phosphorus will be bound to those three phenyl group
this carbon down here and now a new bonds to this oxygen
so here we have this new cyclic this four-membered ring intermediate
and this is where something very interesting happens, the bonds
can rearrange to go back to the way they were or they can cyclize in a
different way
and go like this where we're get basically gonna
gonna flip these bonds in such a way
so as to have this bond go and form
a pi bond here between the two carbon atoms that had the new
sigma bond in the first place so that will snap shut there
forming the alkene product and then this one can flip up this way to form a
double bond
between phosphorus and oxygen to generate the byproduct
so thats or the key step in giving us our alkene product so both
of the new bonds the sigma and the pi bond
are being formed between these two carbons so when you look at
a Wittig reaction and are looking at the substrate and
the Wittig reagent what we want to understand is that we are always forming
two new carbon-carbon bonds and both of them are occurring between
the carbonyl carbon and the
negatively charged carbon in the Wittig reaction so all this other stuff
that just stays there right these two carbons are still there if this was a
cyclohexane
and this would be a cyclohexane there was more whatever it is that would all
still be there
the only chemistry that is occurring is occurring between that carbon and that
carbon everything else is staying the same
and so through this mechanism and because of this four-membered ring
intermediate these groups are able to rearrange to generate an
alkene product so we want to understand
about what the key players are what a Wittig reagent is
what it means to be a ylide, why that's important for the Wittig
reagent and then finally the mechanism
to arrive at the alkene product.
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