Georg Wittig

Transcription

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. thanks for watching guys, subscribe to my channel for more tutorials and as always feel free email me with questions