in this video we're going to go over the wittig reaction and so here we have a ketone and we are going to react it with a phosphonium illid which looks like that what do you think the major product for this reaction will be the wittig reaction converts ketones into alkenes and one thing i like to do is rotate the ketone in such a way that the carbonyl group faces the ch2 group all you need to do is replace the oxygen with the ch2 group or whatever is attached to that double bond and so the product for this reaction looks like this which you can draw this way if you want and so as you can see the wittig reaction is very useful for making alkenes from ketones now let's try another example so let's say we have this ketone and we wish to react it with an illid that looks like this go ahead and predict the major product for this reaction so what we need to do is replace the oxygen with this group and so the product will look like this now i'm gonna draw differently if you want to you can rotate this molecule you don't have to but if you want to so we have an ethyl group on one side and the methyl group on the other side and then it's going to react with the illid and so let's replace the oxygen with this group for me i think it's easier to see it this way now let's redraw the product and we're going to center our drawn on the cc double bond so first we have a carbon-carbon double bond here we have an ethyl group attached to it so that's ch3 ch2 and then here we have a methyl group so just ch3 now attached to this carbon is a hydrogen and a methyl group so we could put the h at the top or we could put the h on the bottom and so as you can see the wittig reaction can give you a mixture of enz isomers so on the left side this is the highest priority group and on the right side the methyl group has more priority than the hydrogen so this is the e isomer and here we have the z isomer now let's look at another example so here we have cyclohexanone and let's react it with an illid that can be presented in a different form so sometimes you'll see a positive charge on the phosphorous atom and a negative charge on the carbon atom if you see this this is one way in which you could represent the illid but if you take the lone pair and form a pi bond then you can get the inlet that you're used to seeing which looks like this and so let's react that with cyclohexanone which i'm going to draw this way so let's replace the oxygen with the group that we see here so the product is going to look like this and that's one way in which you can draw now let's focus on the cc bond so on the left side that carbon is part of the cyclohexane ring on the right side this carbon atom is bonded to a hydrogen and an ethyl group so there are no cis or trans or e and d isomers in this example because for the carbon on the left the top is the same as the bottom so in this case we only get just one product so now let's go over the mechanism for the wittig reaction now what we need to start with is triphenylphosphine it's basically a phosphorus atom that is attached to three benzene rings okay that is a terrible bending ring let's do that again and the phosphorus has a lone pair so that's ph3p and of course let's not forget the double bonds in this ring so what we're going to do is we're going to react triphenylphosphine with an alkyl halide now this alkyl halide you want it to be either methyl bromide or a primary alkyl halide because this is an sn2 reaction secondary halides could work but just be careful with those tertiary halides they don't work very well for sn2 reactions so you need to avoid that but ideally you want to use methyl halides and primary halides because they work very well for an sn2 reaction so here we have ethyl bromide so what we're going to do is add an ethyl group to the phosphorus so right now this is a ch2 and this is a ch3 so we can write it as ph3p dash ch2 ch3 now the phosphorus atom has four bonds so it has a positive formal charge so i'm going to redraw the structure like this so i'm going to write out the ch2 now the next step is to react it with butyl lithium whenever you have a carbon atom that is attached to a metal that carbon atom is nucleophilic and so that carbon is going to abstract a proton from the carbon that's attached to the phosphorus group because this hydrogen is more acidic than these hydrogens the conjugate base is stabilized by resonance and so once we take away this hydrogen we're going to have a negative charge on this carbon and so this is one resonance form of the illid and to draw the other resonance form we could take a lone pair and form a pi bond and so these are the two resonance forms of the ilit now for the next step i'm going to use this resonance form to react with the ketone or and aldehyde so let's use acetaldehyde as an example and let's put the illid in the right position to react with it so the illa that we're using the carbon had a hydrogen and a method group attached to it so this carbon has a negative formal charge and the phosphorous atom has a positive formal charge now the carbonyl carbon is partially positive and the oxygen has a partial negative charge so therefore this nucleophilic carbon is attracted to the electrophilic carbon and so it's going to attack this carbon causing the pi bond to break preventing carbon from having five bonds and so these pi electrons will be used to form a bond between oxygen and phosphorus and so what we're gonna get is a four membered ring that looks like this so this is what we now have this is called an axa phosphatane now what do you think is going to happen next we know we need to generate the alkene we need to form a pi bond here and so therefore this bond and this bond has to break in order to get the alkene so let's focus on the carbon phosphorus bond when that bond breaks where are the electrons going to go are they going to go towards the carbon atom or are they going to go towards the phosphorus atom now if you go to google images and you look up electronegativity table you'll see that carbon is more electronegative than phosphorus so carbon bears the partial negative charge relative to phosphorus and phosphorus bears the partial positive charge so when the carbon phosphorus bond breaks those electrons are going to go towards the more electronegative carbon now what about the carbon oxygen bond we know that oxygen is more electronegative than carbon so when a carbon oxygen bond breaks those electrons will go towards the oxygen and so what we're going to get is a triphenylphosphine oxide as a side product and our main product will be the alkene and so we have a hydrogen and a methyl group on both sides so this is the cis isomer we can also get the trans-isomer and typically the trans isomer is more stable than the cis isomer and so you want the bulky groups to be further apart from each other so this is going to be easier to form but you do get a mixture of both and so that is the mechanism for the vidic reaction you