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Transcription de la vidéo

what I want to do in this video is to talk about nucleo Felicity nucleo nucleo fearless fearless city Felicity nucleophilicity and this is really just how good of a nucleophile something is or we could I'll just make up a definition right now the ability the ability for and Adam let me say an atom let me write this atom / ion / molecule molecule to act as a nucleophile or to give away to give away extra all right that in quotes extra electron and bond and bond with a nucleus or with something else with I'll say with a nucleus with a nucleus let me say with with with and i'll just say end bond but that's you know I want to say with a nucleus because that's what nucleophilicity is saying it loves nucleus is especially positive ones then it can give its extra electron to it now as a first cut if you want to identify a good nucleophile it should have extra electrons to give away so it should have extra electrons to give away so have have extra electrons to give away to give away and the best things that have extra electrons to give away our negative ions or anions so if you're just you know just at a very high level something like something like the fluoride anion normally fluorine has seven valence electrons one two three four five six seven but it's so electronegative it might be able to swipe off and a leather electron from something else and then it becomes the fluoride anion so then it becomes the fluoride anion with a negative charge and you can do that for all of the halides you could do that for chlorine can become chloride bromine can be bromide iodine could be iodide let me do what I too so iodine once again it's a halide it has seven valence electrons it has many many more electrons than fluorine but if you just look at its valence shell it has seven electrons and then it is also reasonably electronegative not as electronegative as fluorine remember the trend goes like this from the bottom left to the top right so fluorine is is extremely electronegative but iodine is still pretty electronegative it is a it is a halogen so it will it also might be able to swipe off an electron from someone else and become iodide and become iodide so in general things with negative extra with extra electrons lone pairs of electrons and especially a negative charge are going to be pretty good are going to be pretty good nucleophiles and you know another example that's not a halide is the hydroxide anion so oh h and this is an example of something that is a molecule so oh h maybe you know traditionally water would look like this so traditionally this is just neutral water and oxygen has two lone pairs like that but oxygen is pretty electronegative it is already kind of taking the electrons away from one of the hydrogen's and at some point it might just take it all together and then you have the hydroxide and then you have the hydroxide anion so this will look like this so you have your original two lone pairs just like that and then you have this pair that's going to be taken it already had that electron it takes that electron from the hydrogen so now it has two more electrons let me color code it so you see what it took it took this electron from the hydrogen and now this also has a negative charge that's also a reasonably good nucleophile and of course you have your hydrogen now it lost its one electron it only has a proton its nucleus so it's just when you ever see h+ this is really just a proton there's nothing else to that hydrogen so that's hydroxide so these are all reasonably good nucleophiles and that they can give they have something to give away they have extra charge now what i want to do is think about between these how do you think about what's going to be a stronger or weaker nucleophile in here it becomes a little bit more nuanced we're going to do is differentiate what happens in a protic solvent versus what happens in an a protic solvent so let me write down let me start with a protic solvent protic predict solvent I'll make two columns here protic solvent and then we'll do a protic solvent a protic a protic solvent solvent right over here and once again these are fancy words but they mean something pretty simple protic solvent is something that has hydrogens that can be taken away or it might have free protons flowing around an example of a protic solvent is water or really any alcohol so water is the simplest example or maybe the most common and the reason why we call it while you might have protons floating around is exactly this little reaction i showed right here maybe every now and then hydroxide anion forms or even more likely maybe a water takes a hydrogen from from one of the other waters water molecules takes a turn from but one of the other water molecules and becomes a hydronium where it's where it is it's not a proton necessarily but it has it's an oxygen it's an oxygen that has so let's say if you if you start with water and this one of these electrons were to be given to some proton floating around it would look like this and it has a positive charge and then this proton is very available it's kind of you can almost imagine it's almost floating around because that oxygen really wants to take back that electron so a protic solvent is water in water you might see a little bit of hydroxide a little bit of proton a little bit of hydronium you see all of it in there but the bottom line is that there are protons that can react with other things you have protons that can react with other things let me clear this away so that I have some real estate so let me just write down water so water is a protic solvent now I say hey wait is that always the case you know it seems like hydrogen's are everywhere well no it's not always the case let me show you an a protic solvent so diethyl ether looks like this diethyl ether looks like this and just so you know the naming it's an ether because it has an oxygen and it's diethyl because it has two ethyl groups that's one ethyl group right there and that is the second so it's diethyl ether now you might say hey this cut this guy's got hydrogens lying around as well maybe those can get released but no these hydrogen's are bonded to the carbon and carbon is not anywhere near as electronegative as oxygen so these are not going to be carbon is unlikely to steal these hydrogen's electrons and these hydrogen's to be loose if they were bonded to the water the oxygen that would have been a possibility so in water you have you know obviously you have h 0 h and alcohols alcohols you have some maybe carbon chain bonded to an oxygen which is then bonded to a hydrogen so in either of these cases in either water or alcohol you have these hydrogen's where the electron might be taken by the oxygen because it's so electronegative and then the hydrogen floats around anyway that's a review of product versus a product now in a protic solvent and this is actually a general rule of thumb if a nucleophile is likely to react with its solvent it will be bad at being a nucleophile and think about it if it's reacting with the solvent it's not going to be able to do this it's not going to be able to give its electrons away to what it needs to give it away to maybe you know what we saw and say an sn2 type reaction so in a protic solvent what happens is is the things that are really electronegative and really small like a fluoride anion so let me draw a fluoride anion a fluoride anion in in a protic solvent in a protic solvent what's going to happen is it's going to be blocked by hydrogen bonds so it's very negative right it has a negative charge and it's also tightly packed as you can see right here as its its electrons are very close tied in its much it's a much smaller atom or ion in this case and if we looked at iodide iodide has 53 electrons many orbitals fluoride or actually iodide would have 54 it had the same as I owe dine plus 1 fluoride will have 10 electrons 9 from Florian plus it gains another one so it's much smaller it's a much smaller Adam and so when you have water hanging around it let's say you have something like water that has a negative charge water water is polar water is polar and actually both of these are polar so I should write down polar for both of this this is a polar polar protic solvent this is a polar a protic solvent in this case water is still more electronegative than the carbon so it still has a partial negative charge these parts still have a partial negative water still has partial negative the hydrogen has a partial positive charge so it is going to be attracted to the fluorine and this is going to happen all around the fluorine and if these waters are attracted to the flooring and kind of forming a shell a tight shell around it it makes it hard for flooring to react hard hard to react hard to react so it's a worse nucleophile then say iodide or hydroxide in a protic in a polar protic solvent hydroxide has the same issue it's still forming hydrogen bonds but if you wanted to compare them hydroxide is sorry iodide is a much is a much bigger you know maybe I draw it like this you know I'll draw its valence shell like this it's a much it's a much bigger much bigger I on you know it has all these electrons in here and so it still will form hydrogen bonds with the water it still will form hydrogen bonds with the hydrogen end of the water because they're partially positive but it's going to be less tightly packed and on top of that iodide is more polarizable it's more polarizable so this is polar polarizable which means that its electron crowded so big and you know the valence electrons are so far away from the nucleus that they can be influenced by things and then be more likely to react so let's say this iodide is getting close to a carbon that has a partial positive charge so let's say a carbon is attached to I don't know let's attach it's attached to a bromine and then it's attached to three hydrogen's we've seen this will have a partial negative charge it's more electronegative than the carbon which will have a partial positive charge and so when this guy this big guy with the electrons really far away gets close to this the electrons are going to more of the electron cloud is going to be attracted to the partial positive charge so it'll get distorted a little bit and so it is more likely to react in a polar protic solvent floor it floor ride on the other hand very tightly packed blocked by blocked by the hydrogen bonds less likely to react so if you were to look at if you look at the periodic table if you look at say just the halogens just the halogens in a polar and a polar protic solvent the halides so this would be the ion version of the halogens the halides iodide is will be the best nucleophile fluoride will be the worst so in a polar protic solvent let me write this down and polar protic solvent in a new color polar protic solvent then we have a situation where the iodide is the best nucleophile iodide followed by bromide followed by followed by chloride and then last of all is the fluoride now the exact opposite true is true in a a protic solvent and in a protic solvent the fluoride with which is you know fluorine is far more electronegative fluoride is more basic it wants it will be more stable if it is able to form a bond with something then iodide iodide is pretty stable it's actually if you look at a hydrogen hydrogen iodide it's actually a highly highly acidic molecule so iodide itself the conjugate base of hydrogen iodide is going to be a very bad base so when you're dealing with an a protic solvent you go in the direction of basicity and we're going to learn in the next video that actually basicity and nucleophilicity are related but they are the same concept we're going to talk about that in a little bit so but if you know if you're in an a protic solvent you're no longer as light you're not reacting with the solvent as much and in this situation fluoride is actually the best nucleophile followed by chloride followed by bromide followed by iodine so here you're going in the direction of basicity this is the best this is the worst in a a product and an a protic solvent if it was in a protic solvent this is flipped around this becomes the best and this becomes the worst