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Transcription de la vidéo

what I want to do in this video is differentiate between the ideas of nucleophilicity nucleo Felicity or how strong fearless city or how strong of a nucleophile something is and basicity and basicity and these the difference is on one level kind of subtle but it's actually a very big difference and I'll show you why it's kind of confusing the first time you learn it when we studied sn2 reactions you have a nucleophile that has an extra electron right here it has a negative charge and maybe you have a methyl carbon maybe you have a methyl carbon let me draw it let me draw it maybe you have a hydrogen coming out you have a hydrogen behind it you have a hydrogen up top and then you have a leaving group you have a leaving group right over there and in an sn2 reaction the nucleophile will give this electron to the carbon the carbon has a partial positive charge let me draw that partial positive charge the leaving group has a partial negative charge because it tends to be or it will be more electronegative and so this electron is given to this carbon right when the carbon gets that are simultaneously with it this electronegative leading group is able to take is able to completely take this electron away from the carbon and then after you are done it looks like this we have our methyl carbon so the hydrogen in the back hydrogen in the front hydrogen on top the leaving group has left leaving group has left it had this electron right there but now it also took that magenta electron it took that magenta electron so it now has a negative charge and the nucleophile the nucleophile has given has given this electron right over here and so now it is bonded to the carbon now the whole reason I did this is because okay this is acting as a nucleophile it loves nucleus is it's giving away its extra electron but it is also acting as a lewis base it is also acting as a lewis base and is a bit of a refresher a lewis base which is really the most general or i guess it covers the the most examples of what it means to be a base it lewis base means that you are an electron donor electron donor and that's exactly what's happening here this nucleophile is donating an electron to the carbon so it's acting as a lewis base so the first time you see that you're like well why did chemists even go through the pain of defining something like a nucleophile why don't they just call it a base why are there two different concepts of nucleophilicity and basicity and the difference is is that nucleophilicity is a kinetic concept it is kinetic which means how good is it at reacting how fast is it reacting how little extra energy does it need to react so something has good nucleophilicity it is good at reacting good at reacting it tells you it doesn't tell you anything about how stable or unstable the reactants before and after are just tells you they're good at reacting with each other basicity basicity is a thermodynamic concept thermo dynamic it's telling you how stable are the reactants or the products so it tells you it tells you how badly how badly something would like to react something would would like to react for example we saw the kitten situation of fluorine we saw the let's let's think about this we saw the exaggeration we actually I say fluoride so fluoride looks like this seven valence electrons for fluorine and then you have it swiped one extra electron away you get fluoride so floor ride is reasonably basic it is more basic than iodide so this is more basic than iodide more basic then then iodide but in a protic solution put in let me write it here but less less nucleophilic less nucleophilic in parodic in parodic solution in a protic solution once again has hydrogen protons around and the reason why this is is fluoride it wants to bond with let's say a carbon or something else more badly or maybe even a hydrogen proton it wants to bond with it more badly than an iodide anion and if it did it actually will be a stronger bond than the iodide and anion will form that the fluoride anion is actually less stable in this form then the iodide is if you were really able to get a pro to proton or give its electron away it will be happier but it's less nucleophilic it's less good at reacting in a product solution and the whole reason it's less nucleophilic is because there are other things that are keeping from reacting and we saw on the video on what makes a good nucleophile then in the case of fluoride it's because it forms hydrogen it's a very small atom actually say a very small ion and so it forms so it's very closely held the electron clouds very is it's very tight and so what it allows is the hydrogen's from the water to form a very tight shell a very tight shell around these all have partial positive charges so they are attracted to the negative anion and so they form a very tight shell kind of protecting the fluoride anion which makes it harder for the harder for it to react in a protic solution so doesn't react as well doesn't react as well if it was able to react it actually will form a stronger bond then the iodide anion so that's the big difference and just so we see the difference in trends so basicity it does not matter what your actual solvent is it is a thermodynamic property of the molecule or the or the anion so if you looked at just pure basicity basicity the strongest base you see and I'll just write hydroxide here it's normally something like sodium hydroxide or potassium hydroxide but when you dissolve it in say something like water they the sodium and the hydroxide separate and it's really the hydroxide that is acting as a base something that wants to donate electrons so hydroxide is a much stronger base than fluoride which is a stronger base than chloride which is a stronger base than bromide which is a stronger base which is a stronger base than iodide now if you were to look at if you were to look at nucleophilicity just to see the difference nucleo félicité we saw that there actually matters what's the solvent because the solvent will affect how good something is at reacting so nucleophilicity either is a difference between a protic solvent and an a protic solvent a protic solvent in a protic solvent the most nucleophile the thing that has the best nucleophilicity is actually iodide because it's not hindered by these hydrogen bonds as much it doesn't have a tight shell has this big molecular cloud and some people think it also has kind of a softness it has this polarizability where that cloud can be pulled towards the carbon and do what it needs to do so in this case iodide is is a better nucleophile let me just say then hydroxide which is a better nucleophile then fluoride now in an a protic solution where all of a sudden where all of a sudden the the the interactions with the solvent are not going to be as significant then things change in this situation basicity matters so in an a predict in an a product solution basicity basicity and and nucleophilicity nucleophilicity correlate and i'll put I'll put out put a and asterisks and asterisks here because there's also one other aspect of new we'll see - I haven't talked about yet but I'll talk about it in a second but in this type of a situation hydroxide hydroxide will be better at reacting than floor fluoride which will be better at reacting then iodide and the whole reason why in both situations in both situations hydroxide hydroxide is I mean even in even weather it can interact with the solvent it's still a pretty good nucleophile because if you think about hydroxide if you think about hydroxide I have to think about this a lot it has an extra electron right if you think about it it's you could imagine it's water that took away let me draw it this way you can imagine it's it's water that where proton left or where an electron was taken from a proton so normally you have two pairs and now you have a third pair right here this oxygen has 1 2 3 4 5 6 7 valence electrons one more than neutral oxygen so it has a negative charge so it already has an extra it already has an extra electron that gives us negative charge but oxygen is also more electronegative than hydrogen so it's also able to get this guy involved a little bit it's also able to get this guy involved a little bit anyway so it's very very it's a very basic very basic molecule so even when it might be interfered a little bit by a protic environment like water it's still a better nucleophile it's still a better nucleophile than something like fluoride if you take the solvent out of the picture it's a super strong base and so it's also going to be a very very good nucleophile now the last aspect of nucleophilicity remember nucleophilicity is how good something reacts now let's imagine let's imagine we have something here so we have we have two two molecules two hydroxide molecules right two hydroxide molecules and let's say that this one is let's just say this one is just a straight up this is just a straight-up hydroxide and let's say this one over here has all sorts of things off of it let's say it's you know it just has this big chain of stuff it just has this big chain of stuff I don't know which one now if you were to look at these two molecules if you were to try to guess which one is going to be a better clea file you should just remember nucleophilicity is how good something reacts how good is it getting in there and and making a reaction happen now this thing has this big molecule all around it it might actually make it very hard if you go back to this circumstance up here it might be make it very hard for it to get in there we've talked about steric hindrance from the point of view of the carbon but we haven't really talked about it from the point of view of the nucleophile in this nucleophile right here it might be hard it might be hard for this extra electron right here for this electra extra electron right there to actually get to the target nucleus it will be hindered while in this situation it will be much easier even though even though the group that's reacting this oxygen would that has a negative charge this extra electron is on some level fairly fairly equivalent but this one right here will be much more it looks a much smaller molecule it'll be less hindered easier to get in so this will be a better nucleophile better nucleophile and that's what I wanted to make that's why I did I wanted to I did want to make the same state the strong statement that an a protic solution basicity and nucleophilicity are completely correlated because nucleophilicity still has that other element of you know how hindered is it is it in an environment where or is it part of a molecule that will keep it from reacting even though it might be a very strong base if it actually forms a bond it'll be very strong so the big thing to remember is that they're just two fundamentally different concepts and that's why they're two different terms for them nucleophilicity how good is it at reacting saying nothing about how good the resulting bond is basicity is how good is the bond how badly does it want to react but it doesn't say how good is it at reacting itself