Going down the periodic table, another factor comes into play next. In so doing, the nucleophile is considerably less reactive; everywhere it goes, its lone pairs of electrons are interacting with the electron-poor hydrogen atoms of the solvent. The ability of nucleophiles to participate in hydrogen bonding decreases as we go down the periodic table.
Hence fluoride is the strongest hydrogen bond acceptor, and iodide is the weakest. A polar aprotic solvent does not hydrogen bond to nucleophiles to a significant extent, meaning that the nucleophiles have greater freedom in solution.
Under these conditions, nucleophilicity correlates well with basicity — and fluoride ion, being the most unstable of the halide ions, reacts fastest with electrophiles. If we want a reaction to take place, we need to use solvents that will actually dissolve our nucleophile. The bottom line here is that the bulkier a given nucleophile is, the slower the rate of its reactions [and therefore the lower its nucleophilicity].
Note: Are there other factors? This list of four covers the basics, but several other factors are worth noting. Iodide, being larger, will have a lower charge density and interactions with hydrogen will be weaker.
Does that make sense to you? Its actually the opposite. Since flourine is smaller, its charge is confined to a smaller space and it therefore has a higher electron density. They are correlated most of the time but not always. How is polarizability related to nucleophilic strength? Would it fit into any of these categories? Polarizability plays a role when you take the solvent into account.
In polar protic solvents, hydrogen bonding occurs between the partial positive hydrogen H attached to N or O usually and the nucleophile. Smaller nucleophiles become more solvated than larger nucleophiles, which means that smaller nucleophiles in polar protic solvents will not be able to react as well and thus are poorer nucleophiles.
For example, Florine anions become so heavily solvated in polar protic solvents that they wont even react, but Iodine, being much larger, is much less solvated and can still react. In aprotic solvents, hydrogen bonding does not occur to any significant extent and the stronger base is usually the stronger nucleophile.
If this list does not take into account all the factors that make a good nucleophile, where is a more detailed treatment of the ones that are remaining? Also, in the case of polar aprotic solvents, one may mention the idea of the cation being solvated, while the anion nucleophile less so, and so it is more reactive. One example is the differing selectivity of enolates for C vs. O alkylation; depending on the nature of the solvent, counter-ion, and electrophile, either dominant O vs.
C alkylation can be achieved. I understand that the presence of electron donating groups eg. Great question. These types of tradeoffs are what can make organic chemistry difficult.
How do we decide that from anisole, nitrobenzene and benzene, what will be the correct order of rate of electrophillic substitution? And can you possibly link me to an article related to it? You are soon gonna get a lot of Indian visitors. In OCH3 oxygen has got a lone pair due to which can be shared with the benzene ring through resonance hence increasing its charge density whereas nitrogen in NO2 has not got lone pair due to formation of dative bond with O and has higher electronegativity than C so it withdraws charge from benzene ring.
Hope it helps! Hello, Is it accurate to say that primary amines are more strongly nucleophilic than carboxylic acids? Many thanks for your help and time. Which one is it? It seems to me that you are contradicting yourself and making me more confused than I was previous to visiting this page….
Maybe I should have made this clearer. Also, that rule only applies for polar protic solvents. F - tends to H-bond with the solvent more, making it less reactive as a nucleophile, as compared to a nucleophile containing carbon. The reverse of the rule is what actually applies in polar aprotic solvents. Since the solvent does not H-bond to the halide nucleophiles, fluorine basically becomes the most reactive among the halides.
It took me way too long before I finally understood this whole nucleophile thing, but I hope my answer helped. Pyridine or Morpholine? Anyway, I think the answer is morpholine but I do not know how to explain it. Could anyone please help me on this? I expect that, in pyridine nitrogen atom surrounded by three bons all of them with carbon atoms while in morpholine there are three bonds two of them with crbon and the thired one with hydrogen which is lower electronegative than carbon so the availability of unshered electron pair in morpholine more than that in pyridine.
I mean which polar aprotic solvent may retard reaction? That drives the equilibrium forward. Not quite. But steric hindrance due to the fact that a sigma star orbital is being attacked on carbon, versus an S orbital on hydrogen is the key difference.
My question is why flouride ion behaves as a strong nucleophile in aprotic polar solvent when nucleophilisity is related to polarizability.
Then why not iodide is a strong nucleophile in aprotic polar solvent and also iodide is less electronegative than fluoride so it should easily donate its lone pair of elctron. It does not prefer to accommodate any other atom with it. Therefore fluoride is a better nucleophile than iodide. It says in my textbook that in methanol, RS- is a better nucleophile than iodide.
I mean, sulfur is smaller and the R group is probably making it more basic by electron-donating effect, thus making it a stronger base. This means that RS- should be the most solvated one and therefore less nucleophilic.
But according to Ms. Paula Bruice, no, RS- is the better one. So, RS- would still have the stronger base, that would still be more solvated. I used to like Ochem a lot, but now I think I will never undestand it and it bums me out. Another thing, in a SN2 reaction, ammonia is the nucleophile and it is asked which solvent will make the reaction faster: ether or ethanol.
I would say ethanol, because it would stabitize the transition state by solvation, right? But, no, according to my professor, ether would be the best one because it would not make hydrogen bonds to ammonia. Please, help. Hi Symara — there are many factors involved in nucleophilicity. Hydrogen bonding is most important for atoms O, N, and F because of the large difference in electronegativity between these atoms and H.
When you compare that to S electronegativity 2. So the above answer RS- beats I- and ether being a better solvent than ethanol probably makes the most sense.
As an aside, when talking about different variables like basicity or polarizability it is much clearer if you are going across a row or up and down a column of the periodic table. Comparing both at once e. Cl- and H2N- really requires looking at experimental evidence.
Being challenged, need reasons why H2O instead of Br- became nucleophiles. Textbooks with huge price tags just presents drawings of kind of skeletons, H2O and arrows. Here I get it. Thank you James. Great article! But I still have a question. The leaving group has a partial negative charge because it tends to be or will be more electronegative. So this electron is given to this carbon right when the carbon gets that, or simultaneously with it, this electronegative leaving group is able to completely take this electron away from the carbon.
Then after you are done, it looks like this. We have our methyl carbon so the hydrogen is in the back, hydrogen in the front, hydrogen on top. The leaving group has left. It had this electron right there, but now it also took that magenta electron so it now has a negative charge and the nucleophile has given this electron right over here and so now it is bonded to the carbon. The whole reason I did this is because this is acting as a nucleophile.
It loves nucleuses. It's giving away its extra electron, but it is also acting as a Lewis base. This is a bit of a refresher. A Lewis base, which is really the most general, or I guess it covers the most examples of what it means to be a base. That's exactly what's happening here. This nucleophile is donating an electron to the carbon. So, it's acting like a Lewis base.
So for 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? The difference is that nucleophilicity is a kinetic concept, which means how good is it at reacting?
How fast is it at reacting? How little extra energy does it need to react? When something has good nucleophilicity, it is good it reacting. It doesn't tell you anything about how stable or unstable the reactants before and after are, It just tells you they're good at reacting with each other. Basicity is a thermodynamic concept.
It's telling you how stable the reactants or the products are. It tells you how badly something would like to react. For example, we saw the situation of fluorine. Let's think about this. We saw the situation-- actually, I should say fluoride, so fluoride looks like this. Seven valence electrons for fluorine and then it swiped one extra electron away.
You get fluoride. So fluoride is reasonably basic. It is more basic than iodide. But in a protic solution-- let me write it here. But less nucleophilic in protic solution. And a protic solution, once again, has hydrogen protons around. And the reason why this is, is fluoride, it wants to bond with 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. If it did, it actually will be a stronger bond than the iodide anion will form, that the fluoride anion is actually less stable in this form than the iodide is.
If it were to be able to get a proton or give its electron away, it will be happier, but it's less nucleophilic. It's less good at reacting in a protic solution. Let me write down. Let me start with a protic solvent. I'll make two columns here, protic solvent and then we'll do aprotic solvent right over here. 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 might have free protons floating around.
An example of a protic solvent is water or really any alcohol. Water is the simplest example or maybe the most common. The reason why you might have protons floating around is exactly this reaction I showed right here. Maybe every now and then a hydroxide anion forms. Even more likely, maybe a water takes a hydrogen from one of the other waters. One of the water molecules takes hydrogen from one of the other water molecules and becomes a hydronium, where it's not a proton necessarily, but it's an oxygen.
Let's say if you start with water and 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. You can almost imagine it's almost floating around because that oxygen really wants to take back that electron.
So protic solvent is water. In water, you might see a little bit of hydroxide, a little bit of proton, a little bit a hydronium.
You see all of it in there, but the bottom line is that there are protons that can react with other things. Let me clear this away so that I have some real estate.
Let me just write down water. Water is a protic solvent. Now, wait. Is that always the case? It seems like hydrogens are everywhere. Well, no, it's not always the case. Let me show you an aprotic solvent. Diethyl ether looks like this. And just so you know the naming, it's an ether because it has 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 guy's got hydrogens lying around as well. Maybe those can get released. But, no, these hydrogens are bonded to the carbon and carbon is not anywhere near as electronegative as oxygen.
Carbon is unlikely to steal these hydrogens' electrons and these hydrogens to be loose. If they were bonded to the oxygen, that would have been a possibility. With water, you have obviously H-O-H. In 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 hydrogens 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 protic versus aprotic. 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. 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 what we saw in an Sn2-type reaction. In a protic solvent, what happens is that the things that are really electronegative and really small, like a fluoride anion-- let me draw a fluoride anion.
In a protic solvent, what's going to happen is it's going to be blocked by hydrogen bonds. It's very negative, right? It has a negative charge. And it's also tightly packed. As you can see right here, its electrons are very close, tied in. It's a much smaller atom or ion, in this case. If we looked at iodide, iodide has 53 electrons, many orbitals.
Actually, iodide would have It would have the same as iodine plus one. Fluoride will have 10 electrons, nine from fluorine plus it gains another one, so it's a much smaller atom.
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