Nucleophile - Chemistry LibreTexts
While nucleophilicity and basicity are two very similar properties in that Compare the two pairs of reactions mechanisms shown below to convince yourself. Nucleophilicity and Basicity Factors in Organic. Reactions. General Principals. Recall the definitions of electrophile and nucleophile: Electrophile: An electron. Relationship between basicity and nucleophilicity. Paula Jaramillo,1* Patricia Pe´ rez1 and Patricio Fuentealba2. 1Departamento de Ciencias.
Reply james Good question.
Lewis acidity is essentially electrophilicity. Acidity Bronsted acidity is what we call it when the electrophile is a proton. What is the link between being too basic and the hydride in NaH not being able to carry out the nucleophilic attack? NaH is generally not observed to add to aldehydes and ketones, although it will add to Lewis acids such as borane BH3. You can think about the rates of competing reactions — 1 addition of hydride to aldehyde slow versus 2 deprotonation of the alpha-carbon by hydride fast and the latter reaction prevails.
Reply Subhasismahapatro NaH acts as a non nucleophilic base so basically involves in deprotonation rather in addition of the hydride to the carbonyl. In general all non nucleophilic base acts by the same mechanism. Reply James Ashenhurst The best guide to basicity is by looking at a pKa table.
The pka of water is 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.
Nucleophilicity vs. Basicity
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.
The whole reason it's less nucleophilic is because there are other things that are keeping it from reacting. We saw in the video on what makes a good nucleophile, and in the case of fluoride, it's because it's a very small atom. It's actually a very small ion so it's very closely held. The electron cloud is very tight, and so what it allows is the hydrogens from the water to form a very tight shell around. These all have partial positive charges so they're attracted to the negative anion.
They form a very tight shell protecting the fluoride anion, which makes it harder for it to react in a protic solution, so it doesn't react as well.
Ch 8 : Nucleophilicity vs Basicity
If it was able to react, it actually will form a stronger bond than the iodide anion. So that's the big difference, 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 atom of the anion. So if you looked at pure 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 something like water the sodium and the hydroxide separates, and it's really the hydroxide that 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 than iodide.
- Nucleophilicity vs. basicity
Now, if you were to look at nucleophilicity just to see the difference, we saw that what the solvent is actually matters because the solvent will affect how good something is at reacting.
So in nucleophilicity, there's a difference between a protic solvent and an aprotic solvent. In a protic solvent, 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. It has this big molecular cloud, and some people think it also has kind of a softness. It has this polarized ability where that cloud can be pulled towards the carbon and do what it needs to do. So in this case, iodide is a better nucleophile, let me just say, than hydroxide, which is a better nucleophile than fluorine.
Nucleophilicity vs. Basicity — Master Organic Chemistry
Now, in an aprotic solution, where all of a sudden the interactions with the solvent are not going to be as significant, then things change. In this situation, basicity matters. So in an aprotic solution, basicity and nucleophilicity correlate.
I'll put an asterisk here because there's also one other aspect of nucleophilicty that I haven't talked about yet, but I'll talk about it in a second. In this type of a situation, hydroxide will be better at reacting than fluoride, which would be better at reacting than iodide.
And the whole reason why in both situations hydroxide is-- I mean, even when it can interact with the solvent, it's still a pretty good nucleophile, because if you think about hydroxide, and I have to think about this a lot, it has an extra electron. If you think about it, you could imagine it's water that took away-- let me draw it this way. You can imagine it's water where a proton left or where an electron was taken from a proton, so normally, you'd have two pairs and now you have a third pair right here.
This oxygen has one, two, three, four, five, six, seven valence electrons, one more than neutral oxygen, so it has a negative charge.
It already has an extra electron that gives this negative charge, but oxygen is also more electronegative than hydrogen, so it's also able to get this guy involved a little bit anyway. It's a 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 than something like fluoride.
If you take the solvent out of the picture, it's a super strong base. 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 we have something here. We have two hydroxide molecules, right? Let's say that this one 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 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 nucleophile, you should just remember: 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 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 the nucleophile. In this nucleophile right here, it might be hard for this extra electron right here to actually get to the target nucleus.