Ch 8 : Nucleophilicity vs Basicity
To say that nucleophilicity follows basicity across a row means that, In the section Nucleophilic Substitution, we assigned a relationship to. While nucleophilicity and basicity are two very similar properties in that Compare the two pairs of reactions mechanisms shown below to convince yourself. First of all, remember that basicity is a subset of nucleophilicity. What's the difference between nucleophicity and basicity? What is the link between being too basic and the hydride in NaH not being able to carry out the.
In our discussion on the effect of protic solvents on nucleophilicity, we learned that solvation weakens the nucleophile, having the greatest effect on smaller anions. In effect, when using protic solvents, nucleophilicity does not follow basicity when moving up and down a column. In fact, it's the exact opposite: Aprotic Solvents An aprotic solvent is a solvent that lacks a positively polarized hydrogen.
The next diagram illustrates several polar aprotic solvents that you should become familiar with. Aprotic solvents, like protic solvents, are polar but, because they lack a positively polarized hydrogen, they do not form hydrogen bonds with the anionic nucleophile. The result, with respect to solvation, is a relatively weak interaction between the aprotic solvent and the nucleophile.
The consequence of this weakened interaction is two-fold. First, by using an aprotic solvent we can raise the reactivity of the nucleophile. This can sometimes have dramatic effects on the rate at which a nucleophilic substitution reaction can occur.
For example, if we consider the reaction between bromoethane and potassium iodide, the reaction occurs times faster in acetone than in methanol. A second consequence that results from the weak interaction that occurs between aprotic solvents and nucleophiles is that, under some conditions, there can be an inversion of the reactivity order.
Nucleophile - Chemistry LibreTexts
An inversion would result in nucleophilicity following basicity up and down a column, as shown in the following diagram. When we considered the effects of protic solvents, remember that the iodide anion was the strongest nucleophile. Now, in considering aprotic solvents under some conditions, the fluoride anion is the strongest nucelophile.
Increasing Atomic Size Increases Nucleophilicity Thus far, our discussion on nucleophilicity and solvent choice has been limited to negatively charged nucleophiles, such as F- Cl- Br- and I.
Nucleophilicity vs. basicity
With respect to these anions we learned that, when using protic solvents, nucleophilicity does not follow basicity, and when using aprotic solvents, the same relationship can occur, or there could be an inversion in the order of reactivity. What happens as we move up and down a column when considering uncharged nucleophiles?
It turns out that, in the case of uncharged nucleophiles, size dictates nucleophilicity. This is because larger elements have bigger, more diffuse, and more polarizable electron clouds.
This cloud facilitates the formation of a more effective orbital overlap in the transition state of bimolecular nucleophilic substitution SN2 reactions, resulting in a transition state that is lower in energy and a nucleophilic substitution that occurs at a faster rate.
James This was not a clear cut example where one could point to periodic trends. However with nitrogen being coordinated to C in -CN using that principle, they would draw the wrong conclusion.
Nucleophilicity vs. Basicity
Because multiple variables are in play [we are changing the basic atom as well as the substituents connected to that atom] the only recourse is to check a pKa table because the effect of changing two variables at once is not easily predictable. This site is not deficient in describing why certain species are stronger acids and bases according to a set of principles.Basicity vs Nucleophilicity, Steric Hindrance / Effects, Base vs Nucleophile Strength, Organic Chem
I have a whole series of articles where I discuss acidity trends and refer to electronegativity, polarizability, resonance, adjacent electron withdrawing groups, and even aromaticity. The point of the current article is to mention that basicity is measured by pKa — it is an equilibrium — whereas nucleophilicity is measured by rate.
Maybe you have a hydrogen coming out. You have a hydrogen behind it. You have a hydrogen up top. Then you have a leaving group right over there. In an Sn2 reaction, the nucleophile will give this electron to the carbon.
The carbon has a partial positive charge. Let me draw that. 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.
Nucleophilicity vs. Basicity — Master Organic Chemistry
The whole reason I did this is because this is acting as a nucleophile. 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.
So fluoride is reasonably basic.