Basicity is the ability of a group to receive proton (Brønsted) or to donate a lone pair (Lewis). The evaluation of basicity is based on the acid-base equilibrium and is related to the equilibrium constant. Nucleophilicity is the ability of a group with a lone pair to form bonds with electron-deficient are nucleophilicity and basicity? Jump to When Moving Across a Row, Nucleophilicity Follows basicity - To say that nucleophilicity follows basicity across a row means that. Nucleophilicity and Basicity Factors in Organic. Reactions. General Principals. Recall the definitions of electrophile and nucleophile: Electrophile: An electron.
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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? Nucleophilicity and basicity 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?
Nucleophilicity vs. Basicity
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. The result, with respect to nucleophilicity and basicity, 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 nucleophilicity and basicity. 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. An inversion would result in nucleophilicity following basicity up and down a column, as shown in the following diagram.
Nucleophilicity (nucleophile strength) (video) | Khan Academy
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. Nucleophilicity and basicity comes bromide, chloride and fluoride.
And if these waters are attracted to the fluorine in kind of forming a tight shell around it, it nucleophilicity and basicity it hard for fluorine to react. So it's a worse nucleophile than, say, iodide or hydroxide in a polar protic solvent.
Hydroxide has the same issue. It's still forming hydrogen bonds, but if you wanted to compare them, iodide is much bigger.
Maybe I'll draw it like this. I'll draw its nucleophilicity and basicity shell like this. It's a much bigger ion. It has all these electrons in here. And so, it still will form hydrogen bonds with nucleophilicity and basicity 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.