Which reactant in each of the following pairs is more nucleophilic? Explain. (a) -NH2 Or NH3 (c)

Which reactant in each of the following pairs is more...

Key Concepts

Basicity vs. Nucleophilicity

Basicity is a thermodynamic property associated with the equilibrium position of proton transfer, whereas nucleophilicity is a kinetic property related to the rate of attack on an electrophile. Although often related (more basic species can be more nucleophilic), they do not always correlate directly, and factors like steric effects and solvation can cause divergence between the two.

Periodic Trends and Polarizability

Periodic trends, such as increasing atomic size and polarizability down a group, can affect nucleophilicity. Larger, more polarizable atoms can more readily donate their electron density, making them more effective nucleophiles under certain conditions despite sometimes being less basic in a thermodynamic sense.

Solvent Effects

The solvent can significantly influence nucleophilicity. In protic solvents, hydrogen bonding can stabilize nucleophiles (especially small, highly charged ones), which can slow down their reaction with electrophiles. Aprotic solvents, on the other hand, do not provide such strong stabilization, often enhancing the apparent nucleophilicity of charged species.

Resonance Stabilization

Resonance stabilization involves the delocalization of a negative charge over multiple atoms, which can reduce the reactivity of a molecule as a nucleophile. When the negative charge is spread out by resonance, it is less available for donation to an electrophile, thus lowering nucleophilicity in comparison to species where the charge is localized.

Charge Effects

The presence of a negative charge often increases nucleophilicity because extra electron density is available to be donated. Neutral molecules generally have lower nucleophilicity compared to their anionic counterparts, although other factors such as solvation and steric hindrance may modulate this effect.

Nucleophilicity

Nucleophilicity is a measure of a species’ ability to donate an electron pair to an electrophile during a chemical reaction. It is a kinetic concept, meaning it relates to how fast a nucleophile reacts, and it depends on factors such as electron density, orbital availability, and the surrounding environment rather than just thermodynamic stability.

Steric Hindrance

Steric hindrance refers to the bulk or spatial congestion around the reactive center of a nucleophile. When large groups surround the nucleophilic site, they can physically impede the approach of an electrophile, reducing the rate of reaction even if the electron density is high.

Transcript

00:01 So we have the following pairs of chemical species and we want to know which is the better nucleophile.

00:07 So this question is a really good question because it really encourages us to think about trends in terms of nucleophilicity.

00:13 So we want to know what characteristics make up of a good nucleophile.

00:18 So looking at example a, we have an azanide or ammonium.

00:24 And a really good trend to think about in terms of nucleophilicity is that often negatively charged species, are more nucleophilic.

00:33 So we can go ahead and write that down here, say that negatively charged species and to be more nucleophilic.

00:53 So when comparing these two species here, we'll see that our aesonide is negatively charged, and therefore we can say that this is going to be more nucleophilic.

01:03 We can do the same line of reasoning here, for example b, when we compare water with our acetate ion.

01:09 Water is actually a very, very poor nucleophile, and we see that our acetate ion, which is negatively charged, it will actually be more nucleophilic here.

01:20 And then we can also do the same thing here.

01:23 For example, c, we have boron trifluoride or our fluoride ion, since boron triphylide is a neutrally charged species, and we have our fluoride ion, which is actually a pretty strong nucleophile.

01:37 You would say that our fluoride ion here is going to be our more.

01:43 Nucleophilic species.

01:47 Now going to example d, this is actually encouraging us to think about a different trend in terms of nucleophilicity.

01:54 So we can actually think about electronegativity.

01:58 And the more electronegative an atom tends to be, the less nucleophilic it is.

02:05 And we can think about it because, so we can think about it in terms of electronegativity because we know that electro -negativity is an atom's tendency to attract electrons to itself, but what makes a good nucleophile is an atom's ability or a nucleophile's ability to share its electrons.

02:27 So it's almost like an opposite effect...

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