Nucleophilicity vs. Basicity

[mmktiger]

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From my (perhaps flawed) understanding...

It seems like the same general factors govern nucleophilicity (kinetic term related to reaction rate) and basicity (thermodynamic term related to acid/base equilibrium position). Of course, the same molecule can act as either a nucleophile (nucleophilic attack) and a base (abstracting a proton), depending on the reaction. (Is this right?)

But molecules can be strong nucleophiles and weak bases (e.g. I-), or weak nucleophiles and strong bases (e.g. F-). How to tell???? Especially when it seems like strong nucleophiles and strong bases both usually have the same characteristics: a negative charge, smaller atomic size, less electronegativity, less bulky (steric hindrance) and electron donating groups. Also, any other examples to clarify (since I- is the conjugate base of a strong acid, it is clearly a weak weak base)?

Thanks!

 

[aldol16]

Nucleophilicy and basicity do usually parallel each other because bases are a subset of nucleophiles. Nucleophiles donate a pair of electrons to something else. We call a nucleophile a "base" when that "something else" is a proton. Okay, so then what's the difference? Well, the main difference is that basicity is a thermodynamic property whereas nucleophilicity is a kinetic one. That is, one measures basicity by the relative stability of a base's conjugate acid. A strong base will have a weak conjugate acid and vice versa. The relative energetics are all that matters. On the other hand, nucleophilicity is how well a nucleophile can donate its pair of electrons to an electrophile. As you might imagine, this must take into account the steric hindrance about the nucleophile and how that relates to its approach to the electrophile - in other words, a kinetic effect.

 

[mmktiger]

Thanks

Nucleophilicy and basicity do usually parallel each other because bases are a subset of nucleophiles. Nucleophiles donate a pair of electrons to something else. We call a nucleophile a "base" when that "something else" is a proton. Okay, so then what's the difference? Well, the main difference is that basicity is a thermodynamic property whereas nucleophilicity is a kinetic one. That is, one measures basicity by the relative stability of a base's conjugate acid. A strong base will have a weak conjugate acid and vice versa. The relative energetics are all that matters. On the other hand, nucleophilicity is how well a nucleophile can donate its pair of electrons to an electrophile. As you might imagine, this must take into account the steric hindrance about the nucleophile and how that relates to its approach to the electrophile - in other words, a kinetic effect.

Thanks! I get that. But if given a molecule, how can you tell if it's a strong nucleotide/weak base or weak nucleotide/strong base, or strong nucleotide/strong base?

 

[aldol16]

Thanks! I get that. But if given a molecule, how can you tell if it's a strong nucleotide/weak base or weak nucleotide/strong base, or strong nucleotide/strong base?

Well, for basicity, draw the molecule and its conjugate acid. These are in equilibrium. Your job is to determine the position of that equilibrium. So take trifluoroacetate. Draw the protonated and deprotonated form. Do you think the conjugate acid will be stabilized? Yes. Why? Because of the strong electron-withdrawing effect of the trifluoromethyl group. So trifluoroacetate is a weak base and trifluoroacetic acid is a very strong acid.

Now, take something like t-butoxide and compare that with ethoxide. Draw these out. Which is the better nucleophile? Ethoxide, because it has less steric hindrance.

Nucleophilicity and basicity are relative terms. You're comparing two things. Nowhere will you be expected to know an "absolute value" for nucleophilicity or basicity. You should understand how to tell if something is nucleophilic or basic as compared to some reference.