Nucleophiles in aprotic sol'n

[ziggy1104]

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Can someone explain why smaller nucleophiles are stronger in an aprotic sol'n? I understand why smaller nucleophiles are weaker in a protic sol'n, but I'm not getting the reasoning behind the opposite effect.

Thanks!

 

[Daneosaurus]

What do you mean by "smaller"?

 

[CoFe2]

Can someone explain why smaller nucleophiles are stronger in an aprotic sol'n? I understand why smaller nucleophiles are weaker in a protic sol'n, but I'm not getting the reasoning behind the opposite effect.

Thanks!

This probably isn't accurate but its how I think of it to help me remember: nucleophiles attack, right? So it is probably more difficult for a big and bulky atom to attack something and maneuver around all of the adjacent bonds than it is for a smaller atom. I think it actually has more to do with the fact that the smaller atoms are more easily attracted but I like to think of tiny little, nucleophilic ninjas sneaking into place where they want to attack 🙂

 

[discensdentibus]

If by smaller you mean (smallest to largest due to e- cloud size) F- <Cl- <Br- < I-
-Weak bases (ie I-) react weakly with your protic solvents (alcohols and waters).. this is a good thing because you want them to be attacking an electron deficient species and being more nucleophilic rather than forming interactions with the solvents
- F- is a stronger base than I-. (you can think pkas here, polarizability, the fact the HI is a strong acid) and is more willing to pluck a proton (H+) than an I-. In general F- does not like to be by itself and will react more strongly with the protic solvents (the dudes who got H's) and the protic solvent shields the F- and stabilizes his charge to make him happier. The species under attack can dodge the bullet here.

Now for the distinction: in aprotic solvents (no Hs to pluck), base strength and nucleophilic strength are on the same team. This means that F- wont be interacting with the aprotic solvent because they wont make him happy and wants his charged satisfied more than the I- does (because hes a big boy and doesnt need to hold hands with anyone). This is how F-, the small guy, becomes a stronger nucleophile in an aprotic solvent

 

[ziggy1104]

This probably isn't accurate but its how I think of it to help me remember: nucleophiles attack, right? So it is probably more difficult for a big and bulky atom to attack something and maneuver around all of the adjacent bonds than it is for a smaller atom. I think it actually has more to do with the fact that the smaller atoms are more easily attracted but I like to think of tiny little, nucleophilic ninjas sneaking into place where they want to attack 🙂

lol awesome explaination! I probably will never forget this now haha

 

[ziggy1104]

What do you mean by "smaller"?

By smaller, I am referring to atomic number. The smaller the atomic num, the stronger the atom is in an aprotic sol'n

 

[ziggy1104]

If by smaller you mean (smallest to largest due to e- cloud size) F- <Cl- <Br- < I-
-Weak bases (ie I-) react weakly with your protic solvents (alcohols and waters).. this is a good thing because you want them to be attacking an electron deficient species and being more nucleophilic rather than forming interactions with the solvents
- F- is a stronger base than I-. (you can think pkas here, polarizability, the fact the HI is a strong acid) and is more willing to pluck a proton (H+) than an I-. In general F- does not like to be by itself and will react more strongly with the protic solvents (the dudes who got H's) and the protic solvent shields the F- and stabilizes his charge to make him happier. The species under attack can dodge the bullet here.

Now for the distinction: in aprotic solvents (no Hs to pluck), base strength and nucleophilic strength are on the same team. This means that F- wont be interacting with the aprotic solvent because they wont make him happy and wants his charged satisfied more than the I- does (because hes a big boy and doesnt need to hold hands with anyone). This is how F-, the small guy, becomes a stronger nucleophile in an aprotic solvent

Also a very good explanation! Thanks 🙂