Fluoride or Iodide anion?

[heatwheat]

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Which is the strongest base? Fluoride or Iodide anion?

 

[phungy]

To find the stronger base, find out which is the weaker acid. HI is the stronger acid thus weakest base.

F- is the answer.

Acidity and Size trend:
- (strongest bond, least acidic) HF < HCl < HBr < HI (most acidic, largest size)
- F- < Cl- < Br- < I- (most stable, largest size)
- strong bonds = weak acid

 

[doc3232]

To find the stronger base, find out which is the weaker acid. HI is the stronger acid thus weakest base.

F- is the answer.

Acidity and Size trend:
- (strongest bond, least acidic) HF < HCl < HBr < HI (most acidic, largest size)
- F- < Cl- < Br- < I- (most stable, largest size)
- strong bonds = weak acid

Ya, I's large size allows it to stabalize the neg charge best through declocalization.

 

[flycd05]

ok so F- is the stronger base b/c its acid HF is a weaker acid compared to HI. i understand that.

HOWEVER, what if they ask u - what's the better nucleophile out of HI and HF?? i know u want to pick the one which donates it e- the easiest. so i think HI would be a better nucleophile b/c the I- is larger and less electronegative and doesn't have a strong hold on its e- so it can donate its e- easier than HF.

is this right? so basically even though F- is the stronger base, it's not the best nucleophile (so that just proves that strong basicity doesn't equal strong nucleophile) WTF that makes so sense to me now haha - both strong bases and nucleophiles want to give up their e- so why is this example disproving that?!!!

 

[futuredent22]

for atoms of similar size, nucleophilicity mirrors basicity. That explains why -OH is a better nucleophile than -F. However, larger atoms will be better nucleophiles because they have better polarization.

So for -F vs. -I, the question becomes: Does -F being a much strong base make up for the fact that -I has much better polarization? This depends on the solvent. In aprotic solvent, -F is the better nucleophile. However, in a protic solvent, the more polarizable -I is able to brush off the solvent molecules better and will be the better nucleophile. I like to picture the basic -F atom being surrounded by the protic solvent molecules, which are very attracted to a strong base like F-.

 

[jdog10]

a weaker base makes the best nucleophile, better than a strong base, especially in sn1 and sn2 reactions. A strong base will have a tendancy to remove a proton to induce elimination reactions where as a weak base has enough free electrons without being overly aggressive, like a fluoride anion. So I- is a better nucleophile than F- because it is a weaker base. Stronger base does not mean better nucleophile. Normally weaker bases are the better ones.

ok so F- is the stronger base b/c its acid HF is a weaker acid compared to HI. i understand that.

HOWEVER, what if they ask u - what's the better nucleophile out of HI and HF?? i know u want to pick the one which donates it e- the easiest. so i think HI would be a better nucleophile b/c the I- is larger and less electronegative and doesn't have a strong hold on its e- so it can donate its e- easier than HF.

is this right? so basically even though F- is the stronger base, it's not the best nucleophile (so that just proves that strong basicity doesn't equal strong nucleophile) WTF that makes so sense to me now haha - both strong bases and nucleophiles want to give up their e- so why is this example disproving that?!!!

 

[flycd05]

a weaker base makes the best nucleophile, better than a strong base, especially in sn1 and sn2 reactions. A strong base will have a tendancy to remove a proton to induce elimination reactions where as a weak base has enough free electrons without being overly aggressive, like a fluoride anion. So I- is a better nucleophile than F- because it is a weaker base. Stronger base does not mean better nucleophile. Normally weaker bases are the better ones.

your referring to nuclophiles in protic solvents here right? in a protic solvent, the I- is a better nucleophile b/c it can brush off the solvent's H's off better b/c it is a weaker base and doesn't want a H+ as much as a strong base would. therefore I- is the better nucleophile ONLY in protic solvents.

in aprotic solvents, as Emory said - the F- would be a better nucleophile b/c there's no H+ to decrease the nucelophilicity of F-. F- would be a better nucelophile than I- b/c its a stronger base and wants to be attached to something therefore it will induce a Sn2 or E2 rxn.

aprotic solvents (Sn2 E2): strong bases = strong nucleophiles (example F-)
protic solvents (Sn1 E1): weak bases = strong nucleophiles (example I-)

 

[futuredent22]

So I- is a better nucleophile than F- because it is a weaker base. Stronger base does not mean better nucleophile. Normally weaker bases are the better ones.

That is only half-correct, because its not that simple. In an aprotic solvent, the better base is the better nucleophile. Thus, F- is a much better nucleophile than I-.

It all depends on the solvent.