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Discussion in 'MCAT Study Question Q&A' started by DrMattOglesby, May 26, 2008.
why is the fluorine anion NOT a good nucleophile?
I would say because Fluorine is very electronegative and since the fluoride ion is relatively stable, it is less likely to donate its electrons to other atoms.
the larger the atom, the more polarizable (ie the greater its ability to spread out its charg). the nucleophilicity of an atom measures how polarizable the atoms electron cloud is. good nucleophiles will be highly polarizable. if you go down halogen column you increase the size of the atom and therefore increase its polarizability. since F is at the top, you know it has to have the least amount of polarizability (its the smallest) and therefore is the worst nucleophile
if you havent already take a look at post 4 on this link, its very helpful. i basicallly just summarized it.
I thought the fluoride ion was a bad leaving group, but decent nucleophile. I remember it was a bad leaving group because, although it is the most electronegative halogen, it is also the smallest, and cannot smear out its charge as effectively as Cl, Br, and I (i.e., it is not stable). In terms of nucelophilicity, I don't know why it is a stronger nucleophile than the other halogens (probably because of its high electronegativity).
i think that is better used to explain why it is a weak base comparing it with other elements going across the periodic table from left to right. i dunno tho, im confusing myself.
As a general rule, halogens are good leaving groups, but (comparatively) poor nucleophiles. They make better bases. (Fluorine's a worse leaving group than say bromine because it's smaller, but the idea still holds true.)
I'm trying to remember what reaction it was that involved using HX acids exclusively (I think either HBr or HI), instead of say, H2SO4, because the anion X- would not tend to do a nucleophilic substitution reaction, but would tend to act as a base, which is what that substitution reaction required. Was it the iodoform reaction? I'll have to look it up.
But anyway, compared to all the other halogens, fluorine's going to be an even worse nucleophile because it's smaller, so it needs to be closer to an electrophile in order to react with it. But the electrophile could be surrounded by some bulky R groups (steric hinderance), etc., which would make that difficult.
Depending on what you want, the iodoform reaction either uses I2/OH or I2/H30...
Halogens in general are excellent leaving groups. Think of the "six" strong acids...Most of them are halogens.
Hydrogen Fluoride is not one of the strong acids. It's a weak base...The affinity it has for hydrogen is what makes it a poor nucleophile. It would rather grab a hydrogen and leave than attack an electrophilic site.