[deutdeut]

BF3 is a non-polar molecule and it contains lone pairs, it should be a nucleophile, right? And for example, ammonia, it has an lone pair on N atom, and it's a polar molecule, then it can act as both an electrophile and nucleophile?

[english]

Boron has no lone pairs in that molecule.  It has an empty p orbital, and therefore is a strong electrophile.

[deutdeut]

But isn't the fluorine atoms has lone pairs so that BF3 can also behave as a nucleophile when it reacts with another electrophile?

[english]

But isn't the fluorine atoms has lone pairs so that BF3 can also behave as a nucleophile when it reacts with another electrophile?

Don't worry about that.

[deutdeut]

So, that molecule can act as both electrophile and nucleophile depending on the conditions?

[english]

So, that molecule can act as both electrophile and nucleophile depending on the conditions?

Not exactly.  Fluorine is a very poor nucleophile.  I tell you not to worry about it because to get into an explanation of such things would eventually expand to ideas beyond my own knowledge and much likely yours as well.

Keep things simple.  Don't worry about it.

[deutdeut]

So, for a molecule with partial postive and negative charges, they possess both electrophilic and nucleophilic sites, hence can't they be both electrophiles and nucleophiles?

[english]

Yes and no. Boron is an electrophile because it has a vacant orbital.  Different classes of reactions follow their own set of rules.  For alkene addition, for example, pi electrons are the nucleophile.  Partial charges aren't necessarily the reason for nucleophilicity.

In general it's what atom is electron rich that acts as a nucleophile.  But you also have to consider how good of a nucleophile it is.  Fluorine is a weak nucleophile, but a nucleophile nonetheless.  It would take so much energy to have it act as an appropriate nucleophile that it is chemically unreasonable to consider it a nucleophile.

(see post below for clarification)

[deutdeut]

That means, in a BF3 molecule, both nucleophilic and electrophilic parts are present, but it usually act as a electrophile, rather than an nucleophile. But theoretically, it can be both?

[english]

Technically it depends on what solvent you are using.  I don't know what reaction you are using BF3 for, but I would assume if it has anything to do with organic molecules it would be in an aprotic solvent.

I'm sorry if I mislead you, I wasn't taking solvent into account.    In an aprotic solvent, F^- would be a strong nucleophile. 


The point is aside from this fact.  You can only focus on one reaction at a time. 

If your reaction is with boron acting as an electrophile, and your nucleophile comes from a solvent atom for example, you must ignore fluorine's potential as a nucleophile (from another BF3 molecule) in this reaction. 

You have to pick a reaction to focus on.

[Ψ×Ψ]

Probably you don't need to worry about fluorine.  It's rarely mentioned in freshman/sophomore organic classes except for its presence in some electron-withdrawing groups.
Just remember this: "F is for freaky."  (Courtesy of a fluorine chemist at my university.)  It doesn't quite behave like the other halogens. 

[Yggdrasil]

Furthermore, in BF₃ the nucleophilicity of the fluorine is severely decreased by the presence of the electron deficient boron.  The empty p-orbital on the boron pulls a lot of the electron density away from the fluorines so that they don't have a lot of electron density to donate in a reaction with an electrophile.  So, it will be a very poor nucleophile and for all intents and purposes you will not have to worry about the fluorines acting as nucleophiles.

[deutdeut]

But isn't NH3 an electrophile as it also possess electrophilic H atoms? How come HBr is an electrophile?

[maakii]

Furthermore, in BF₃ the nucleophilicity of the fluorine is severely decreased by the presence of the electron deficient boron.  The empty p-orbital on the boron pulls a lot of the electron density away from the fluorines so that they don't have a lot of electron density to donate in a reaction with an electrophile.  So, it will be a very poor nucleophile and for all intents and purposes you will not have to worry about the fluorines acting as nucleophiles.

Hey Yggdrasil, I don't think that the boron empty p-orbital will pull electron density from fluorine, fluorine is much more electronegative than boron, and thus the electron density will be surrounding them instead of the boron, even though it has an empty p-orbital.

This is also why the fluorine atoms are poor nucleophiles, they are much more reluctant to part away with their electrons as compared as to boron is willing to accept electrons, thus BF3 acts as an electrophile instead.

[english]

But isn't NH3 an electrophile as it also possess electrophilic H atoms? How come HBr is an electrophile?

Both do have dipole moments, one considerably stronger than the other; however, ammonia is a very weak acid.  It's pK_a is around 35 or so.  In order for it to act as a good electrophile it must have sufficiently acid protons.

HBr, on the other hand, has a sufficiently acidic proton.