[hvalarcon]

Hi right now i'm seeing in organic class that some nucleophilic groups are substituted with others. Nonetheless, i tried to come up with a logical explanation and couldn'tt. I mean in some of the reactions that i have been reviewing i see the leaving group being
more electronegative that the one being recived by the elctron deficient carbon. I thought that maybe in trying to get a more even share of electron was the reason of this exchange, but then i found rections in which the leaving group was more electronegative throwing away my analysis. Can someone help me with this?

[dexangeles]

could you give an example?

[Asmodeus]

Sounds more like Sn1 and Sn2 reactions instead of oxidations.

Maybe this will help you out a bit.

In nucleophilic substitution, the carbon being attacked has a slightly positive charge on it.  The electronegative leaving group is a good leaving group because it just picks up that pair of electron, getting a full octet and off it goes.  I'm not going to talk about radicals because that's a whole other ball game.

But examples of what you are looking at would really help.

[GCT]

You need to understand the difference between a nucleophile and a base (from what I remember).  Some elements may be more electronegative, yet they are not good nucleophiles; you text should explain this very clearly.  

At times, the solvent contributes to this effect.  A more electronegative element, in a polar protic solvent such as water will have a more difficult time reacting through an SN2 pathway since it is primarily attracted to water, thus the less electronegative element will be more available; this is also one of the main reasons that less electronegative elements (less effective nuclear charge) are able to act better as nucleophiles...electrons are more available.

yeah, again read up on the text

[dexangeles]

ok....as GCT as already explained, some nucleophiles get solvated.

Although it may seem that Iodine (as an example) is both a better leaving group, but at the same time reacts faster that Flourine (seems to contradict), you have to look at the other constants.  Fluorine is still a better nucleophile, but because of it's charge to size ratio, it get's easily solvated and is hindered in a reaction.  Iodine is less solvated; thus, it seems to react faster.  But this doesn't mean it's a stronger nucleophile.  This would be the same problem with other atoms.  In a situation wherein solvation is non-existant, Flourine actually reacts faster than Iodine.