leaving group/base question

[Tina324]

Ok so I know that I- is a better leaving group than Br- because Br has a higher elextronegativity and ionization energy, so it is less likely to leave the molecule it is attached to, BUT I read that another explanation for a good leaving group is an ion that is stable once it leaves a molecule. ummm, so... isn't I- less stable?? It is a larger ion than Br and therefore has less of a hold on its electrons.

ugh see how i confused myself?...
can anyone clarify...please

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[artanis]

hey.
here's a good example for what it is getting at. Think about an -OH group attached to a tertiary carbon. That OH group is a poor leaving group because if it were to leave it will have a negative charge. However, if you protonate it to make it -OH2 (+), then that leaves easy as it is stable (no charges) and is water. Just know the general trend

OH-, NH2-, OR- < F < Cl < Br < I < OTos

in terms of leaving groups. F will not leave The best leaving group of all time is -OTos and its derivatives. As for stability of the I and Br once they leave, they both have a negative charge but thats how they are in solution anyway, existing as ions

 

[doc3232]

I - is BIG so it can DELOCALIZE the neg charge better. Memorize these words.
Br- is more electronegative though and pulls electrons more.
So which is the stronger acid, I gave you an example in how Br is a better acid, but also a way in which I -is the better acid.

Now the question is which has a stronger effect? The answer is HI.The fact that I- can delocalize the neg charge is more important than electronegativity.
This idea works exactly the same with leaving groups, not just acidity. It is the same concept.

 

[Tina324]

hmmm ok i THINK i get it. i might have more q's for u guys later if u don't mind =) ..but for now, just to clarify, if i were given a question: "which is more stable Br- or I-. I would say I- becuase...even though it it slightly less electronegative than Br-, it has a bigger radius and can therefore delocalize the charge better?

Now, if we were asked which is more stable between Br2 and I2 we would say Br2, because it has a smaller radius and therefore the the nucleus has a better hold on them?

let me knowww thankss!
acids/bases and intermolecular interactions realllllly confuse me

 

[Contach]

so we are saying:
worst leaving group ----> best leaving group
OH-, NH2-, OR- < F < Cl < Br < I < OTos
this also meanss
best base/nucleophile ----> worst base/nucleophile

correct?
but doesn't the nucleophilicity also relate to the environment? ie if its polar protic or polar aprotic? can som1 explain this?

i'm wondering if anyone is memorizing pka values for the popular acids, or the pkb values for the popular bases... that could help a lot...
does anyone have a list?

thanks