What is consider a good nucleophile and a poor nucleophile??

[dentalgirlyay]

I am very confused on nucleophiles..... how does one know if its a good nucleophile or a poor nucleophile? I know good nucleophile is use in SN2 reaction and poor nucleophile is use in SN1 reaction but how do u determine if its good or poor?

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[topdent1]

First of all, a nucleophile is defined as a lewis base, a species that donates electrons. So, a good nucleophile is a good lewis base, that which can donate its electrons fairly easily. Anything with lone pair(s) of electrons is a good nucleophile. What is a better nucleophile. Br- or I-? I- because it is a larger atom and can donate its electrons better than Br-.

 

[doc3232]

Good: OH- NH3 NH2 CN- CH3O- CHCH2O- CH3NH2 CH3CH2-MgBr the list goes on...
Bad nucleophile: tbutyloxide LDA Not many after these two that aren't obvious.

 

[doc3232]

First of all, a nucleophile is defined as a lewis base, a species that donates electrons. So, a good nucleophile is a good lewis base, that which can donate its electrons fairly easily. Anything with lone pair(s) of electrons is a good nucleophile. What is a better nucleophile. Br- or I-? I- because it is a larger atom and can donate its electrons better than Br-.

I is bigger and in some cases steric hindrance may make Br the better nucleophile. Also I- is more stable than Br- because it can delocalize the neg charge (because it is bigger it can delocalize better).

 

[topdent1]

I is bigger and in some cases steric hindrance may make Br the better nucleophile. Also I- is more stable than Br- because it can delocalize the neg charge (because it is bigger it can delocalize better).

But, Br holds on to its electrons very tightly. It is more reluctant to donate its electrons.

 

[doc3232]

But, Br holds on to its electrons very tightly. It is more reluctant to donate its electrons.

That is also true.

 

[topdent1]

That is also true.

lol..so which one is better? Or does it depend on the situation.

 

[TeamGuo]

But, Br holds on to its electrons very tightly. It is more reluctant to donate its electrons.

not really... Br and I have very similar electronegativity. It really has nothing to do with Br holding electrons much tighter than I.

That reasoning is valid for atoms such as F.