[kamiyu]

The three butyllithium: n-butyllithium, sec-butyllithium and tert-butyllithium.

What is the difference between them? (I mean in terms of reactivitiy) For example, what determines when to use which one???

[TheUnassuming]

Haha... I've asked my PI this before and the basic response is literature precedent and experience determines which you use right off the bat.
In general if you can get away with using n-BuLi, use that.  Its what I use for my metal/halogen exchange reactions, transmetalations, and simple deprotonations.  The only problem I have had from n-BuLi is it can act as a nucleophile. 
t-BuLi I only ever use for ortho lithiations or other very high pka deprotonations.  It's not nucleophillic to speak of but substantially more basic than n-BuLi, which can be good or bad depending on what you want to happen.  Also it is far less forgiving to mistakes in handling or accidents like a needle breaking.
Sec-BuLi seems to be used more rarely and usually for very specific variations of reactions.  For example there are report in the literature of doing o-lithiations with benzoic acid which can then be reacted with various electrophiles, something I don't think you can do with t-BuLi though I have no understanding of why.

[orgopete]

Although more complicated than this, the reactivity is the reverse of carbocation stability.

[snorkack]

Remember that there are five, not three. You omitted isobutyllithium. And there are two sec-butyllithiums (because H, Me, Et and Li are 4 different substituents).

[discodermolide]

sec-butyllithium is an ionic species in which the carbanion part is stabilised by solvent coordination. So there are not really two of them.

[kamiyu]

Thanks for you all guys for your input, I learn a lot from them!

As to the chemical nature of butyllithiums, I think this is quite controversial. To me, I consider the bond between C and Li as covalent with varying ionic character.

Also, I would like to add that some organolithium species is clearly ionic. For example, fluorenyllithium is so stable that it can be isolate and checked by X-ray. The result is that it is clearly ionic and the fluorenyl anion and lithium cation adopt sandwich type packing.

There is a paper for this but I forgot which..

[discodermolide]

see here for methyl lithium, I guess the butyllithiums may well be similar.
http://en.wikipedia.org/wiki/Methyllithium
don't forget the compound is solvated.

[kamiyu]

Thanks for your reply, discodermolide.

Could you please tell me the point you would like to raise by referring to methyllithium??

see here for methyl lithium, I guess the butyllithiums may well be similar.
http://en.wikipedia.org/wiki/Methyllithium
don't forget the compound is solvated.

[discodermolide]

I was replying to the post made by snorkack.

[snorkack]

see here for methyl lithium, I guess the butyllithiums may well be similar.
http://en.wikipedia.org/wiki/Methyllithium
don't forget the compound is solvated.

The sterical size of the carbanion differs.
So does sec-buthyllithium racemize or keep is configuration?

[discodermolide]

I have no idea if it racemises or not. You can generate chiral bases which are configurationally stable, I suppose it depends on the compound.