[herrhansen]

Hi again.

When doing an metal-halogen exchange on arylbromides with either n-BuLi or MeLi the formed alkylbromide has been reported to react with the formed aryllithium again. Methane would have a pKa over 50, whereas the phenyl would have a pKa value around 43. So the reverse reaction seems unfavourable to me. Could it be that a small fraction attacks the aryllithium, giving the toluene in the case of MeLi, and the formed LiBr simple precipitates out, pushing that side reaction to the right? Otherwise I cant explain how a less stable base could be formed again?

Thanks in advance.

[orgopete]

You should write the mechanism for this one and I think it will then make sense.

ArBr + MeLi  ArLi + MeBr  ArMe + LiBr

[Sepelio]

I try to remember these ones in that the alkali metal is always going to want to try and grab the halogen if it can.