[stevet]

Someone asked the following question in the lab, and after asking around, we realised no one had a good answer:

Why doesn't t-BuLi react with pentanes (the solvent you buy t-BuLi in)?

t-BuLi is more reactive than sec-BuLi which is more reactive than n-BuLi, due to the instability of the anion due to the electron donating effects of the increasing number of methyl groups. The pKa of isobutane is around ~53, while that of methane is ~48 (I find conflicting reports of the pKa of butane, but some where around ~50 seems right). This seems to indicate that t-Bu anion should be the more basic species, so why does t-BuLi exist happily in pentanes?

Any ideas?

[Dan]

Good question.

My first guess is that ^t BuLi is kinetically stable in pentane, so even though the reaction to form pentyllithium + ^tBuH would be thermodynamically favoured, it has a very high activation energy barrier.