tert-Butyllithium

tert-Butyllithium is a chemical compound with the formula (CH₃)₃CLi. As an organometallic compound, it has applications in organic synthesis (see also Grignard reaction) since it is a sufficiently strong base to deprotonate many carbon acids, including benzene. In regard to molecular aggregation state, the compound exists in clusters.

tert-Butyllithium is readily available commercially as hydrocarbon solutions; it is not usually prepared in the laboratory.

Chemical properties

The lithium carbon bond in tert-butyllithium is highly polarized, having about 40 percent ionic character. The molecule reacts like a carbanion, as is represented by these two resonance structures.^[1] (Given the polarity calculations on the C-Li bond, the "real" structure of a single molecule of t-butyllithium is likely a near-average of the two resonance contributors shown, in which the central carbon atom has a ~50% partial negative charge while the lithium atom has a ~50% partial positive charge.)

Similar to n-butyllithium, tert-butyllithium can be used for the exchange of lithium with halogens and for the deprotonation of amines and activated C—H compounds.

This compound and other alkyllithium compounds are known to be unstable in ether solvents; the half life of tert-butyllithium is 60 minutes at 0 °C in diethyl ether, 40 minutes at -20 °C in THF,^[2] and about 11 minutes at -70 °C in dimethoxyethane.^[3] In this example, the reaction of tert-butyllithium with tetrahydrofuran (THF) is shown:

Safety

tert-Butyllithium is a pyrophoric substance, meaning one that easily catches fire on exposure to air. (A precise definition of a pyrophoric material is one "that ignite[s] spontaneously in air at or below 54.55 °C (130.19 °F)".^[4]) The solvents used in common commercial preparations are themselves flammable. While it is possible to work with this compound using cannula transfer, traces of tert-butyllithium at the tip of the needle or cannula may catch fire and clog the cannula with lithium salts. Some workers prefer to enclose the needle tip or cannula in a short glass tube which is flushed with an inert gas and sealed via two septa.^[5] Serious laboratory accidents involving tert-butyllithium have occurred. For example, in 2009 a staff research assistant, Sheharbano Sangji in the lab of Patrick Harran^[6] at the University of California, Los Angeles, died after being severely burned while working with this compound when her synthetic sweater caught on fire.^[7][8][9]

Large scale reactions may lead to runaway reactions, fires, and explosions when tert-butyllithium is mixed with ethers such as diethyl ether, tetrahydrofuran, and glymes. The use of hydrocarbon solvents may be preferred.

Air-free techniques are important so as to prevent this compound from reacting violently with oxygen and moisture in the air:

t-BuLi + O₂ → t-BuOOLi

t-BuLi + H₂O → t-BuH + LiOH

References