Butyl lithium Formula
Butyl lithium or butyillithium is a strong organic base, widely used in chemical synthesis. In organic reactions, butyl lithium is abbreviated as BuLi.
Formula and structure: The chemical formula of butyl lithium is CH3CH2CH2CH2Li. Its molecular formula is C4H9Li and its molar mass is 64.06 g mol-1. Butyl lithium is a organometalic compound, specifically of the organolithium group. Butyl lithium forms tetrameric or hexameric (depending on solvent) strutcures by delocalization of covalent bonds between the lithium of a molecule and the butyl chain of other molecule. Its most common chemical structure is the n-butyl lirhium and it can be written as bellow:
Occurrence: Butyl lithium is not present in nature. It is just obtained by chemical synthesis as showed bellow.
Preparation: Butyl lithium is synthesized by reaction of 1-bromobutane or 1-chlorobutane with Lithium metal. In general, benzene, ciclohexane or diethylether are used as solvents in these reactions. The synthesis also produces LiBr or LiCl, which are in a homogeneous solution together with BuLi.
2 Li + C4H9Br → C4H9Li + LiBr or 2 Li + C4H9Cl → C4H9Li + LiCl
Physical properties: Butyl lithium is commonly a colorless or slightly yellow liquid, but it also can be found as a powder. Its boiling/decomposition point is 80-90 ºC. Its density is 0.68 g mol-1 and it is soluble in most organic solvent, including hydrocarbons and ethers. In water, BuLi reacts vigorously.
Chemical properties: Butyl lithium is a strong base (metalation), with a pKAH ~ 50. Moreover, BuLi may be, in certain conditions, a powerful nucleophile and reductant. It can form complex with aprotic Lewis bases, such as ethers, amines and sulfides. Consequently with the difference between the electronativities of carbon and lithium (2.55 against 0.98), it reaction mechanisms considering BuLi as the butylanion, n-Bu-, and a lithium cation Li+.
Uses: The most extended butyl lithium application is as strong base in organic synthesis in the chemical and pharmaceutical industry due its capacity to deprotonate weak Bronsted acids:
C4H9Li + R-H → C4H10 + R-Li
Addiotionally, BuLi is used as polymerization catalyst in the production of elastomers such as styre-butadiene-styrene or polybutadiene:
C4H9Li + CH2=CH-CH=CH2 → C4H9-CH2-CH=CH-CH2Li
Other type of reactions promoted by BuLi are the transmetalations, where two organometalic compound exchange their metals:
C4H9Li + R-M → C4H9M + R-Li
Health effects/safety hazards: Butyl lithium reacts violently with water and in concetration high to 20%, it can have spontaneus combustion in the air. It is explosive when having polymerization with the styrene and in contact with heat and fire.