Lithium Compounds Ethyllithium

An electropositive metal in organic compounds of alkali metals is replaced by a more electropositive one in series of reversible reactions. More electronegative, i. e. more acid, hydrocarbon groups or whole molecules replace those which are less acid [140]. Caesium replaces lithium in ethyllithium. Benzene, which is a stronger acid than ethane, replaces ethyl in ethyllithium. Toluene and H2 are more acid than benzene, and they can therefore replace phenyl in phenylsodium [141, 142],... 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

A is approximately twice the observed covalent radius of lithium, and both the Li—C distance and the Li—C—Li angle are reminiscent of the trimethylaluminum structure previously described. In the two tetra-mers for which structural data are available, the Li—Li distances are substantially shorter in the range of 2.4 to 2.6 A with Li—C distances in the 2.2-2.5 A range. Crystallization effects may play a substantial role in the ethyllithium system since this compound is hexameric in noninteracting solvents but crystallizes in the tetrameric form. 

The properties of lithium metal are well known, but the properties of its alkyls have until recently received much less attention. The lowest member of the series, methyllithium, is a non-volatile microcrystalline powder insoluble in hydrocarbons. Ethyllithium is a colourless crystalline compound melting at 95°. n-Propyl and n-butyllithium are almost colourless fairly viscous non-volatile oils soluble in hydrocarbons and ethers. These properties are to be compared with those of the corresponding sodium alkyls which are all colourless, non-volatile crystalline solids, insoluble in hydrocarbons. The difference in properties is usually attributed to differences in the type of bond between lithium and sodium alkyls, the former being considered covalent and the latter ionic compounds. Thus Coates (17) distinguishes between two types of compounds ... 

Yur ev et al. 2 prepared deuteriated selenophenes from the respective halogenated compounds by halogen-deuterium replacement. Thus, 2-deuterio-, 2,5-dideuterio-, tetradeuterioselenophene, 3-methyl-, and 5-methyl-2-deuterioselenophene were obtained from the corresponding iodoselenophenes on reduction by zinc in deuteriated acetic acid. 3-Deuterioselenophene was obtained from 3-selenienyl-lithium hydrolyzed with deuteriated acetic acid at — 7 0°C the lithium derivative was obtained from 3-bromoselenophene and ethyllithium, also at — 70°C. The deuterium content was determined by combustion, the water being analyzed by the drop method.43... 

Substances that catch fire spontaneously in air without an ignition source are called pyrophoric. These include several elements— white phosphorus, the alkali metals (group lA), and powdered forms of magnesium, calcium, cobalt, manganese, iron, zirconium, and aluminum. Also included are some organometallic compounds, such as ethyllithium (LiC2H5) and phenyllithium (LiQHj), and some metal carbonyl compounds such as iron pentacarbonyl, Fe(CO)5. Another major class of pyrophoric compounds consists of metal and metalloid hydrides, including lithium hydride, LiH ... 

Reaction scheme (f) has been most widely subscribed to by polymer chemists. In this writer s opinion it is not a likely pathway for reaction. The enthalpy change for dissociation of hexamer into monomer molecules, Eq. (7), is not known, but is probably on the order of 100 kcal/mole of hexamer, at a minimum. This, coupled with an unfavorable entropy effect, should make the free energy change for Eq. (7) so large (positive) that no reasonable assumption about the kinetic behavior of the various candidate species will suffice to make the concentration of monomer rate-determining. It must be recalled that the mass spectral data for ethyllithium vapor at 87° C failed to show the presence of any monomer. Intermolecular exchange of alkyl groups, observed in NMR studies of ethyllithium (47), almost certainly proceeds by dissociation of hexamer into tetramer and dimer. The enthalpy of dissociation of polyisoprenyllithium dimer has been estimated to be about 37 kcal/mole 31b). If a similar value obtains for short chain n-alkyllithium compounds (and there is reason to believe it does), the rate of formation of monomer could not possibly be as fast as the observed rates of initiation. Finally, it is difficult to see how a monomeric alkyl-lithium species could confer the required stereospecificity in reaction with an olefin 73). 

Alkylalkali compounds readily eliminate metal hydride to give the alkene this reaction is especially pronounced with the higher alkali metals. The microscopic reverse process has been modeled by the addition of LiH to ethylene. As shown in Figure 6, the reaction first involves a r-complex between ethylene and lithium hydride in which the structures of ethylene and of LiH are changed but little from the separated reactants. In the cyclic transition structure the C-Li bond has significantly shortened towards its value in the product ethyllithium. The C-H bond is still quite long in the transition structure but is on its way to a normal C-H bond. The same type of... 

Lithium reagents can trace their roots back almost 100 years to the pioneering work of Wilhelm Schlenk [2, 3], who initially prepared ethyllithium, propylUthium and phenyUithium from the corresponding alkyl/aryl mercury compound and lithium metal (Fig. 1.) [4]. 

---