Sodium tetraethylborate

Sodium triethylhydroborate is very air- and moisture-sensitive and must be handled in an inert atmosphere. In completely pure form, it is a colorless crystalline solid, m.p. 30°. However, the salt is often obtained as a brown-black oil because of impurities in the sodium hydride. Nevertheless, such products can be used without further purification in reactions such as preparation of sodium tetraethylborate. 

Sodium tetraethylborate is obtained from sodium triethylhydroborate 140-180° under ethylene pressure (65-100 atmospheres).7,8 The addition of triethylborane7 is desirable but not absolutely necessary.8 ( Caution. The entire operation must be carried out in an inert gas atmosphere with strict exclusion of moisture.) 

A 500-ml. steel autoclave (suitable for use at 500 atmospheres at 300°) with manometer is charged with 201 g. (1.65 moles) of sodium triethylhydroborate and pressured to 80 atmospheres with ethylene. The mixture is heated with shaking at 140-150° for 2 hours. The pressure falls from 100 to 30 atmospheres at 150°. After cooling to 20°, an additional 60 atmospheres of ethylene pressure is added, and the mixture is agitated a further 2 hours at 140-150°. 

After cooling to 20°, the unreacted ethylene is vented and the autoclave is emptied by injection of two 300-ml. portions of hot (80-100°) toluene and release of the hot solution through a glass tube by means of nitrogen pressure. Colorless sodium tetraethyl-borate crystallizes from the combined solutions on cooling. After filtration and vacuum drying, 232.8 g. (94% yield) of the salt is obtained, m.p. 145°. Anal. Calcd. for C8H20BNa C, 64.04 H, 13.44 B, 7.20 Na, 15.32. Found C, 63.94 H, 13.23 B, 7.01 Na, 14.87. 

The salt can also be obtained directly as a solid product by opening the autoclave under an inert atmosphere. The product is obtained as a hard, faintly yellow mass, m.p. 142°. Recrystallization from toluene gives white crystals, m.p. 145°. 

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There have been a few research papers reporting the use of other vapour generation techniques to volatilize analytes that form unstable hydrides, or had previously been thought not to form vapours at room temperature. Examples include the use of sodium tetraethylborate to form volatile ethyl compounds of cadmium, lead and thallium. 

Zuliaurre, R., Pons, B. and Nerin, C. (1997) Speciation of trimethyllead in rainwater by gas chromatography mass spectrometry after ethylation with sodium tetraethylborate-comparison with other alkylation methods./. Chromatogr. A, 779, 299-306. 

The use of sodium tetraethylborate overcomes the problem of hydrolytic instability of the Grignard reagents, allowing ethylation to be carried out in an aqueous medium. This reaction has been employed for the analysis of alkyl-Pb-, Sn- and Hg-species (Ashby and Craig, 1989). Aqueous phase ethylation is growing in popularity because extraction into an organic phase is avoided and one source... 

A technique for the determination of methylmercury in aqueous samples (natural and seawater) involved the conversion of methylmercury to gaseous methyl-ethylmercury by reaction with sodium tetraethylborate (Bloom, 1989 Bloom and Watras, 1989). The volatile derivative was purged from the solution and concentrated on a graphitic column at room temperature. The derivative was thermally desorbed from the column, and then analysed by cryogenic gas chromatography with cold vapour atomic fluorescence detection. In addition to methylmercury, labile Hg11 species could be determined (as diethylmercury) as well as dimethylmercury (which is not ethylated). The detection limit for... 

Fischer, R., Rapsomanikis, S. and Andreae, M.O. (1993) Determination of methylmercury in fish samples using GC/AA and sodium tetraethylborate derivatization. Anal. Chem., 65, 763-766. 

Schubert, P., E. Rosenberg, and M. Grasserbauer. 2000. Comparison of sodium tetraethylborate and sodium tetra(n-propyl)borate as derivatization reagent for the speciation of organotin and organolead compounds in water samples. Fresenius J. Anal. Chem. 366 356-360. 

Methyl mercury is of much greater concern when health effects are considered, as it is much more toxic than ionic mercury or free mercury. Methyl mercury is also much more likely to be bioaccumulated, leading to serious contaminations, especially of fish. The speciation for mercury can be accomplished by derivatizing the methyl mercury and Hg2+ with sodium tetraethylborate, NaBEt4. The volatile MeHgEt, from methyl mercury, and HgEt2, from Hg2+, species formed are purged from the sample solution and separated in a GC column. An atomic emission spectrometer is used as a detector. 

De la Calle Guntinas et al. [769] volatilised selenium from natural water samples by reaction with sodium tetraethylborate and measured the volatilised selenium by gas chromatography microwave-induced plasma atomic emission spectrometry. The detection limit for a 5mL sample was 8ppt. 

A surprising addition has recently been made to the list of elements which may be usefully determined by vapour generation techniques, namely cadmium.5 Sodium tetraethylborate was used to produce a volatile cadmium species, with citrate being used to mask interference from nickel and copper. Using an argon-diluted hydrogen diffusion flame as atomizer, the detection limit by AFS was 20 ng l-1. 

Rapsomanikis, S., Craig, P.J. Speciation of mercury and methylmercury compounds in aqueous samples by chromatography-atomic absorption specUometry after ethylation with sodium tetraethylborate. Anal. Chim. Acta 248, 563-567 (1991)... 

One major criticism of all previous US-assisted methods is that they have not been compared with their unassisted counterparts, so the influence of US on the results has not been quantified. This, however, is not the case with the method for pesticides in honey and that for the determination of mercury in biological materials proposed by Tu et al., which is based on acid leaching for 5 min, followed by simultaneous in situ derivatization and LLE for 40 min in the presence of sodium tetraethylborate and nonane, buffered at pH 7.0, under US [10]. The mixture was shaken by hand and sonicated at 40°C at a fixed US amplitude of 100% however, the characteristics of the US bath used were omitted. Phase separation was effected by centrifugation at 5400 rpm for 5 min. The improvement provided by US assistance is apparent from Fig. 6.1. 

The restricted versatility can, to a certain degree, be overcome by replacing NaBH4 by alkyl or arylborates [1]. The most common derivatization procedure relies on ethylation with sodium tetraethylborate, which was initiated for determining methyllead ionic compounds [9] ... 

For measuring at low concentrations, the derivatisation of the extracted organotin compound with sodium tetraethylborate is an established procedure to make a more sensitive gas chromatographic analysis possible. Derivatisation can be combined with the extraction as prescribed in the German method but can also be carried out after the extraction. Selective detection is possible using GC-MS. For organo-metallic compounds atomic emission or atomic absorption detection is not only very sensitive, but very selective. The steps necessary for the chromatographic separation and detection are described in ISO/CD 17353 (2001). Application of this well-described method for soil samples does not mean that the difficulties with the extraction have been overcome. 

Cai Y, Rapsomanikis S, Andreae MO (1993) Determination of butyltin compounds in sediment using gas chromatography - atomic absorption spectrometry comparison of sodium tetrahydroborate and sodium tetraethylborate derivatization methods. Anal Chim Acta 274, 243-251. 

Rapsomanikis S, Donard OF, Weber JH (1986) Speciation of lead and methyllead ions in water by chromatography/Atomic Absorption Spectrometry after ethylation with sodium tetraethylborate. Anal Chem 58, 35-38. 

SODIUM TRIETHYLHYDROBORATE, SODIUM TETRAETHYLBORATE, AND SODIUM TRIETHYL-1-PROPYNYLBORATE... 

The preparation of the salts through combination of the sodium compound and the trialkyl- or triarylborane generally proceeds smoothly. Only in a few instances are other procedures used. For example, sodium tetraalkylborates can be prepared from sodium, an alkyl chloride, and a trialkylborane.6,7 In particular, sodium tetraethylborate is prepared most easily from sodium triethyl-hydroborate and ethylene.7,8... 

Sodium tetraethylborate is a white, crystalline compound of m.p. 145°. The light-sensitive and highly hygroscopic salt is insoluble in aliphatic hydrocarbons and poorly soluble in cold benzene or toluene. It is readily soluble in diethyl ether, tetrahydrofuran, and diglyme. Aqueous solutions react distinctly alkaline and liberate 1 mole of ethane on acidification. 

Multi = multi-elemental method, STEB = sodium tetraethylborate, STPB = sodium tetrapropylborate, SBH = sodium borohydride, PFPD = pulsed flame photometric deteotion,... 

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