Boron Halides

Abstract

AbstractThe boron trihalides boron trifluoride, BF3, boron trichloride, BCl3, and boron tribromide, BBr3, are important industrial chemicals having increased usage as Lewis acid catalysts and in chemical vapor deposition (CVD) processes. Boron halides are widely used in the laboratory as catalysts and reagents in numerous types of organic reactions and as starting material for many organoboron and inorganic boron compounds. Reactions of boron trihalides that are of commercial importance are those of BCl3and, to a lesser extent, BBr3, with gases in chemical vapor deposition (CVD). Boron trichloride is prepared on a large scale by the reaction of Cl2and a heated mixture of borax,\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{Na}}{_{2}}{\rm{BO}}_{4}{\rm{O}}{_{7}}{\hskip-0.167em}{\hskip-0.167em}{\cdot{}}{\hskip-0.167em}{\hskip-0.167em}10{\rm{H}}{_{2}}{\rm{O}}}$\end{document}, and crude oil residue in a rotary kiln heated to 1038°C. Boron trihalides, BX3, are trigonal planar molecules which are (sp)2hybridized. The X–B–X angles are 120°. The boron trihalides are strong Lewis acids; however, the order of relative acid strengths,\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{BI}}{_{3}}\geq {\rm{BBr}}{_{3}}>{\rm{BCl}}{_{3}}>{\rm{BF}}{_{3}}}$\end{document}, is contrary to that expected based on the electronegativities and atomic sizes of the halogen atoms. Boron tribromide is produced on a large scale by the reaction of Br2and granulated B4C at 850–1000°C or by the reaction of HBr with CaB6at high temperatures. Approximately 75–95% of the BCl3consumed in the United States is used to prepare boron filaments by CVD. Another important use of BCl3is as a Friedel‐Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses. BCl3is also used for the production of halosilanes, in the preparation of many boron compounds and in the production of optical wave guides. Approximately 30% of BBr3produced in the United States is consumed in the manufacture of proprietory pharmaceuticals. BBr3is used in the manufacture of isotopically enriched crystalline boron, as a Friedel‐Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Boron subhalides are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohalides and boron dihalide radicals have been studied. Diboron tetraflouride, B2F4, diboron tetrachloride, B2Cl4, diboron tetrabromide, B2Br4, and diborontetraiodide, B2I4, are well‐known but thermally unstable compounds.