boron trichloride

Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by chlorination of boron oxide and carbon at 501 °C.

:B₂O₃ + 3 C + 3 Cl₂ → 2 BCl₃ + 3 CO

The carbothermic reaction is analogous to the Kroll process for the conversion of titanium dioxide to titanium tetrachloride. One consequence of this synthesis route is that samples of boron trichloride are often contaminated with phosgene.{{cite book |doi=10.1002/14356007.a04_309 |chapter=Boron Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Brotherton |first1=Robert J. |last2=Weber |first2=C. Joseph |last3=Guibert |first3=Clarence R. |last4=Little |first4=John L. |isbn=978-3-527-30385-4}}

In the laboratory BCl₃ can be prepared by treating with AlCl₃ with BF₃, a halide exchange reaction.{{Greenwood&Earnshaw}}

BCl₃ is a trigonal planar molecule like the other boron trihalides. The B–Cl bond length is 175 pm. A degree of π-bonding has been proposed to explain the short B− Cl distance, although there is some debate as to its extent. BCl₃ does not dimerize, although NMR studies of mixtures of boron trihalides shows the presence of mixed halides. The absence of dimerisation contrasts with the tendencies of AlCl₃ and GaCl₃, which form dimers or polymers with 4 or 6 coordinate metal centres.

BCl₃ hydrolyzes readily to give hydrochloric acid and boric acid:

:BCl₃ + 3 H₂O → B(OH)₃ + 3 HCl

Alcohols behave analogously giving the borate esters, e.g. trimethyl borate.

File:NH3-BCl3-adduct-bond-lengthening-2D.png forms a Lewis adduct with boron trichloride.]]

As a strong Lewis acid, BCl₃ forms adducts with tertiary amines, phosphines, ethers, thioethers, and halide ions.{{cite journal | author = Gerrard, W. |author2=Lappert, M. F. | title = Reactions Of Boron Trichloride With Organic Compounds | journal = Chemical Reviews | year = 1958 | volume = 58 | issue = 6 | pages = 1081–1111 | doi = 10.1021/cr50024a003}} Adduct formation is often accompanied by an increase in B-Cl bond length. BCl₃•S(CH₃)₂ (CAS# 5523-19-3) is often employed as a conveniently handled source of BCl₃ because this solid (m.p. 88-90 °C) releases BCl₃:

:(CH₃)₂S·BCl₃ ⇌ (CH₃)₂S + BCl₃

The mixed aryl and alkyl boron chlorides are also of known. Phenylboron dichloride is commercially available. Such species can be prepared by the redistribution reaction of BCl₃ with organotin reagents:

:2 BCl₃ + R₄Sn → 2 RBCl₂ + R₂SnCl₂

Reduction of BCl₃ to elemental boron is conducted commercially in the laboratory, when boron trichloride can be converted to diboron tetrachloride by heating with copper metal:{{cite book |author1=Wartik, T. |author2=Rosenberg, R. |author3=Fox, W. B. | chapter = Diboron Tetrachloride | title = Inorganic Syntheses | year = 1967 | volume = 10 | pages = 118–125 | doi = 10.1002/9780470132418.ch18 |isbn=978-0-470-13241-8}}

:2 BCl₃ + 2 Cu → B₂Cl₄ + 2 CuCl

B₄Cl₄ can also be prepared in this way. Colourless diboron tetrachloride (m.p. −93 °C) is a planar molecule in the solid, (similar to dinitrogen tetroxide, but in the gas phase the structure is staggered. It decomposes (disproportionates) at room temperatures to give a series of monochlorides having the general formula (BCl)_n, in which n may be 8, 9, 10, or 11.

:n B₂Cl₄ → B_{_{n}}Cl_{_{n}} + n BCl₃

The compounds with formulas B₈Cl₈ and B₉Cl₉ are known to contain closed cages of boron atoms.

Boron trichloride is a starting material for the production of elemental boron. It is also used in the refining of aluminium, magnesium, zinc, and copper alloys to remove nitrides, carbides, and oxides from molten metal. It has been used as a soldering flux for alloys of aluminium, iron, zinc, tungsten, and monel. Aluminium castings can be improved by treating the melt with boron trichloride vapors. In the manufacture of electrical resistors, a uniform and lasting adhesive carbon film can be put over a ceramic base using BCl₃. It has been used in the field of high energy fuels and rocket propellants as a source of boron to raise BTU value. BCl₃ is also used in plasma etching in semiconductor manufacturing. This gas etches metal oxides by formation of a volatile BOCl_x and M_xO_yCl_z compounds.

BCl₃ is used as a reagent in the synthesis of organic compounds. Like the corresponding bromide, it cleaves C-O bonds in ethers.{{cite encyclopedia |author1=Yamamoto, Y.|author2-link=Norio Miyaura |author2=Miyaura, N. | encyclopedia = Encyclopedia of Reagents for Organic Synthesis | editor = Paquette, L. | year = 2004 | publisher = J. Wiley & Sons | location = New York | doi = 10.1002/047084289X.rb245.pub2 | isbn = 0-471-93623-5|entry=Boron Trichloride}}{{cite journal|title=Trichloroboron-promoted Deprotection of Phenolic Benzyl Ether Using Pentamethylbenzene as a Non Lewis-Basic Cation Scavenger |author=Shun Okaya |author2=Keiichiro Okuyama |author3=Kentaro Okano |author4=Hidetoshi Tokuyama|journal=Org. Synth.|year=2016|volume=93|pages=63–74|doi=10.15227/orgsyn.093.0063|doi-access=free}}

BCl₃ is an aggressive reagent that can form hydrogen chloride upon exposure to moisture or alcohols. The dimethyl sulfide adduct (BCl₃SMe₂), which is a solid, is much safer to use,{{Cite journal | doi = 10.1016/0040-4039(80)80164-X | title = Boron trihalide-methyl sulfide complexes as convenient reagents for dealkylation of aryl ethers | year = 1980 | last1 = Williard | first1 = Paul G. | last2 = Fryhle | first2 = Craig B. | journal = Tetrahedron Letters | volume = 21 | issue = 39 | page = 3731}} when possible, but H₂O will destroy the BCl₃ portion while leaving dimethyl sulfide in solution.

• List of highly toxic gases

{{Reflist}}

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• {{cite journal | author = Martin, D. R. | title = Coordination Compounds of Boron Trichloride. I. - A Review | journal = Chemical Reviews | year = 1944 | volume = 34 | issue = 3 | pages = 461–473 | doi = 10.1021/cr60109a005}}
• {{cite journal |author1=Kabalka, G. W. |author2=Wu, Z. Z. |author3=Ju, Y. H. | title = The Use of Organoboron Chlorides and Bromides in Organic Synthesis | journal = Journal of Organometallic Chemistry | year = 2003 | volume = 680 | issue = 1–2 | pages = 12–22 | doi = 10.1016/S0022-328X(03)00209-2}}

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Category:Boron compounds

Category:Chlorides

Category:Boron halides