Boron trichloride

Names

IUPAC name

Boron trichloride

Other names

Boron(III) chloride
Trichloroborane

Identifiers

CAS Number

10294-34-5^Y

3D model (JSmol)

Interactive image

ChemSpider

23480^Y

ECHA InfoCard

100.030.586

EC Number

233-658-4

PubChem CID

25135

RTECS number

ED1925000

UNII

K748471RAG^Y

CompTox Dashboard (EPA)

DTXSID2041676

InChI

InChI=1S/B.3ClH/h;3*1H/q+3;;;/p-3^Y
Key: PYQQLJUXVKZOPJ-UHFFFAOYSA-K^Y
InChI=1/B.3ClH/h;3*1H/q+3;;;/p-3
Key: PYQQLJUXVKZOPJ-DFZHHIFOAV

SMILES

ClB(Cl)Cl

Properties

Chemical formula

BCl₃

Molar mass

117.17 g/mol

Appearance

Colorless gas,
fumes in air

Density

1.326 g/cm³

Melting point

−107.3 °C (−161.1 °F; 165.8 K)

Boiling point

12.6 °C (54.7 °F; 285.8 K)^[1]

Solubility in water

hydrolysis

Solubility

soluble in CCl₄, ethanol

Magnetic susceptibility (χ)

-59.9·10⁻⁶ cm³/mol

Refractive index (n_D)

1.00139

Structure

Molecular shape

Trigonal planar (D_3h)

Dipole moment

zero

Thermochemistry

Heat capacity (C)

107 J/mol K

Std molar
entropy (S^⦵₂₉₈)

206 J/mol K

Std enthalpy of
formation (Δ_fH^⦵₂₉₈)

-427 kJ/mol

Gibbs free energy (Δ_fG^⦵)

-387.2 kJ/mol

Hazards^[2]

Occupational safety and health (OHS/OSH):

Main hazards

May be fatal if swallowed or if inhaled
Causes serious burns to eyes, skin, mouth, lungs, etc.
Contact with water gives HCl

GHS labelling:

Pictograms

Signal word

Danger

Hazard statements

H300, H314, H330^{[note 1]}

NFPA 704 (fire diamond)

Flash point

Non-flammable

Safety data sheet (SDS)

ICSC 0616

Related compounds

Other anions

Boron trifluoride
Boron tribromide
Boron triiodide

Other cations

Aluminium trichloride
Gallium trichloride

Related compounds

Boron trioxide
Carbon tetrachloride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Y verify (what is ^YN ?)

Infobox references

Chemical compound

Boron trichloride is the inorganic compound with the formula BCl₃. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water.

Production and structure

Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by direct 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. In the laboratory BF₃ reacted with AlCl₃ gives BCl₃ via halogen exchange.^[3] BCl₃ is a trigonal planar molecule like the other boron trihalides, and has a bond length of 175pm.

A degree of π-bonding has been proposed to explain the short B− Cl distance although there is some debate as to its extent.^[3] It 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.

Reactions

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.

Ammonia forms a Lewis adduct with boron trichloride.

As a strong Lewis acid, BCl₃ forms adducts with tertiary amines, phosphines, ethers, thioethers, and halide ions.^[4] 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

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:^[5]

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.^[3] 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_nCl_n + n BCl₃

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

Uses

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.^[1]^[6]

Safety

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,^[7] when possible, but H₂O will destroy the BCl₃ portion while leaving dimethyl sulfide in solution.

References

^ ^a ^b Yamamoto, Y.; Miyaura, N. (2004). "Boron Trichloride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rb245.pub2. ISBN 0-471-93623-5.
^ Index no. 005-002-00-5 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 341.
^ ^a ^b ^c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ Gerrard, W.; Lappert, M. F. (1958). "Reactions Of Boron Trichloride With Organic Compounds". Chemical Reviews. 58 (6): 1081–1111. doi:10.1021/cr50024a003.
^ Wartik, T.; Rosenberg, R.; Fox, W. B. (1967). "Diboron Tetrachloride". Inorganic Syntheses. Vol. 10. pp. 118–125. doi:10.1002/9780470132418.ch18. ISBN 978-0-470-13241-8.
^ Shun Okaya; Keiichiro Okuyama; Kentaro Okano; Hidetoshi Tokuyama (2016). "Trichloroboron-promoted Deprotection of Phenolic Benzyl Ether Using Pentamethylbenzene as a Non Lewis-Basic Cation Scavenger". Org. Synth. 93: 63–74. doi:10.15227/orgsyn.093.0063.
^ Williard, Paul G.; Fryhle, Craig B. (1980). "Boron trihalide-methyl sulfide complexes as convenient reagents for dealkylation of aryl ethers". Tetrahedron Letters. 21 (39): 3731. doi:10.1016/0040-4039(80)80164-X.