Boron trichloride

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Boron trichloride
Boron trichloride
Boron trichloride
IUPAC name
Boron trichloride
Other names
Boron(III) chloride
3D model (Jmol)
ECHA InfoCard 100.030.586
EC Number 233-658-4
RTECS number ED1925000
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 soluble in CCl4, ethanol
-59.9·10−6 cm3/mol
Trigonal planar (D3h)
107 J/mol K
206 J/mol K
-427 kJ/mol
-387.2 kJ/mol
Main hazards May be fatal if swallowed or if inhaled
Causes serious burns to eyes, skin, mouth, lungs, etc.
Contact with water gives HCl
Safety data sheet ICSC 0616
GHS pictograms Press. GasAcute Tox. 2Skin Corr. 1B
GHS signal word DANGER
H330, H300, H314[note 1]
Very Toxic T+ (T+)
Corrosive C (C)
R-phrases R14, R26/28, R34
S-phrases (S1/2), S9, S26, S28,
S36/37/39, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Flash point Non-flammable
Related compounds
Other anions
Boron trifluoride
Boron tribromide
Boron triiodide
Other cations
Aluminium chloride
Gallium chloride
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).
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Infobox references

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

Production and structure[edit]

Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by direct chlorination of boron oxide and carbon at 500 °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.


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 of BCl3 to elemental boron is conducted commercially (see below). In the laboratory, when boron trichloride can be converted to diboron tetrachloride by heating with copper metal:[5]

2 BCl3 + 2 Cu → B2Cl4 + 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 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. 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. Aluminum 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 BOClx 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]


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

See also[edit]


  1. ^ a b Yamamoto, Y.; Miyaura, N. (2004). "Boron Trichloride". In Paquette, L. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rb245.pub2. ISBN 0471936235. 
  2. ^ 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.
  3. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 
  4. ^ Gerrard, W.; Lappert, M. F. (1958). "Reactions Of Boron Trichloride With Organic Compounds". Chemical Reviews. 58 (6): 1081–1111. doi:10.1021/cr50024a003. 
  5. ^ Wartik, T.; Rosenberg, R.; Fox, W. B. (1967). "Diboron Tetrachloride". Inorganic Syntheses. 10. pp. 118–125. doi:10.1002/9780470132418.ch18. 
  6. ^ 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. 


  1. ^ Within the European Union, the following additional hazard statement (EUH014) must also be displayed on labelling: Reacts violently with water.

Further reading[edit]

  • Martin, D. R. (1944). "Coordination Compounds of Boron Trichloride. I. - A Review". Chemical Reviews. 34 (3): 461–473. doi:10.1021/cr60109a005. 
  • Kabalka, G. W.; Wu, Z. Z.; Ju, Y. H. (2003). "The Use of Organoboron Chlorides and Bromides in Organic Synthesis". Journal of Organometallic Chemistry. 680 (1–2): 12–22. doi:10.1016/S0022-328X(03)00209-2. 

External links[edit]