- Boron trifluoride
Name = Boron trifluoride
ImageFileL1 = Boron-trifluoride-2D.png
ImageNameL1 = Boron trifluoride in 2D
ImageFileR1 = Boron-trifluoride-3D-vdW.png
ImageNameR1 = Boron trifluoride in 3D
Section1 = Chembox Identifiers
CASNo = 7637-07-2
UNNumber = "Compressed: "1008. "Complex with acetic acid: "1742. "Complex with propionic acid: "1743. "Boron trifuoride diethyl etherate: "2604. "Boron trifluoride dimethyl etherate: "2965. "Boron trifluoride dihydrate: "2851.
Section2 = Chembox Properties
Formula = BF3
MolarMass = 67.8062 g mol−1
Density = 2.178 g dm−3
Solvent = other solvents
SolubleOther = decomposes
MeltingPt = −126°C
BoilingPt = −100.3°C
Boron trifluoride is the
chemical compoundwith the formula BF3. This pungent colourless toxicgas forms white fumes in moist air. It is a useful Lewis acidand a versatile building block for other boroncompounds.
tructure and bonding
Unlike the aluminium trihalides, the boron trihalides are all monomeric. They do undergo rapid reversible dimerization as indicated by the high rate of the halide exchange reactions::BF3 + BCl3 → BF2Cl + BCl2FBecause of the facility of this exchange process, the mixed halides cannot be obtained in pure form.
The geometry of a
moleculeof BF3 is described as trigonal planar. The D3h symmetry conforms with the prediction of VSEPR theory. Although featuring three polar covalent bonds, the molecule has no dipole moment by virtue of its high symmetry. Although isoelectronicwith carbonate, CO32−, BF3 is commonly referred to as " electron deficient," a description that is reinforced by its exothermicreactivity toward Lewis bases.
In the boron trihalides, BX3, the length of the B-F bonds (1.30 Å) is shorter than would be expected for single bonds,Greenwood, N. N.; A. Earnshaw (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.] and this shortness may indicate stronger B-X π-bonding in the fluoride. A facile explanation invokes the symmetry-allowed overlap of a p orbital on the boron atom with the in-phase combination of the three similarly oriented p orbitals on fluorine atoms.
BF3 is manufactured by the reaction of boron oxides with
hydrogen fluoride::B2O3 + 6 HF → 2 BF3 + 3 H2OTypically the HF is produced in situ from sulfuric acid and fluorite(CaF2). [Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.]
On a laboratory scale, BF3 is produced by the thermal decomposition of diazonium salts: [OrgSynth | author = Flood, D. T. | title = Fluorobenzene | collvol = 2 | collvolpages = 295 | prep = CV2P0295] :PhN2BF4 → PhF + BF3 + N2
Lewis acidity and related reactions
Boron trifluoride is a versatile Lewis acid that forms
adducts with such Lewis bases as fluorideand ethers::CsF + BF3 → CsBF4:O(C2H5)2 + BF3 → BF3O(C2H5)2 Tetrafluoroborate salts are commonly employed as non-coordinating anions. The adduct with diethyl ether is a conveniently handled liquidand consequently is a widely encountered as a laboratory source of BF3.
Comparative Lewis acidity
All three lighter boron trihalides, BX3 (X = F, Cl, Br) form stable adducts with common Lewis bases. Their relative Lewis acidities can be evaluated in terms of the relative exothermicities of the adduct-forming reaction. Such measurements have revealed the following sequence for the Lewis acidity::BF3< BCl3< BBr3 (strongest Lewis acid)This trend commonly attributed to the degree of π-bonding in the planar boron trihalide that would be lost upon pyramidalization of the BX3 molecule. [Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. (1999). Advanced Inorganic Chemistry (6th Edn.) New York: Wiley-Interscience. ISBN 0-471-19957-5.] which follows this trend::BF3 > BCl3 > BBr3 (most easily pyramidalized)
The criteria for evaluating the relative strength of π-bonding are not clear, however.
One of the suggestion is that F atom is small compared to I atom, the lone pair electron in pz of F readily and easily donated and overlapped to empty pz orbital of boron.
As a result, the back donation of F is greater than that of I.
In an alternative explanation, the low Lewis acidity for BF3 is attributed to the relative weakness of the bond in the adducts F3B-L. ["Group V Chalcogenide Complexes of Boron Trihalides" Boorman, P. M.; Potts, D. Canadian. Journal of Chemistry (Rev. can. chim.) volume 52, (1974) pp 2016-2020] [cite journal | author = T. Brinck, J. S. Murray and P. Politzer | title = A computational analysis of the bonding in boron trifluoride and boron trichloride and their complexes with ammonia | year = 1993 | journal =
Inorg. Chem.| volume = 32 | issue = 12 | pages = 2622–2625 | doi = 10.1021/ic00064a008]
Boron trifluoride reacts with water to give
boric acid,and fluoroboric acid: and [HF] The reaction commences with the formation of the aquo adduct, H2O-BF3, which then loses HF::4 BF3 + 3 H2O → 3 HBF4 + "B(OH)3"
The heavier trihalides do not undergo analogous reactions, possibly the lower stability of the tetrahedral ions BX4- (X = Cl, Br). Because of the high acidity of fluoroboric acid, the fluoroborate ion can be used to isolate particularly electrophilic cations, such as
diazoniumions, that are otherwise difficult to isolate as solids.
Boron trifluoride is corrosive. Suitable metals for equipment handling boron trifluoride include
stainless steel, monel, and hastelloy. In presence of moisture it corrodes steel, including stainless steel. It reacts with polyamides. Polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, and polypropyleneshow satisfactory resistance. The grease used in the equipment should be fluorocarbonbased, as boron trifluoride reacts with the hydrocarbon-based ones. [cite web | publisher = Air Liquide| url = http://encyclopedia.airliquide.com/encyclopedia.asp?GasID=68 | work = Gas Encyclopedia | title = Boron trifluoride]
* applied as
dopantin ion implantation
* p-type dopant for
epitaxially grown silicon
polymerisationreactions of unsaturated compounds. Example polyethers
* as a catalyst in some
isomerization, alkylation, esterification, condensation, Mukaiyama aldol addition, and other reactions.
* used in sensitive
* [http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory - Boron and compounds fact sheet]
* [http://www.npi.gov.au/database/substance-info/profiles/44.html National Pollutant Inventory - Fluoride and compounds fact sheet]
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