Boron trifluoride
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2008-06-23T01:51:31Z
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{{Chembox new
| Name = Boron trifluoride
| ImageFileL1 = Boron-trifluoride-2D.png
<!-- | ImageSizeL1 = 150px -->
| ImageNameL1 = Boron trifluoride in 2D
| ImageFileR1 = Boron-trifluoride-3D-vdW.png
<!-- | ImageSizeR1 = 150px -->
| ImageNameR1 = Boron trifluoride in 3D
| Section1 = {{Chembox Identifiers
| CASNo = 7637-07-2
| UNNumber = ''Compressed: ''[[List of UN Numbers 1001 to 1100|1008]].<br/>''Complex with acetic acid: ''[[List of UN Numbers 1701 to 1800|1742]].<br/>''Complex with propionic acid: ''[[List of UN Numbers 1701 to 1800|1743]].<br/>''Boron trifuoride diethyl etherate: ''[[List of UN Numbers 2601 to 2700|2604]].<br/>''Boron trifluoride dimethyl etherate: ''[[List of UN Numbers 2901 to 3000|2965]].<br/>''Boron trifluoride dihydrate: ''[[List of UN Numbers 2801 to 2900|2851]].
}}
| Section2 = {{Chembox Properties
| Formula = BF<sub>3</sub>
| MolarMass = 67.8062 g mol<sup>−1</sup>
| Density = 2.178 g dm<sup>−3</sup>
| Solvent = other solvents
| SolubleOther = decomposes
| MeltingPt = −126°C
| BoilingPt = −100.3°C}}
}}
'''Boron trifluoride''' is the [[chemical compound]] with the [[chemical formula|formula]] BF<sub>3</sub>. This pungent colourless [[toxic]] gas forms white fumes in moist air. It is a useful [[Lewis acid]] and a versatile building block for other [[boron]] compounds.
==Structure 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:
:BF<sub>3</sub> + BCl<sub>3</sub> → BF<sub>2</sub>Cl + BCl<sub>2</sub>F
Because of the facility of this exchange process, the mixed halides cannot be obtained in pure form.
The geometry of a [[molecule]] of BF<sub>3</sub> is described as [[trigonal planar]]. The D<sub>3h</sub> [[symmetry group|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 [[isoelectronic]] with carbonate, CO<sub>3</sub><sup>2−</sup>, BF<sub>3</sub> is commonly referred to as " [[electron deficiency|electron deficient]]," a description that is reinforced by its [[exothermic]] reactivity toward [[Lewis base]]s.
In the boron trihalides, BX<sub>3</sub>, the length of the B-F bonds (1.30 Å) is shorter than would be expected for single bonds,<ref name = greenwood>Greenwood, N. N.; A. Earnshaw (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.</ref> and this shortness may indicate stronger B-X [[pi bond|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.<ref name = greenwood/>
==Synthesis==
BF<sub>3</sub> is manufactured by the reaction of boron oxides with [[hydrogen fluoride]]:
:B<sub>2</sub>O<sub>3</sub> + 6 HF → 2 BF<sub>3</sub> + 3 H<sub>2</sub>O
Typically the HF is produced in situ from sulfuric acid and [[fluorite]] (CaF<sub>2</sub>).<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>
On a laboratory scale, BF<sub>3</sub> is produced by the thermal decomposition of diazonium salts:<ref>{{OrgSynth | author = Flood, D. T. | title = Fluorobenzene | collvol = 2 | collvolpages = 295 | prep = CV2P0295}}</ref>
:PhN<sub>2</sub>BF<sub>4</sub> → [[Fluorobenzene|PhF]] + BF<sub>3</sub> + [[nitrogen|N<sub>2</sub>]]
==Lewis acidity and related reactions==
Boron trifluoride is a versatile Lewis acid that forms [[adduct]]s with such [[Lewis base]]s as [[fluoride]] and [[ether]]s:
:[[Caesium fluoride|CsF]] + BF<sub>3</sub> → CsBF<sub>4</sub>
:[[Diethyl ether|O(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>]] + BF<sub>3</sub> → BF<sub>3</sub>O(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>
[[Tetrafluoroborate]] [[salt]]s are commonly employed as [[non-coordinating anion]]s. The adduct with diethyl ether is a conveniently handled [[liquid]] and consequently is a widely encountered as a laboratory source of BF<sub>3</sub>.
====Comparative Lewis acidity====
All three lighter boron trihalides, BX<sub>3</sub> (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:
:BF<sub>3</sub>< BCl<sub>3</sub>< BBr<sub>3</sub> (strongest Lewis acid)
This trend commonly attributed to the degree of [[pi bond|π-bonding]] in the planar boron trihalide that would be lost upon pyramidalization of the BX<sub>3</sub> molecule.<ref>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.</ref> which follows this trend:
:BF<sub>3</sub> > BCl<sub>3</sub> > BBr<sub>3</sub> (most easily pyramidalized)
The criteria for evaluating the relative strength of [[pi bond|π-bonding]] are not clear, however.<ref name = greenwood/><br />
One of the suggestion is that F atom is small compared to I atom, the lone pair electron in P<sub>z</sub> of F readily and easily donated and overlapped to empty P<sub>z</sub> 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 BF<sub>3</sub> is attributed to the relative weakness of the bond in the adducts F<sub>3</sub>B-L.<ref>''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</ref><ref>{{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}}</ref>
====Hydrolysis====
Boron trifluoride reacts with water to give [[boric acid]],and [[fluoroboric acid]]: and [HF]
The reaction commences with the formation of the aquo adduct, H<sub>2</sub>O-BF<sub>3</sub>, which then loses HF:
:4 BF<sub>3</sub> + 3 H<sub>2</sub>O → 3 HBF<sub>4</sub> + "B(OH)<sub>3</sub>"
The heavier trihalides do not undergo analogous reactions, possibly the lower stability of the tetrahedral ions BX<sub>4</sub><sup>-</sup> (X = Cl, Br). Because of the high acidity of fluoroboric acid, the fluoroborate ion can be used to isolate particularly electrophilic cations, such as [[diazonium]] ions, that are otherwise difficult to isolate as solids.
==Handling==
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 [[polyamide]]s. [[Polytetrafluoroethylene]], [[polychlorotrifluoroethylene]], [[polyvinylidene fluoride]], and [[polypropylene]] show satisfactory resistance. The [[grease (lubricant)#Laboratory grease|grease]] used in the equipment should be [[fluorocarbon]] based, as boron trifluoride reacts with the hydrocarbon-based ones.<ref>{{cite web | publisher = [[Air Liquide]] | url = http://encyclopedia.airliquide.com/encyclopedia.asp?GasID=68 | work = Gas Encyclopedia | title = Boron trifluoride}}</ref>
==Uses==
* applied as [[dopant]] in [[ion implantation]]
* [[p-type semiconductor|p-type]] dopant for [[epitaxial]]ly grown [[silicon]]
* initiates [[polymerisation]] reactions of [[saturation (chemistry)|unsaturated]] [[chemical compounds|compounds]]. Example [[polyether]]s
* as a catalyst in some [[isomerization]], [[alkylation]], [[esterification]], [[Condensation reaction|condensation]], [[Mukaiyama aldol addition]], and other reactions.
* used in sensitive [[neutron]] detectors
{{Sectstub|date=May 2008}}
==References==
<references/>
==External links==
* http://www.osha.gov/dts/chemicalsampling/data/CH_221700.html
* http://www.cdc.gov/niosh/ipcsneng/neng0231.html
*[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]
[[Category:Fluorides]]
[[Category:Boron compounds]]
[[Category:Nonmetal halides]]
[[cs:Fluorid boritý]]
[[de:Bortrifluorid]]
[[fr:Trifluorure de bore]]
[[nl:Boortrifluoride]]
[[ja:三フッ化ホウ素]]
[[zh:三氟化硼]]