Boron trifluoride
Encyclopedia
Boron trifluoride is the chemical compound
with the formula
BF3. 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.
of BF3 is trigonal planar
. The D3h symmetry
conforms with the prediction of VSEPR theory
. The molecule has no dipole moment by virtue of its high symmetry. The molecule is isoelectronic with the carbonate anion, CO32−.
BF3 is commonly referred to as "electron deficient
," a description that is reinforced by its exothermic
reactivity 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, 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.
:
Typically the HF is produced in situ from sulfuric acid and fluorite
(CaF2). Approximately 2300-4500 tonnes of boron trifluoride are produced every year.
On a laboratory scale, BF3 is produced by the thermal decomposition of diazonium salts:
Anhydrous boron trifluoride has a normal boiling temperature of −100.3 C and a critical temperature of −12.3 C, so that it can be stored as a refrigerated liquid only between those temperatures. Storage or transport vessels should be designed to withstand internal pressure, since a refrigeration system failure could cause pressures to rise to the critical pressure of 49.85 bar (4.985 MPa).
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 used in the equipment should be fluorocarbon
based, as boron trifluoride reacts with the hydrocarbon-based ones.
Because of the facility of this exchange process, the mixed halides cannot be obtained in pure form.
Boron trifluoride is a versatile Lewis acid that forms adduct
s with such Lewis bases as fluoride
and ether
s:
Tetrafluoroborate
salts are commonly employed as non-coordinating anion
s. The adduct with diethyl ether
, boron trifluoride diethyl etherate or just boron trifluoride etherate (BF3·O(Et)2) is a conveniently handled liquid
and consequently is a widely encountered as a laboratory source of BF3. It is stable as a solution in ether, but not stoichiometrically. Another common adduct is the adduct with dimethyl sulfide
(BF3·S(Me)2), which can be handled as a neat liquid.
This trend is commonly attributed to the degree of π-bonding
in the planar boron trihalide that would be lost upon pyramidalization of the BX3 molecule. which follows this trend:
The criteria for evaluating the relative strength of π-bonding
are not clear, however. One suggestion is that the F atom is small compared to the larger Cl and Br atoms, and the lone pair electron in pz of F is readily and easily donated and overlapped to empty pz orbital of boron. As a result, the pi donation of F is greater than that of Cl or Br.
In an alternative explanation, the low Lewis acidity for BF3 is attributed to the relative weakness of the bond in the adducts F3B-L.
and fluoroboric acid
. The reaction commences with the formation of the aquo adduct, H2O-BF3, which then loses HF that gives fluoboric acid with boron trifluoride.
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 diazonium ions, that are otherwise difficult to isolate as solids.
, typically as a Lewis acid. Examples:
Other uses:
and Louis Jacques Thénard
, who were trying to isolate "fluoric acid" (i.e. hydrofluoric acid
) by combining calcium fluoride
with vitrified boric acid
; the resulting vapours failed to etch glass, so they named it fluoboric gas.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
BF3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
and a versatile building block for other boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
compounds.
Structure and bonding
The geometry of a moleculeMolecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
of BF3 is trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...
. The D3h symmetry
Symmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
conforms with the prediction of VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
. The molecule has no dipole moment by virtue of its high symmetry. The molecule is isoelectronic with the carbonate anion, CO32−.
BF3 is commonly referred to as "electron deficient
Electron deficiency
Electron deficiency occurs when a compound has too few valence electrons for the connections between atoms to be described as covalent bonds. Electron deficient bonds are often better described as 3-center-2-electron bonds...
," a description that is reinforced by its exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
reactivity 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, and this shortness may indicate stronger B-X π-bonding
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
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.
Synthesis and handling
BF3 is manufactured by the reaction of boron oxides with hydrogen fluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
:
- B2O3 + 6 HF → 2 BF3 + 3 H2O
Typically the HF is produced in situ from sulfuric acid and fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
(CaF2). Approximately 2300-4500 tonnes of boron trifluoride are produced every year.
On a laboratory scale, BF3 is produced by the thermal decomposition of diazonium salts:
- PhN2BF4 → PhFFluorobenzeneFluorobenzene is the chemical compound with the formula C6H5F, often abbreviated PhF. This species is a derivative of benzene, with a single fluorine atom attached. Its melting point is 44 °C lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular...
+ BF3 + N2NitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
Anhydrous boron trifluoride has a normal boiling temperature of −100.3 C and a critical temperature of −12.3 C, so that it can be stored as a refrigerated liquid only between those temperatures. Storage or transport vessels should be designed to withstand internal pressure, since a refrigeration system failure could cause pressures to rise to the critical pressure of 49.85 bar (4.985 MPa).
Boron trifluoride is corrosive. Suitable metals for equipment handling boron trifluoride include stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, monel
Monel
Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...
, and hastelloy
Hastelloy
Hastelloy is the registered trademark name of Haynes International, Inc. The trademark is applied as the prefix name of a range of twenty two different highly corrosion-resistant metal alloys loosely grouped by the metallurgical industry under the material term “superalloys” or “high-performance...
. In presence of moisture it corrodes steel, including stainless steel. It reacts with polyamide
Polyamide
A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...
s. Polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
, polychlorotrifluoroethylene
Polychlorotrifluoroethylene
Polychlorotrifluoroethylene is a fluoropolymer with the molecular formula n. It is chemically related to PTFE....
, polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
, and polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
show satisfactory resistance. The grease used in the equipment should be fluorocarbon
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...
based, as boron trifluoride reacts with the hydrocarbon-based ones.
Reactions
Unlike the aluminium trihalides, the boron trihalides are all monomeric. They undergo rapid halide exchange reactions:- BF3 + BCl3Boron trichlorideBoron trichloride is a chemical compound with the formula BCl3. This colorless gas is a valuable reagent in organic synthesis. It is also dangerously reactive.-Production and properties:Boron reacts with halogens to give the corresponding trihalides...
→ BF2Cl + BCl2F
Because of the facility of this exchange process, the mixed halides cannot be obtained in pure form.
Boron trifluoride is a versatile Lewis acid that forms adduct
Adduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
s with such Lewis bases as fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
and ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
s:
- CsFCaesium fluorideCaesium fluoride , is an inorganic compound usually encountered as a hygroscopic white solid. It is more soluble and more readily dissociated than sodium fluoride or potassium fluoride. It is available in anhydrous form, and if water has been absorbed it is easy to dry by heating at 100 °C for...
+ BF3 → CsBF4 - O(C2H5)2Diethyl etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
+ BF3 → BF3O(C2H5)2
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
salts are commonly employed as non-coordinating anion
Non-coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is 'weakly coordinating anion'. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes...
s. The adduct with diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, boron trifluoride diethyl etherate or just boron trifluoride etherate (BF3·O(Et)2) is a conveniently handled liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
and consequently is a widely encountered as a laboratory source of BF3. It is stable as a solution in ether, but not stoichiometrically. Another common adduct is the adduct with dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...
(BF3·S(Me)2), which can be handled as a neat liquid.
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 is commonly attributed to the degree of π-bonding
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
in the planar boron trihalide that would be lost upon pyramidalization of the BX3 molecule. which follows this trend:
- BF3 > BCl3 > BBr3 (most easily pyramidalized)
The criteria for evaluating the relative strength of π-bonding
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
are not clear, however. One suggestion is that the F atom is small compared to the larger Cl and Br atoms, and the lone pair electron in pz of F is readily and easily donated and overlapped to empty pz orbital of boron. As a result, the pi donation of F is greater than that of Cl or Br.
In an alternative explanation, the low Lewis acidity for BF3 is attributed to the relative weakness of the bond in the adducts F3B-L.
Hydrolysis
Boron trifluoride reacts with water to give boric acidBoric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
and fluoroboric acid
Fluoroboric acid
Fluoroboric acid is the chemical compound with the formula HBF4. It is the conjugate acid of tetrafluoroborate. It is available commercially as a solution in water and other solvents such as diethyl ether...
. The reaction commences with the formation of the aquo adduct, H2O-BF3, which then loses HF that gives fluoboric acid with boron trifluoride.
- 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 diazonium ions, that are otherwise difficult to isolate as solids.
Uses
Boron trifluoride is most importantly used as a reagent in organic chemistryOrganic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, typically as a Lewis acid. Examples:
- initiates polymerisation reactions of unsaturatedSaturation (chemistry)In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
compounds such as polyethers - as a catalyst in some isomerization, alkylationAlkylationAlkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene . Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in...
, esterification, condensationCondensation reactionA condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...
, Mukaiyama aldol additionMukaiyama aldol additionThe Mukaiyama aldol addition is an organic reaction and a type of aldol reaction between a silyl enol ether and an aldehyde catalyzed by a Lewis acid. This choice of reactants allows for a crossed aldol reaction between an aldehyde and a ketone or a different aldehyde without self-condensation of...
, and other reactions
Other uses:
- applied as dopantDopantA dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
in ion implantationIon implantationIon implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid... - p-typeP-type semiconductorA P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....
dopant for epitaxially grown siliconSiliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table... - used in sensitive neutronNeutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
detectors in ionization chamberIonization chamberThe ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation...
s and devices to monitor radiation levels in the Earth's atmosphereAtmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low... - in fumigationFumigationFumigation is a method of pest control that completely fills an area with gaseous pesticides—or fumigants—to suffocate or poison the pests within. It is utilized for control of pests in buildings , soil, grain, and produce, and is also used during processing of goods to be imported or...
- as a fluxFlux (metallurgy)In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
for solderingSolderingSoldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
magnesium - to prepare diboraneDiboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
Discovery
Boron trifluoride was discovered in 1808 by Joseph Louis Gay-LussacJoseph Louis Gay-Lussac
- External links :* from the American Chemical Society* from the Encyclopædia Britannica, 10th Edition * , Paris...
and Louis Jacques Thénard
Louis Jacques Thénard
Louis Jacques Thénard , was a French chemist.His father, a poor peasant, managed to have him educated at the academy of Sens, and sent him at the age of sixteen to study pharmacy in Paris. There he attended the lectures of Antoine François Fourcroy and Louis Nicolas Vauquelin...
, who were trying to isolate "fluoric acid" (i.e. hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
) by combining calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
with vitrified boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
; the resulting vapours failed to etch glass, so they named it fluoboric gas.
External links
- http://www.osha.gov/dts/chemicalsampling/data/CH_221700.html
- http://www.cdc.gov/niosh/ipcsneng/neng0231.html
- National Pollutant Inventory - Boron and compounds fact sheet
- National Pollutant Inventory - Fluoride and compounds fact sheet
- WebBook page for BF3