Boron trifluoride is the
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
BF
3. This pungent, colourless, and
toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a sub ...
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
The geometry of a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
of BF
3 is
trigonal planar. Its D
3h symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
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, .
BF
3 is commonly referred to as "
electron deficient," a description that is reinforced by its
exothermic reactivity toward
Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
s.
In the
boron trihalides, BX
3, the length of the B–X 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.
[ Others point to the ionic nature of the bonds in BF3.
]
Synthesis and handling
BF3 is manufactured by the reaction of boron oxides with hydrogen fluoride
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...
:
:B2O3 + 6 HF → 2 BF3 + 3 H2O
Typically the HF is produced ''in situ'' from sulfuric acid and fluorite (CaF2). Approximately 2300-4500 tonnes of boron trifluoride are produced every year.
Laboratory scale
For laboratory scale reactions, BF3 is usually produced in situ using boron trifluoride etherate
Boron trifluoride etherate, strictly boron trifluoride diethyl etherate, or boron trifluoride–ether complex, is the chemical compound with the formula BF3O(C2H5)2, often abbreviated BF3OEt2. It is a colorless liquid, although older samples can a ...
, which is a commercially available liquid.
Laboratory routes to the solvent-free materials are numerous. A well documented route involves the thermal decomposition of diazonium salts of :
:PhN2BF4 → PhF + BF3 + N2
Alternatively it arises from the reaction of sodium tetrafluoroborate, boron trioxide, and sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
:
:6 NaBF4 + B2O3 + 6 H2SO4 → 8 BF3 + 6 NaHSO4 + 3 H2O
Properties
Anhydrous boron trifluoride has a boiling point
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid varies depending upon the surrounding env ...
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 is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
, monel, and hastelloy. In presence of moisture it corrodes steel, including stainless steel. It reacts with polyamide
A polyamide is a polymer with repeating units linked by amide bonds.
Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made th ...
s. Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemo ...
, polychlorotrifluoroethylene
Polychlorotrifluoroethylene (PCTFE or PTFCE) is a thermoplastic chlorofluoropolymer with the molecular formula , where ''n'' is the number of monomer units in the polymer molecule. It is similar to polytetrafluoroethene (PTFE), except that it i ...
, polyvinylidene fluoride, and polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
show satisfactory resistance. The grease used in the equipment should be fluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
based, as boron trifluoride reacts with the hydrocarbon-based ones.
Reactions
Unlike the aluminium and gallium trihalides, the boron trihalides are all monomeric. They undergo rapid halide exchange reactions:
:BF3 + BCl3 → 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 adducts with such Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
s as fluoride and ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again ...
s:
: CsF + BF3 → CsBF4
: O(C2H5)2 + BF3 → BF3·O(C2H5)2
Tetrafluoroborate salts are commonly employed as non-coordinating anions. The adduct with diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
, boron trifluoride diethyl etherate, or just boron trifluoride etherate
Boron trifluoride etherate, strictly boron trifluoride diethyl etherate, or boron trifluoride–ether complex, is the chemical compound with the formula BF3O(C2H5)2, often abbreviated BF3OEt2. It is a colorless liquid, although older samples can a ...
, (BF3·O(Et)2) is a conveniently handled liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
and consequently is widely encountered as a laboratory source of BF3. Another common adduct is the adduct with dimethyl sulfide
Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH3)2S. Dimethyl sulfide is a flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from co ...
(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 < BI3(strongest Lewis acid)
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:
:BF3 > BCl3 > BBr3 < BI3 (most easily pyramidalized)
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 p''z'' 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 acid and fluoroboric acid
Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...
. The reaction commences with the formation of the aquo adduct, H2O−BF3, which then loses HF that gives fluoroboric acid with boron trifluoride.
:4 BF3 + 3 H2O → 3 HBF4 + B(OH)3
The heavier trihalides do not undergo analogous reactions, possibly due to the lower stability of the tetrahedral ions and . 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
Organic chemistry
Boron trifluoride is most importantly used as a reagent in organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
, typically as a Lewis acid.[ Examples include:
* initiates polymerisation reactions of ]unsaturated compound
In chemistry, a saturated compound is a chemical compound (or ion) that resists the addition reactions, such as hydrogenation, oxidative addition, and binding of a Lewis base. The term is used in many contexts and for many classes of chemical c ...
s, such as polyether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...
s
* as a catalyst in some isomerization, acylation, alkylation
Alkylation 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 (or their equivalents). Alkylating agents are reagents for effectin ...
, esterification, dehydration
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
, condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapo ...
, Mukaiyama aldol addition
The Mukaiyama aldol addition is an organic reaction and a type of aldol reaction between a silyl enol ether and an aldehyde or formate. The reaction was discovered by Teruaki Mukaiyama (1927–2018) in 1973. His choice of reactants allows ...
, and other reactions
Niche uses
Other, less common uses for boron trifluoride include:
* applied as dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. Whe ...
in ion implantation
* p-type dopant for epitaxially grown silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
* used in sensitive neutron detectors in ionization chambers and devices to monitor radiation levels in the Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
* in fumigation
* as a flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
for soldering
Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal ( solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not inv ...
magnesium
* to prepare diborane
Discovery
Boron trifluoride was discovered in 1808 by Joseph Louis Gay-Lussac and Louis Jacques Thénard, who were trying to isolate "fluoric acid" (i.e., hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...
) by combining calcium fluoride with vitrified boric acid. The resulting vapours failed to etch glass, so they named it ''fluoboric gas''.
See also
*List of highly toxic gases
Many gases have toxic properties, which are often assessed using the LC50 (median lethal dose) measure. In the United States, many of these gases have been assigned an NFPA 704 health rating of 4 (may be fatal) or 3 (may cause serious or perman ...
References
External links
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{{DEFAULTSORT:Boron Trifluoride
Fluorides
Boron compounds
Boron halides
Industrial gases
Acid catalysts