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Home >> Chemistry Dictionary >> Boron Halides Bredts Rule
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Boron fluorides. The normal fluoride is BF3 CaF2 plus B2O3 plus H2SO4; heat on a diazonium fluoroborate) a colourless gas b.p. - 100°C which fumes in moist air and gives boric acid and fluoroboric acid, HBF4, with water. BF3 is a strong Lewis acid and forms adducts with many donors (e.g. F3B. NMe4. It is very powerful catalyst in Friedel-Crafts and other reactions and it used as a starting point for the preparation of organoboron derivatives. Reacts with B2O3 to (FBO)3 . Lower fluorided are also known. B2F4 (BO plus SF4) contains a single B-B bond. B plus BF3 at very high temperatures gives unstable BF which may be used to prepare B3F5, B8F12, etc.
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Boron halides. Boron forms BX3, B2X4 and BX4- with each halogen and also some lower halides. The order of Lewis acidity is BBr3 BCI3 BF3.Each of the halides is rapidly hydrolysed by waster. See under individual entries.
Boron carbide (B4C). A very hard black crystalline compound. m.p. 2450°C. It is used in nuclear reactor as moderator and as an abrasive.
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Boron Nitride (BN). A slippery white solid obtained by the action of nitrogen on boron. It exists in two structurally different forms; hexagonal and cubic form. It is used as refractory material, in thermal insulation of furnaces, as lubricant and electrical insulator for high voltages.
Boron trichloride. Boron trichloride is obtained industrially by the exothermic chlorination of boron carbide at above 700°C followed by fractional distillation. Boron trichloride is a Lewis acid. The compound is important industrially as a source of pure boron for the electronics industry.
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Borosilicates. Boron-containing silicates found naturally. e.g. danburite, CaB2Si2O8, and prepared as glasses by fusing together B2O3, SiO2 and a metal oxide (the latter is not a necessary constituent). Pyrex glasses are borosilicate glasses.
Boyle’s law. At constant temperature the volume of a given mass of gas is inversely proportional to the pressure. Although exact at low pressures, the law is not accurately obeyed at high pressures because of the finite size of molecules and the existence of intermolecular forces.
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Boyle temperature. The Temperature at which a gas most closely obeys Boyle’s law.
Br. Bromine.
Bragg’s law. The interatomic distances of crystals are of the same order of magnitude as the wavelengths of X-rays.
Branched chain. A chain of carbon atoms in which one or more carbon atoms are joined to at least three other carbon atoms, such that the structure is not one continuous straight chain.
Brass. A group of alloys consisting of copper and zinc. A typical yellow brass might contain about 67% copper and 33% zinc.
Bredt’s rule. A double bond cannot be introduced at the bridgehead carbon of a bridged bicyclic or polocyclic system with small or medium-sized rings.
Bridge. An atom which is joining two other atoms in a molecule.
Brine. Concentrated NaCI in water. NaCI is generally extracted as brine.
Brittle. Describes a solid which breaks into pieces when a stress is applied.
Bromacil, 5-bromo-6-methyl-3-see-butyluracil. C9H13BrN2O2. soil-acting herbicide. White crystalline solid, m.p. 158-159°C. It is non-selective inhibitor of photosynthesis used for weed control in citrus and cane fruit plantations. It is relatively non-toxic to animal life.
Bromates. Salts containing bromine oxyanions; hypobromates, [BrO]-, bromate (1); bromates (the usual meaning), [BrO3]-, bromates (V); perbromates), [BrO1]-, bromates (VII). Bromates (1) are formed from Br2 and base, but BrO- disproportionate to Br- and [BrO3]- above 0°C. Bromates (V) are stable at elevated temperatures (Ba(BrO3)2 plus H2SO4 give HBrO3), Bromates (VII) are formed by electrolystic, XeF2 or F2 oxidation of [BrO3]-, the free acid HBrO4, 2H2O can be crystallized [BrO4]-, is a strong but sluggish oxidizing agent.
E° (acid solution) [BrO4]-→ [BrO3]- + 1.76 volts
E° (acid solution) [BrO3]- → [BrO]- + 0.56 volts
[BrO]- →Br2 + 0.45 volts
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Borone nitride
Borazole
