Borate
Borates are chemical compounds which contain oxoanions of boron in oxidation state +3. The simplest borate ion, BO33−, has a trigonal planar structure. Other borates are made up of trigonal BO3 or tetrahedral BO4 structural units, sharing oxygen atoms.[1] Boron occurs in nature mainly as borate minerals and borosilicates.
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[edit] Structures
When boron forms three covalent bonds, as in the borate ion, BO33-, it has a share in three pairs of electrons. VSEPR theory predicts a trigonal planar structure as this is the configuration that minimizes the energy of repulsion between electrons. It also predicts that in compounds where the boron atom bonds with three oxygen atoms the inter-bond angle will be 120°. In terms of valence bond theory the bonds are formed by using sp2 hybrid orbitals. The fact that there is an incomplete octet means that these compounds are Lewis acids.
When a trigonal boron atom accepts a pair of electrons from a Lewis base, it adopts a tetrahedral configuration (sp3), and the octet rule is satisfied. Both trigonal and tetrahedral units can co-exist in a complex borate, such as the anion in borax (crystal structure below).
[edit] Aqueous chemistry
Boric acid, B(OH)3, while sometimes indicated as dissociate in aqueous solution,[2]
- B(OH)3
BH2O3– + H+; Ka = 5.794 x 10−10 mol/l (computed from pKa = 9.237 in citation).
BH2O3– + H+; Ka = 5.794 x 10−10 mol/l (computed from pKa = 9.237 in citation).it may also be represented as acidic due to its hydrolysis reaction with water molecules, forming tetrahydroxyborate and releasing a proton:[citation needed]
- B(OH)3 + H2O B(OH)−
4 + H+; Ka = 5.8x10−10 mol/l; pKa = 9.24.
In the presence of cis-diols such as mannitol, glucose, sorbitol and glycerol the pK is lowered to about 4.[3]
Under acid conditions boric acid may undergo condensation reactions to form polymeric oxyanions. The equilibrium
- 4 [B(OH)4]- + 2 H+ [B4O5(OH)4]2− + 7 H2O
illustrates the process. The tetrameric anion is present in the mineral borax, as a octahydrate, Na2B4O5(OH)4.8H2O. This compound can be obtained in high purity and so can be used to make a standard solution in titrimetric analysis.[4]
[edit] Polymeric ions
A number of polymeric borate ions are known. They may be made by reacting B(OH)3 or B2O3 with metal oxides. Examples include:[1]
- diborate B2O54− e.g. in Mg2B2O5 (suanite)
- triborate B3O75- in CaAlB3O7 (johachidolite)
- tetraborate B4O96− in e.g. Li6B4O9
- metaborates, such as LiBO2 contain long chains of trigonal BO3 structural units, each sharing two oxygen atoms with adjacent units.
- borates containing 3 and four coordinate boron, including the anion in borax.
- borosilicate Borosilicate glass, also known as pyrex, is useful because it has a very low coefficient of thermal expansion and is resistant to cracking when heated, unlike soda glass. Structurally it is a silicate in which some SiO4 units are replaced by BO4, together with a cation to compensate for the difference in oxidation states of Si(IV) and B(III).
[edit] Minerals and uses
Common borate salts include sodium metaborate, NaBO2, and borax. Borax is quite soluble in water, so mineral deposits only occur in places with very low rainfall. Extensive deposits were found in Death Valley and transported out using the famous twenty-mule teams (1883 to 1889). Later (1925), deposits were found at Boron, California on the edge of the Mojave Desert. The Atacama Desert in Chile also contains mineable borate concentrations.
Lithium metaborate or lithium tetraborate, or a mixture of both, can be used in borate fusion sample preparation of various samples for analysis by XRF, AAS, ICP-OES, ICP-AES and ICP-MS. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used in the analysis of contaminated soils.[5]
Disodium octaborate tetrahydrate is used as wood preservatives or fungicide. Zinc borate is used as a flame retardant.
[edit] Borate esters
Borate esters are organic compounds of the type B(OR)3 where R is an organic residue (for example alkyl or aryl). They are conveniently prepared by a reaction such as
- B(OH)3 + 3 ROH → B(OR)3 +3 H2O
in the presence of a dehydrating agent, such as concentrated sulfuric acid.[6] The borate ester is volatile and can be removed from the reaction mixture by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel.[7] Alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.[8]
Trimethyl borate, B(OCH3)3, is used as a precursor to boronic esters for Suzuki couplings.[9] Borate esters were shown to serve as effective coupling agents in the synthesis of amides from either carboxylic acids or primary amides. [10]
[edit] References
- ^ a b Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
- ^ Goldberg, R.; Kishore, N.; Lennen, R. (2002). "Thermodynamic Quantities for the Ionization Reactions of Buffers". J. Phys. Chem. Ref. Data 31 (2): 231–370. doi:10.1063/1.1416902. http://www.nist.gov/data/PDFfiles/jpcrd615.pdf.
- ^ Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M.J.K.; Denney, R. C.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 357, ISBN 0-582-22628-7
- ^ Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M.J.K.; Denney, R. C.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 316, ISBN 0-582-22628-7
- ^ Hettipathirana, Terrance D. (2004). "Simultaneous determination of parts-per-million level Cr, As, Cd and Pb, and major elements in low level contaminated soils using borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation". Spectrochimica Acta Part B: Atomic Spectroscopy 59 (2): 223–229. doi:10.1016/j.sab.2003.12.013.
- ^ Brown, Herbert C.; Mead, Edward J.; Shoaf,Charles J. (1956). "Convenient Procedures for the Preparation of Alkyl Borate Esters". J. Am. Chem. Soc 78 (15): 3613–3614. doi:10.1021/ja01596a015.
- ^ Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M.J.K.; Denney, R. C.; Thomas, M. J. K. (2000), Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 666, ISBN 0-582-22628-7
- ^ Vogel, Arthur I.; Svehla, G. (1979), Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis (5th ed.), London: Longman, ISBN 0-582-44367-9
- ^ Li, W.; Nelson, D. P.; Jensen, M. S.; Hoerrner, R. S.; Cai, D.; Larsen, R. D.; Reider, P. J. J. Org. Chem. 2002, 67, 5394. "An Improved Protocol for the Preparation of 3-Pyridyl- and Some Arylboronic Acids". http://pubs.acs.org/doi/abs/10.1021/jo025792p. Retrieved 2010-12-16.
- ^ Starkov, P.; Sheppard, T. D. Org. Biomol. Chem. 2011, doi:10.1039/c0ob01069c. "Borate Esters as Convenient Reagents for Direct Amidation of Carboxylic Acids and Transamidation of Primary Amides". http://pubs.rsc.org/en/Content/ArticleLanding/2010/OB/C0OB01069C. Retrieved 2010-12-16.
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