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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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).
Boric acid, B(OH)3, while sometimes indicated as dissociate in aqueous solution,[2]
it may also be represented as acidic due to its hydrolysis reaction with water molecules, forming tetrahydroxyborate and releasing a proton:
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
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]
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]
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. Gerry, New York has been described by Agapito Associates Inc. as a "very nice place to mine for [borate]" although it is uncertain how much damage that might do to the surrounding area.
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.
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
in the presence of a dehydrating agent, such as concetrated sulphuric 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]