Ethyl borate

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

The equihbrium shown in equation 3 normally ties far to the left. Usually the water formed is removed by azeotropic distillation with excess alcohol or a suitable azeotroping solvent such as benzene, toluene, or various petroleum distillate fractions. The procedure used depends on the specific ester desired. Preparation of methyl borate and ethyl borate is compHcated by the formation of low boiling azeotropes (Table 1) which are the lowest boiling constituents in these systems. Consequently, the ester—alcohol azeotrope must be prepared and then separated in another step. Some of the methods that have been used to separate methyl borate from the azeotrope are extraction with sulfuric acid and distillation of the enriched phase (18), treatment with calcium chloride or lithium chloride (19,20), washing with a hydrocarbon and distillation (21), fractional distillation at 709 kPa (7 atmospheres) (22), and addition of a third component that will form a low boiling methanol azeotrope (23). 

From Boric Oxide and Alcohol. To avoid removing water, boric oxide, B2O3, can be used in place of boric acid. The water of reaction (eq. 4) is consumed by the oxide (eq. 5). Because boric acid reacts with borates at high temperatures, it is necessary to filter the reaction mixture prior to distillation of the product. Only 50% of the boron can be converted to ester by this method. In cases where this loss can be tolerated, the boric oxide method is convenient. This is particularly tme for methyl borate and ethyl borate preparation because formation of the undesirable azeotrope is avoided. 

Treatment of 2-lithiofurans with ethyl borate and hydrolysis of the resultant boronic ester yields 2(3//)-furanones (69AK(29)229). 2,5-Dimethyl-3(2i/)-furanone (358) has been prepared by a Curtius reaction on the ester (359) and acid hydrolysis of the intermediate urethane (360) (Scheme 97). This type of reaction has been used to synthesize muscarine and its stereoisomers (61QR153). 

The further new nitroethyl compounds based on boron esters are tris-(2-nitro-ethyl) borate and tris-(2,2,2-trinitroethyl) borate. Especially the trinitroethyl derivative is a suitable candidate for high energy density oxidizers and for smoke-free, green coloring agents in pyrotechnic compositions. Tris-(2-nitroethyl) borate and tris-(2,2,2-trinitroethyl) borate can be obtained from boron oxide with 2-nitroetha-nol and 2,2,2-trinitroethanol, respectively ... 

Borate in the presence of copper and barium salts When carrying out the ethyl borate test for borates (Section IV.23, reaction 2), it must be remembered that copper and barium salts may also impart a green colour to the alcohol... 

Ethyl Borate. See Vol 2 of Encycl, p B247-L, under Borates... 

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