organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, borate esters are organoboron compounds which are conveniently prepared by the stoichiometric

condensation reaction In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...

boric acid Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white ...

with

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

. There are two main classes of borate

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s: orthoborates, and metaborates, . Metaborates contain 6-membered boroxine rings. :B(OH)3 + 3 ROH -> B(OR)3 + 3 H2O :3 B(OH)3 + 3 ROH -> B3O3(OR)3 + 6 H2O A dehydrating agent, such as concentrated

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

is typically added. Borate esters are volatile and can be purified by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel. Like all boron compounds, alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis. Structural formula of trimethyl borate

Borate esters form spontaneously when treated with

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol may also be called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting gro ...

s such as sugars and the reaction with mannitol forms the basis of a titrimetric analytical method for boric acid. Metaborate esters show considerable

Lewis acidity A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty Non-bonding orbital, orbital which is capable of accepting an electron pair from a Lewis Base (chemistry), base to form a Lewis ...

and can initiate epoxide polymerization reactions.M.A. Beckett, G.C. Strickland, J.R. Holland, and K.S. Varma, "A convenient NMR method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity", ''Polymer,'' 1996, 37, 4629–4631. doi: 10.1016/0032-3861(96)00323-0 The Lewis acidity of orthoborate esters, as determined by the Gutmann-Beckett method, is relatively low.

Trimethyl borate Trimethyl borate is the organoboron compound with the formula B(OCH3)3. It is a colourless liquid that burns with a green flame. It is an intermediate in the preparation of sodium borohydride and is a popular reagent in organic chemistry. It i ...

, , is used as a precursor to boronic esters for Suzuki couplings: Unsymmetrical borate esters are prepared from alkylation of trimethyl borate:{{OrgSynth, author=R. L. Kidwell, author2=M. Murphy, author3=S. D. Darling, name-list-style=amp , year=1969 , title=Phenols: 6-Methoxy-2-naphthol , volume=49, pages=90, collvol=10, collvolpages=80, prep=CV5P0918 :ArMgBr + B(OCH3)3 -> MgBrOCH3 + ArB(OCH3)2 :ArB(OCH3)2 + 2H2O -> ArB(OH)2 + 2HOCH3 These esters hydrolyze to boronic acids, which are used in Suzuki couplings.

References