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

, borate esters are

organoboron compound Organoborane or organoboron compounds are chemical compounds of boron and carbon that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds. Organoboron ...

s 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 borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves ...

with

alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

. There are two main classes of borate

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

s: orthoborates, and metaborates, . Metaborates contain 6-membered

boroxine Boroxine () is a 6-membered heterocyclic compound composed of alternating oxygen and singly-hydrogenated boron atoms. Boroxine Derivative (chemistry), derivatives (boronic anhydrides) such as trimethylboroxine and triphenylboroxine also make up a ...

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 ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

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 is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

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 orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any s ...

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 is ...

, , is used as a precursor to

boronic ester A boronic acid is an organic compound related to boric acid () in which one of the three hydroxyl groups () is replaced by an alkyl or aryl group (represented by R in the general formula ). As a compound containing a carbon–boron bond, memb ...

s 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 acid A boronic acid is an organic compound related to boric acid () in which one of the three hydroxyl groups () is replaced by an alkyl or aryl group (represented by R in the general formula ). As a compound containing a carbon–boron bond, membe ...

s, which are used in

Suzuki coupling The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, a ...

References