chemistry

Borinic acid

Borinic acid is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Borinic acid rather than just read about it. In short: Borinic acid, also known as boronous acid, is an oxyacid of boron with formula H2BOH. Borinate is the associated anion of borinic acid with formula H2BO−; however, being a Lewis acid, the form in basic solution is H2B(OH)−2.

Key takeaways

Borinic acid belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Borinic acid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Borinic acid from memory before moving on to harder problems.

Reference excerpt

Borinic acid, also known as boronous acid, is an oxyacid of boron with formula H2BOH. Borinate is the associated anion of borinic acid with formula H2BO−; however, being a Lewis acid, the form in basic solution is H2B(OH)−2. Borinic acids (containing RR'BOH) and borinic esters (RR'BOR") also refer to the families of compounds derived from the parent by replacing hydrogen atoms with more general organyl groups. Their primary application is in academic research.

Naming The IUPAC name borinic acid is a unique name for the acid. The anhydrides are named diboroxanes, H2BOBH2, as the base compound and H being able to be substituted, e.g. tetraethyldiboroxane, as the anhydride for diethylborinic acid. Organic naming standard in the Bluebook allows skeletal replacement naming where the name is shorter, 3-borapentan-3-ol versus diethyl borinic acid. The grouping -BH-O-BH2 is called diboroxanyl. Substituting sulfur for oxygen gives borinothioic acid (H2BSH). (Dimethylboranyl)oxy is used for the group (CH3)2B-O− and methyl(hydroxy)boranyl for the grouping CH3B(OH)-.

Parent compound

Borinic acid can be formed as the first step in the hydrolysis of diborane:

BH3 + H2O → H2BOH + H2 It is also produced by the comproportionation of boric acid and diborane, though this reaction is unfavorable, with an equilibrium constant at 300 K of between 10−5 and 10−4:

⁠1/3⁠B2H6 + ⁠1/3⁠B(OH)3 ⇌ H2BOH Borinic acid itself is unstable and only lasts for a few seconds during the hydrolysis reaction. However, by using microwave spectroscopy various properties can be determined. The B-O distance is 1.352 Å, O-H distance 0.96 Å, B-H length is probably 1.2 Å. The angle between bonds at the oxygen atom BOH = 112° and the angles at boron are cis-HBO 121°, and trans-HBO = 117°. The dipole moment is 1.506 Debye. Borinic acid can form esters such as methoxyborane. This too is unstable only lasting about ten seconds. It can be formed by heating diborane and methanol gas together.

Formation There are several ways to produce substituted borinic acids. Firstly borinic acids can be made from oxidizing trialkylborane starting materials (R3B) with exposure to moist air, or treatment with iodine, which makes a dialkyliodoborane (R2BI). Hydrolysis then results in the boronic acid (R2BOH). Dialkyl boron chloride (R2BCl) with tertiary amine react with ketones to form an enol borinate. Such alcoholysis products require purification, as often boronic esters will also be produced and mixed in with the borinic esters. The method of Letsinger is to dissolve the mixture in ether and precipitate the borinic ester as an ammonia complex. Treatment with ethanolamine ends up making an aminoethylborinate. Secondly, trialkylborates [(RO)3B] or trialkoxyboroxine [(ROBO)3] can be reduced to borinic acid by use of a Grignard reagent. Grignard reagents can also reduce a boronic ester [RB(OR')2] to a borinic ester.

Bu3B + N2CHCOR → BuCH=C(R)OBBu2 Bu3B + CH2=CHCOCH3 → BuCH2CH=C(CH3)OBBu2 RCOC2H5 + R2BOTf → RC(OBR2)=CHCH3 (Tf = Trifluoromethanesulfonate) Likewise, a trialkoxyborane can react with lithium containing organic molecules to eliminate lithium and one or two alkoxy groups to make boronic and borinic esters. Finally, trialkylboranes can reduce starting materials directly.

HB(C6F5)2 + phosphino alcohol → tBu2P+HCH2C(CH3)2OB−H(C6F5)2 → H2 + tBu2PCH2C(CH3)2OB(C6F5)2 and same for tBu2PCH2C(CF3)2OB(C6F5)2 di-Tris(tert-butoxy)siloxy borinic acid HOB[OSi(O(t)Bu)3]2 can be made from tributoxyborate and tributoxysiloxane. It can form a very complex crystal with Cp2Zr(Me)[OB[OSi(O(t)Bu)3]2]2.

Compounds A wide variety of borinic acids, anhydrides ((R2B)2O), and esters (R2BOR′) are known. Diborinic acids have two RBOH groups linked together by an organic connection such as diphenyl or phenyl. Besides those below, other compounds include methoxy(dimethyl)borane, methoxy(methyl)boron, methoxy(methylidene)borane (with a C=B double bond).

2-APB

2-Aminoethoxydiphenyl borate (2-APB) inhibits transient receptor potential channels. This kind of inhibition, particularly inhibition of TRPM7, is being studied to find treatments for prostate cancer. 2-APB can work as a catalyst to add an alkyl group from an alkyl halide to a polyol or carbohydrate that contains a cis-vicinal diol to a precise position. It does this by first combining with the two hydroxy groups to make a ring containing OCCOB−. It can also calatlyse acid chloride or chloroformate reaction a specific region of the diol.

Diphenylborinic acid Diphenylborinic acid was discovered in 1894 by Michaelis who produced it by hydrolyzing the chloride. Letsinger determined its properties in 1955. Diphenylborinic acid can catalyse the condensation of pyruvic acids with aldehydes to yield substituted isotetronic acid. Diphenylborinic acid has an extra high affinity for catechols compared with carbohydrates. Diphenylborinic acid is an inhibitor of several enzymes such as α-chymotrypsin, subtilisin BPN', and trypsin.

Reactions Esters will tend to be stable in acidic conditions, but in alkaline conditions the boron atom can gain a negative charge and attach two hydroxyl groups, or two ester bonds. RR'B−(OH)2 or RR'B−(OR")2. The anionic borinate ion can very easily form esters with diols such as ethylene glycol or sugars. Borinate radicals (RR'BO·) can be formed from peroxyborinate decomposition. Chiral boronates enol esters are applied in academia for enantioselective aldol reactions. (Z)-Enolates give syn products, whereas (E)-enolates give anti products. 1,1,1,3,3,3-Hexafluoroisopropylbis(pentafluorophenyl)borinate can greatly increase solubility of LiF by complexing the F− anion. Borinic esters are being researched as bacterial growth inhibitors due to their ability to disable some bacterial enzymes such as menaquinone methyltransferase and CcrM.

References

Extra reading Dimitrijević, Elena; Taylor, Mark S. (3 May 2013). "Organoboron Acids and Their Derivatives as Catalysts for Organic Synthesis". ACS Catalysis. 3 (5): 945–962. doi:10.1021/cs4000848. Review of use borinic acids as a catalyst Roth, HJ; Miller, B (September 1964). "Synthesis of Phenyl- and Thienyl-Substituted Boronic and Borinic Acids". Archiv der Pharmazie. 297 (9): 513–523. doi:10.1002/ardp.19642970902. PMID 14341914. S2CID 95181696.

Illustrations

Worked examples

Example 1 — a first encounter with Borinic acid

Start with the simplest possible case. Write down what Borinic acid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Borinic acid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Borinic acid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Borinic acid

In research: Borinic acid appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Borinic acid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Borinic acid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Organoboron compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Borinic acid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Borinic acid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Borinic acid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Borinic acid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Borinic acid in simple terms?

Why does Borinic acid matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Borinic acid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Borinic acid.