Trimethyl borate
[CAS# 121-43-7]

Identification

• Classification: Chemical reagent >> Organic reagent >> Bicyclic compound
• Name: Trimethyl borate
• Synonyms: Boric acid trimethyl ester; Boron methoxide; Methyl borate
• Molecular Formula: C₃H₉BO₃
• Molecular Weight: 103.91
• CAS Registry Number: 121-43-7
• EC Number: 204-468-9
• SMILES: B(OC)(OC)OC

Properties

• Density: 0.9±0.1 g/cm³ Calc.^*, 0.931 g/mL (Expl.)
• Melting point: -34 °C (Expl.)
• Boiling point: 67.5 °C 760 mmHg (Calc.)^*, 68 - 69 °C (Expl.)
• Flash point: -8.3 °C (Calc.)^*, -8 °C (Expl.)
• Index of refraction: 1.335 (Calc.)^*, 1.361 (Expl.)
• Water solubility: reacts

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS02;GHS06;GHS07;GHS08 Danger
• Risk Statements: H225-H226-H301-H311-H312-H319-H331-H360FD-H360-H370
• Safety Statements: P203-P210-P233-P240-P241-P242-P243-P260-P261-P262-P264-P264+P265-P270-P271-P280-P301+P316-P302+P352-P303+P361+P353-P304+P340-P305+P351+P338-P308+P316-P316-P317-P318-P321-P330-P337+P317-P361+P364-P362+P364-P370+P378-P403+P233-P403+P235-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute toxicity	Acute Tox.	4	H312
Flammable liquids	Flam. Liq.	3	H226
Flammable liquids	Flam. Liq.	2	H225
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	1	H370
Acute toxicity	Acute Tox.	3	H301
Acute toxicity	Acute Tox.	3	H311
Acute toxicity	Acute Tox.	3	H331
Reproductive toxicity	Repr.	1B	H360FD
Reproductive toxicity	Repr.	1B	H360
Reproductive toxicity	Repr.	2	H361
Reproductive toxicity	Repr.	1A	H360FD
Acute toxicity	Acute Tox.	4	H302
Specific target organ toxicity - repeated exposure	STOT RE	1	H372

• Transport Information: UN 2416

Discovery and Applications

Trimethyl borate is an organoboron compound with the chemical formula B(OCH₃)₃. It appears as a colorless, flammable liquid with a mild odor and is known for its volatility and reactivity. As an ester of boric acid and methanol, trimethyl borate plays an important role both in laboratory synthesis and in industrial applications due to its unique chemical properties, particularly its ability to act as a boron source.

The compound is typically synthesized by the reaction of boric acid (B(OH)₃) with methanol under dehydrating conditions. The removal of water during the reaction shifts the equilibrium toward ester formation. This process is straightforward and economical, making trimethyl borate a readily accessible reagent in both research and commercial settings.

One of the most significant applications of trimethyl borate is in organic synthesis, where it is used as a reagent to introduce boron-containing groups into molecules. It serves as a precursor in the preparation of trialkylboranes and other boron-based intermediates, which are valuable in Suzuki-Miyaura cross-coupling reactions. These cross-coupling reactions are essential tools in modern synthetic organic chemistry, especially in the construction of carbon–carbon bonds for pharmaceuticals, agrochemicals, and advanced materials.

In addition to its use in synthesis, trimethyl borate also finds utility in the production of borosilicate glass and in flame retardants. When used as an additive, it contributes boron to the glass matrix, which improves thermal and chemical stability. The flame retardant properties arise from the compound's ability to form a protective boron oxide layer when exposed to high temperatures, which inhibits combustion.

Trimethyl borate is also employed in analytical chemistry, particularly in the formation of boron-containing complexes for spectroscopic analysis. Its ability to coordinate with various ligands allows it to be used in the preparation of calibration standards for the quantification of boron via techniques such as inductively coupled plasma mass spectrometry (ICP-MS).

From a safety perspective, trimethyl borate is flammable and can form explosive mixtures with air. It hydrolyzes in the presence of moisture, releasing methanol and boric acid. Methanol is toxic and can pose serious health hazards if inhaled, ingested, or absorbed through the skin. Therefore, proper handling and storage in a dry, well-ventilated environment are essential to ensure safe use.

Environmental considerations regarding trimethyl borate primarily involve its hydrolysis products. Boric acid, while relatively low in toxicity to humans, can be harmful to aquatic organisms at high concentrations. As a result, disposal must be managed in accordance with environmental regulations to prevent contamination.

In summary, trimethyl borate is a versatile boron ester widely used in organic synthesis, materials science, and analytical chemistry. Its ease of preparation, reactivity, and function as a boron donor make it a valuable chemical across several fields of research and industry. Despite its benefits, appropriate safety measures must be observed due to its flammable and toxic nature.

References

1931. Quantitative Untersuchungen über die Borsäure-Alkohol-Flammenreaktion. Zeitschrift für analytische Chemie, 85(1).
DOI: 10.1007/bf01359871

2023. Bicyclobutanes as unusual building blocks for complexity generation in organic synthesis. Communications Chemistry, 6(1).
DOI: 10.1038/s42004-022-00811-3

2024. Boron-deficient molecules tailored inorganic-rich cathode/electrolyte interfaces for stable Li LiFe0.2Mn0.8PO4 battery. Ionics, 30(10).
DOI: 10.1007/s11581-024-05792-y