Trimethyl borate

Structural formula
General
Surname	Trimethyl borate
other names	ortho- boric acid trimethyl ester Trimethyl borate
Molecular formula	C 3 H 9 BO 3
Brief description	colorless liquid with a pleasant odor
External identifiers / databases
CAS number 121-43-7 EC number 204-468-9 ECHA InfoCard 100.004.063 PubChem 8470 Wikidata Q423710
properties
Molar mass	103.91 g mol −1
Physical state	liquid
density	0.923 g cm −3 (20 ° C)
Melting point	−29.3 ° C
boiling point	68.7 ° C
Vapor pressure	182 hPa (25 ° C)
solubility	Decomposes with water
Refractive index	1.3568 (20 ° C)
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary danger H and P phrases H: 225-312-319-360FD-370 P: 201-210-260-302 + 352 + 312-308 + 311-337 + 313
Toxicological data	6140 mg kg −1 ( LD 50 ,  rat ,  oral ) 1850 mg kg −1 ( LD 50 ,  rabbit ,  transdermal )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Boric acid trimethyl ester ( trimethyl borate ) is a chemical compound from the group of boric acid esters . It is the methyl ester of boric acid .

presentation

Boric acid trimethyl ester is produced by esterifying boric acid with methanol in the presence of acidic catalysts in an equilibrium reaction :

${\displaystyle \mathrm {B(OH)_{3}+3\ CH_{3}OH\ \rightleftharpoons \ B(OCH_{3})_{3}+3\ H_{2}O} }$

To shift the equilibrium to the right, however, it is necessary to remove one of the products from the reaction mixture ( Le Chatelier's principle ). The most common method for esterification is to bind the water using dehydrating agents such as sulfuric acid , which also acts as an acidic catalyst.

The conversion of boron trioxide leads to the formation of a mixture of different boric acid esters.

A third possibility is to react boron halides with methanol in the presence of a base, as in the following example with boron tribromide :

${\displaystyle \mathrm {BBr_{3}+3\ CH_{3}OH\ \rightleftharpoons \ B(OCH_{3})_{3}+3\ HBr} }$

The resulting hydrogen halide (in the example hydrogen bromide ) is bound by the base.

properties

Green flame of trimethyl borate

Boric acid trimethyl ester is a colorless, volatile liquid. In water, it is quickly hydrolyzed into boric acid and methanol . The hydrolysis is exothermic at −19.3 kJ mol ⁻¹ . It is the simplest boric acid ester . As a compound of boron , it burns with a bright, bright green flame . Because there are no C –C bonds, no soot is produced during combustion, which glows yellow in the flame and thus outshines the color of the flame (a large ethanol flame already has a yellowish flame, alkanes burn more or less brightly). Combustion produces non-volatile, white boron trioxide , the acid anhydride of boric acid, which makes the flame smoke:

${\displaystyle \mathrm {2\ B(OCH_{3})_{3}+9\ O_{2}\ \longrightarrow \ 6\ CO_{2}+9\ H_{2}O+B_{2}O_{3}.} }$

According to Antoine, the vapor pressure function results from log ₁₀ (P) = A− (B / (T + C)) (P in bar, T in K) with A = 4.74371, B = 1476.192 and C = −29 , 0 in the temperature range from 304 K to 340.5 K. The compound forms flammable vapor-air mixtures. The flash point is −18 ° C.

use

Pure boric acid trimethyl ester has little industrial use. It is used to a lesser extent for the surface treatment of steel (formation of a corrosion-inhibiting Fe ₂ B layer) in special plasma processes. Boric acid trimethyl ester is used as the starting substance for boric acid starting materials for the Suzuki coupling . Furthermore, boric acid trimethyl ester is used as a flux for soldering in so-called "gas flux" processes.

The continuously green flame coloration of the ester can analytically serve as an indication of the presence of boric acid / borates or methanol (the boric acid esters of higher alcohols only produce green flame fringes), but it is not clear. However, it is very sensitive and is caused by traces of boric acid (for example from the borosilicate glass often used in laboratories ).

In nuclear technology , the trimethyl borate is used as an absorbent for neutrons .

Web links

• Video demonstrates the flame coloration by methyl borate
• Entry on Trimethyl borate in the Spectral Database for Organic Compounds (SDBS) of the National Institute of Advanced Industrial Science and Technology (AIST)

Individual evidence

1. ↑ ^{a b c d e f g h} Entry on trimethyl borate in the GESTIS substance database of the IFA , accessed on January 10, 2017(JavaScript required) .
2. ↑ ^{a b c} G. Sorbe: Safety characteristics. 100th supplement 6/2004, ecomed-Verlag, ISBN 3-609-73060-9 .
3. ↑ Data sheet boric acid trimethyl ester (PDF) from Merck , accessed on July 24, 2013.
4. ↑ Entry on trimethyl borate in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on August 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
5. ↑ Green flames (formation of boric acid methyl ester) (PDF; 9 kB).
6. ↑ Charnley, T .; Skinner, HA; Smith, NB: Thermochemistry of Organo-boron Compounds. Part I. Esters of Boric Acid , in: J. Chem. Soc. , 1952 , pp. 2288-2291 ( doi : 10.1039 / JR9520002288 ).
7. ↑ PM Christopher, A. Shilman: Vapor Pressures of Trialkyl Borates , in: J. Chem. Eng. Data , 1967 , 12 , pp. 333-335 ( doi : 10.1021 / je60034a012 ).
8. ^ Herder Lexicon Chemistry. Gütersloh 1985, book no. 03838 0, p. 43.