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Trimethylborane Ball and Stick.png
Trimethylborane Space Fill.png
Preferred IUPAC name
Other names
3D model (JSmol)
ECHA InfoCard 100.008.926
EC Number 209-816-3
Molar mass 55.92 g/mol
Appearance Colorless gas or liquid
Density 0.625 g/cm3 at −100 °C[3]
Melting point −161.5 °C (−258.7 °F; 111.6 K)
Boiling point −20.2 °C (−4.4 °F; 253.0 K)
Slight, highly reactive
Main hazards Spontaneously flammable in air; causes burns
GHS pictograms The flame pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) The gas-cylinder pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
H220, H250, H280, H314
P210, P222, P260, P264, P280, P301+330+331, P302+334, P303+361+353, P304+340, P305+351+338, P310, P321, P363, P370+378, P377, P381, P403, P405, P410+403, P422, P501
Flash point Not applicable, pyrophoric gas
−40 °C (−40 °F; 233 K)[4]
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl).


As a liquid it is colourless. The strongest line in the infrared spectrum is at 1330 cm−1 followed by lines at 3010 cm−1 and 1185 cm−1.

Its melting point is −161.5 °C, and its boiling point is −20.2 °C.

Vapour pressure is given by log P = 6.1385 + 1.75 log T − 1393.3/T − 0.007735 T, where T is temperature in kelvins.[5] Molecular weight is 55.914. The heat of vapourisation is 25.6 kJ/mol.[4]


Trimethylborane was first made by Stock and Zeidler. Their method of preparation combined boron trichloride gas with dimethylzinc.[5] Although the substance can be prepared using Grignard reagents the output is contaminated by unwanted products from the solvent. Trimethylborane can be made on a small scale with a 98% yield by reacting trimethylaluminium in hexane with boron tribromide in dibutyl ether as a solvent.[5] Yet other methods are reacting tributyl borate with trimethylaluminium chloride, or potassium tetrafluoroborate with trimethylaluminium.[6] Yet another method is to add boron trifluoride in ether to methyl magnesium iodide.[7]


Trimethylborane spontaneously ignites in air if the concentration is high enough. It burns with a green flame producing soot.[8] Slower oxidation with oxygen in a solvent or in the gas phase can produce dimethyltrioxadiboralane, which contains a ring of two boron and three oxygen atoms. However the major product is dimethylborylmethylperoxide, which rapidly decomposes to dimethoxymethylborane.[9]

Trimethylborane is a strong Lewis acid. It reacts with water and chlorine at room temperature. It also reacts with grease but not with Teflon or glass.[5] Trimethylborane can form an adduct with ammonia: (NH3):B(CH3)3.[10]

Trimethylborane reacts with diborane to disproportionate to form monomethyldiborane and dimethyldiborane: (CH3)BH2.BH3 and (CH3)2BH.BH3.

It reacts as a gas with trimethylphosphine to form a solid Lewis salt with a heat of formation of −41 kcal per mol. This adduct has a heat of sublimation of −24.6 kcal/mol. No reaction occurs with trimethylarsine or trimethylstibine.[7]

Methyl lithium reacting with the Trimethylborane produces a tetramethylborate salt: LiB(CH3)4.[11] The tetramethylborate ion has a negative charge and is isoelectronic with neopentane, tetramethylsilane, and the tetramethylammonium cation.


Trimethylborane has been used as a neutron counter. For this use it has to be very pure.[10] It is also used in chemical vapour deposition where boron and carbon need to be deposited together.


  1. ^ "CHAPTER P-6. Applications to Specific Classes of Compounds". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 974. doi:10.1039/9781849733069-00648. ISBN 978-0-85404-182-4.
  2. ^ Graner, G.; Hirota, E.; Iijima, T.; Kuchitsu, K.; Ramsay, D. A.; Vogt, J.; Vogt, N. (2001). "C3H9B Trimethylborane". Molecules containing Three or Four Carbon Atoms. Landolt-Börnstein - Group II Molecules and Radicals. 25C. p. 1. doi:10.1007/10688787_381. ISBN 978-3-540-66774-2.
  3. ^ MSDS from Voltaix
  4. ^ a b Trimethylborane
  5. ^ a b c d William S. Rees, Jr. and al (1990). Alvin P. Ginsberg, ed. Trimethylborane. Inorganic Syntheses. 27. p. 339.
  6. ^ Roland Koumlstera, Paul Bingera Wilhelm, V. Dahlhof; Binger; Dahlhoff (1973). "A Convenient Preparation of Trimethylborane and Triethylborane". Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry. 3 (4): 359–367. doi:10.1080/00945717308057281.
  7. ^ a b Donald Charles Mente (May 1975). "The Reactions of Trimethyl group Va Lewis Bases with simple Boron Lewis Acids" (PDF).
  8. ^ Herbert Ellern (1968). Military and Civilian Pyrotechnics. Chemical Publishing Company. p. 24. CiteSeerX
  9. ^ Barton, Lawrence; Crump, John M.; Wheatley, Jeffrey B. (June 1974). "Trioxadiborolanes from the oxidation of methyldiborane". Journal of Organometallic Chemistry. 72 (1): C1–C3. doi:10.1016/s0022-328x(00)82027-6.
  10. ^ a b Gaylon S. Ross; et al. (2 October 1961). "Preparation of High Purity Trimethylborane" (PDF). Journal of Research of the National Bureau of Standards Section A. 66 (1).
  11. ^ Georg Wittig in 1958