Trimethylborane
Chemical compound
From Wikipedia, the free encyclopedia
Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl).
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| Names | |||
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| Preferred IUPAC name
Trimethylborane[1] | |||
| Other names
Trimethylborine Trimethylboron | |||
| Identifiers | |||
3D model (JSmol) |
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| ChemSpider | |||
| ECHA InfoCard | 100.008.926 | ||
| EC Number |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |||
| C3H9B | |||
| 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 | |||
| Structure | |||
| Δ | |||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards |
Spontaneously flammable in air; causes burns | ||
| GHS labelling: | |||
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| Danger | |||
| H220, H250, H280, H314 | |||
| P210, P222, P260, P264, P301+P330+P331, P302+P334, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P370+P378, P377, P381, P403, P405, P410+P403, P422, P501 | |||
| NFPA 704 (fire diamond) | |||
| Flash point | Not applicable, pyrophoric gas | ||
| −40 °C (−40 °F; 233 K)[4] | |||
| Safety data sheet (SDS) | MSDS from Voltaix | ||
| Related compounds | |||
Related compounds |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Properties
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]
Preparation
Trimethylborane was first described in 1862 by Edward Frankland,[6] who also mentioned its adduct with ammonia.[7] Due to its dangerous nature the compound was no longer studied until 1921, when Alfred Stock and Friedrich Zeidler took advantage of the reaction between boron trichloride gas and dimethylzinc.[8] 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,[9] or adding boron trifluoride in ether to methyl magnesium iodide.[10]
Reactions
Trimethylborane spontaneously ignites in air if the concentration is high enough. It burns with a green flame producing soot.[11] 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.[12]
Trimethylborane is a strong Lewis acid. B(CH3)3 can form an adduct with ammonia: (NH3):B(CH3)3.[13] as well as other Lewis bases. The Lewis acid properties of B(CH3)3 have been analyzed by the ECW model yielding EA= 2.90 and CA= 3.60. When trimethylborane forms an adduct with trimethylamine, steric repulsion between the methyl groups on the B and N results. The ECW model can provide a measure of this steric effect.
Trimethylborane reacts with water and chlorine at room temperature. It also reacts with grease but not with teflon or glass.[5]
Trimethylborane reacts with diborane to disproportionate to form methyldiborane 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.[10]
Methyl lithium reacting with the Trimethylborane produces a tetramethylborate salt: LiB(CH3)4.[14] The tetramethylborate ion has a negative charge and is isoelectronic with neopentane, tetramethylsilane, and the tetramethylammonium cation.
Use
Trimethylborane has been used as a neutron counter.[15] For this use it has to be very pure.[13] It is also used in chemical vapour deposition where boron and carbon need to be deposited together.