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

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...

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 , where ''T'' is temperature in kelvins. Molecular weight is 55.914. The heat of vapourisation is 25.6 kJ/mol.Trimethylborane
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Preparation

Trimethylborane was first described in 1862 by Edward Frankland, who also mentioned its adduct with ammonia. 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. 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. Yet other methods are reacting tributyl borate with trimethylaluminium chloride, or potassium tetrafluoroborate with trimethylaluminium. Yet another method is to add

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bondin ...

in ether to methyl magnesium iodide.

Reactions

Trimethylborane spontaneously ignites in air if the concentration is high enough. It burns with a green flame producing soot. 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. Trimethylborane is a strong

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

. B(CH₃)₃ can form an adduct with ammonia: (NH₃):B(CH₃)₃. as well as other Lewis bases. The Lewis acid properties of B(CH₃)₃ have been analyzed by the ECW model yielding E_A= 2.90 and C_A= 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. Trimethylborane reacts with diborane to disproportionate to form

methyldiborane Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses ...

and

dimethyldiborane 1,2-Dimethyldiborane is an organoboron compound with the formula CH3)BH2sub>2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alky ...

: (CH₃)BH₂.BH₃ and (CH₃)₂BH.BH₃. It reacts as a gas with

trimethylphosphine Trimethylphosphine is a neutral organophosphorus compound with the formula P(CH3)3, commonly abbreviated as PMe3. This colorless liquid has a strongly unpleasant odor, characteristic of alkylphosphines. The compound is a common ligand in coordin ...

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. Methyl lithium reacting with the Trimethylborane produces a tetramethylborate salt: LiB(CH₃)₄. The tetramethylborate ion has a negative charge and is isoelectronic with

neopentane Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is a flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, ...

, tetramethylsilane, and the tetramethylammonium cation.

Use

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

References

{{Boron compounds Alkylboranes Gases