Trimethylborane (TMB) is a toxic,

pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...

gas with the formula B(CH₃)₃ (which can also be written as Me₃B, with Me representing methyl).

Properties

As a liquid it is colourless. The strongest line in the

infrared spectrum Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

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

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ph ...

s. 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 Sir Edward Frankland, (18 January 18259 August 1899) was an English chemist. He was one of the originators of organometallic chemistry and introduced the concept of combining power or valence. An expert in water quality and analysis, he was ...

, 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 Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive toward water. Production and structure Boron reacts with halogens to give the corresponding trihalides ...

gas and

dimethylzinc Dimethylzinc, also known as Zinc methyl, DMZ, or DMZn is a colorless volatile liquid Zn(CH3)2, formed by the action of methyl iodide on zinc at elevated temperature or on zinc sodium alloy. :2Zn + 2CH3I → Zn(CH3)2 + ZnI2 The sodium assists the ...

. Although the substance can be prepared using

Grignard reagents A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...

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 Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( CH3)6 (abbreviated as Al2Me6 or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industria ...

hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...

with

boron tribromide Boron tribromide, BBr3, is a colorless, fuming liquid compound containing boron and bromine. Commercial samples usually are amber to red/brown, due to weak bromine contamination. It is decomposed by water and alcohols. Chemical properties Boron ...

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 Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

: (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 Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. (It is also sold in pressurized ...

, 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 Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chem ...

or glass. Trimethylborane reacts with

diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...

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 coordi ...

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 Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated As Me3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building blo ...

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 Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

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 Tetramethylsilane (abbreviated as TMS) is the organosilicon compound with the formula Si(CH3)4. It is the simplest tetraorganosilane. Like all silanes, the TMS framework is tetrahedral. TMS is a building block in organometallic chemistry but also ...

, and the

tetramethylammonium Tetramethylammonium (TMA) or (Me4N+) is the simplest quaternary ammonium cation, consisting of four methyl groups attached to a central nitrogen atom, and is isoelectronic with neopentane. It is positively charged and can only be isolated in assoc ...

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 Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...

where boron and carbon need to be deposited together.

References

{{Boron compounds Alkylboranes Gases