Tetramethyldiborane
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| Names | |
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IUPAC name
Tetramethyldiborane(6)
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| Other names
Dimethylborane dimer
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| Identifiers | |
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3D model (JSmol)
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| Properties | |
| (CH 3) 2BH 2B(CH 3) 2 |
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| Molar mass | 83.777 |
| Appearance | Colorless liquid |
| Odor | Pungent; |
| Melting point | −72.5 °C (−98.5 °F; 200.7 K) |
| Boiling point | 68.6 °C (155.5 °F; 341.8 K) |
| Hazards | |
| NFPA 704 | |
| Related compounds | |
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trimethylborane dimethyldiborane diethylborane |
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Related compounds
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Borane tetramethyl aluminium hydride tetramethyl gallium hydride |
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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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| Infobox references | |
Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or TMDB (CH3)2BH)2. Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.
Dimethylborane is formed when lithium dimethylborohydride Li(CH3)2BH2 reacts with an acid. The lithium dimethylborohydride can be made from a dimethylborinic ester and lithium monoethoxy aluminium hydride.
Methylboranes are also formed by the reaction of diborane and trimethylborane. This reaction produces four different substitutions of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. The latter is maximised when trimethylborane is six times the concentration of diborane.
Other methods to form methyldiboranes include reacting hydrogen with trimethylborane between 80 and 200 °C under pressure, or reacting a metal borohydride with trimethylborane in the presence of hydrogen chloride, aluminium chloride or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium then no methane is produced. dimethylchloroborane and methyldichloroborane are also produced as gaseous products.
Atomic hydrogen converts trimethylborane on a graphene monolayer surface to dimethylborane which dimerises to tetramethyldiborane.
Tetramethyldiborane has two boron atoms linked by a two hydrogen atom bridge, and each boron is linked to two methyl groups. A Tetramethyldiborane molecule belongs to the D2hpoint group. Its infrared spectrum shows a strong absorption band at 1602 cm−1 due to bridging hydrogen, a weak band at 1968 cm−1 and lines due to methyl between 900 and 1400 cm−1. In the molecule the boron to hydrogen distance is 1.36 Å, the boron to boron distance is 1.84 Å; the boron to carbon distance is 1.590 Å; the angle of boron-boron to carbon is 120.0°; the boron-carbon-hydrogen angle is 112.0°. The NMR J coupling between two boron-11 nuclei in tetramethyldiborane is 55 Hz.
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