Methyldiborane
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IUPAC name
Methyldiborane
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| Other names
monomethyldiborane
methylated diborane boraethane |
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3D model (JSmol)
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| Properties | |
| CH 3BH 3BH 2 |
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| Molar mass | 41.70 g mol−1 |
| Appearance | Colorless gas |
| Density | 0.546 at -126° |
| Boiling point | −43 °C (−45 °F; 230 K) |
| Related compounds | |
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dimethyldiborane trimethyldiborane tetramethyldiborane trimethylborane ethyldiborane |
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Related compounds
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Diborane |
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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 | |
Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest alkyldiborane, 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 three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3). Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.
The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.
Methylboranes are formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. The reaction is complex. At 0 °C when diborane is in excess, monomethyldiborane is initially produced, coming to a steady but low level, and 1,1-dimethyldiborane level increases over a long time, until all trimethylborane is consumed. Monomethyldiborane ends up at equilibrium with a mixture of diborane and dimethyldiborane. At 0° the equilibrium constant for 2B2H5Me ←→ B2H6 + (BH2Me)2 is around 0.07, so monomethyldiborane will typically be the majority of the mixture, but there will still be a significant amount of diborane and dimethyldiborane present. Monomethyldiborane yield is best with a ratio of 4 of diborane to 1 of trimethylborane. The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.
When methyllithium reacts with diborane, monomethyldiborane is produced in about a 20% yield.
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