Density 1.12 g/cm³ | Appearance Colourless liquid | |
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Related compounds Thermodynamic
data Phase behaviour
solid–liquid–gas | ||
Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination chemistry. It has distinct "garlic"-like smell. Trimethylarsine had been discovered as early as 1854.
Contents
Structure and preparation
As predicted by VSEPR theory, AsMe3 is a pyramidal molecule. The As-C distances average 1.519 Å, and the C-As-C angles are 91.83° This bond angle is a strong indication of a low, if any, hybridisation of the atomic orbitals, leaving the lone pair in the s-orbital buried in the inner regions of the arsenic atom, rather than pointing outward like the lone pair of the ammonia molecule.
Trimethylarsine can be prepared by treatment of arsenic oxide with trimethylaluminium:
As2O3 + 1.5 [AlMe3]2 → 2 AsMe3 + 3/n (MeAl-O)nProperties and reactions
Trimethylarsine is pyrophoric due to the exothermic nature of the following reaction, which initiates combustion:
AsMe3 + 1/2 O2 → OAsMe3 (TMAO)History
Poisoning events due to a gas produced by certain microbes was assumed to be associated with the arsenic in paint. In 1893 the Italian physician Bartolomeo Gosio published his results on "Gosio gas" that was subsequently shown to contain trimethylarsine. Under wet conditions, the mold Scopulariopsis brevicaulis produces significant amounts of methyl arsines via methylation of arsenic-containing inorganic pigments, especially Paris green and Scheele's Green, which were once used in indoor wallpapers. Newer studies show that trimethylarsine has a low toxicity and could therefore not account for the death and the severe health problems observed in the 19th century.
Safety
Trimethylarsine is potentially hazardous, although its toxicity is often overstated.
Importance
Trimethylarsine is the volatile byproduct of microbial action on inorganic forms of arsenic which are naturally occurring in rocks and soils at the parts-per-million level. Trimethylarsine has been reported only at trace levels (parts per billion) in landfill gas from Germany, Canada, and the U.S.A., and is the major arsenic-containing compound in the gas.