AsH3, boiling point ?55°C. A gaseous hydride of arsenic, formed by reducing solutions of arsenic compounds.
Arsine, the simplest compound of arsenic, is AsH3.
At its standard state, arsine appears in the form of a colorless, denser than air gas that is soluble in water (200 ml/l) and in many organic solvents as well.
AsH3 is a pyramidal molecule with H-As-H angles of 91.8° and three equivalent As-H bonds, each of 1.519 Å length.
Discovery
AsH3 was discovered in 1775 by Carl Scheele from the reduction of arsenic(III) oxide with zinc and acid.
Synthesis
AsH3 is generally prepared by the reaction of As equivalents.
4 AsCl3 + 3 NaBH4 → 4 AsH3 + 3 NaCl + 3 BCl3Alternatively, sources of As3− react with protonic reagents to also produce this gas:
Zn3As2 + 6 HReactions
The chemical properties of AsH3 are reasonably well developed and could be anticipated based on an average of the behavior of PH3 and SbH3.
Thermal decomposition
Typical for a heavy hydride (e.g., SbH3, H2Te, SnH4), AsH3 is unstable with respect to its elements. In other words, AsH3 is stable kinetically but not thermodynamically.
2 AsH3 → 3 H2 + 2 AsThis decomposition reaction is the basis of the Marsh Test described below, which detects the metallic As.
Oxidation
Continuing the analogy to SbH3, AsH3 is readily oxidized by O2 or even air:
2 AsH3 + 3 O2 → As2O3 + 3 H2OArsine will react violently in presence of strong oxidizing agents, such as potassium permanganate, sodium hypochlorite or nitric acid.
Precursor to metallic derivatives
AsH3 is used as a precursor to metal complexes of "naked" (or "nearly naked") As.
Gutzeit test
A characteristic test for arsenic involves the reaction of AsH3 with Ag Although this test has become obsolete in analytical chemistry, the underlying reactions further illustrate the affinity of AsH3 for "soft" metal cations. In the Gutzeit test, AsH3 is generated by reduction of aqueous arsenic compounds, typically arsenites, with Zn in the presence of H2SO4. The evolved gaseous AsH3 is then exposed to AgNO3 either as powder or as a solution. With "solid" AgNO3, AsH3 reacts to produce yellow Ag4AsNO3, whereas AsH3 reacts with a "solution" of AgNO3 to give black Ag3As.
Acid-base reactions
The acidic properties of the As-H bond are often exploited. Thus, AsH3 can be deprotonated:
AsH3 + NaNH2 → NaAsH2 + NH3Upon reaction with the aluminium trialkyls, AsH3 gives the trimeric [R2AlAsH2]3, where R = (CH3)3C. This reaction is relevant to the mechanism by which GaAs forms from AsH3 (see below).
AsH3 is generally considered non-basic, but it can be protonated by "super acids" to give isolable salts of the tetrahedral species [AsH4]
Reaction with halogen compounds
Reactions of arsine with the halogens (fluorine and chlorine) or some of their compounds, such as nitrogen trichloride, are extremely dangerous and can result in explosions.
Catenation
In contrast to the behavior of PH3, AsH3 does not form stable chains, although H2As-AsH2 and even H2As-As(H)-AsH2 have been detected.
Microelectronics applications
AsH3 is used in the synthesis of semiconducting materials related to microelectronics and solid-state lasers. More importantly, AsH3 is used to make the semiconductor GaAs by CVD at 700-900 °C:
Ga(CH3)3 + AsH3 → GaAs + 3 CH4Chemical warfare applications
Since before WWII AsH3 was proposed as a possible chemical warfare weapon. On the other hand, several organic compounds based on arsine, such as lewisite (β-chlorovinyldichloroarsine), adamsite (diphenylaminearsine), Clark I (diphenylchlorarsine) and Clark II, (diphenylcyanoarsine) have been effectively developed for use in chemical warfare.
Forensic science and the Marsh test
AsH3 is also well known in forensic science because it is a chemical intermediate in the detection of arsenic poisoning. The old (but extremely sensitive) Marsh test generates AsH3 the presence of arsenic. This procedure, developed around 1836 by James Marsh, is based upon treating a As-containing sample of a victim's body (typically the stomach) with As-free zinc and dilute sulphuric acid: if the sample contains arsenic, gaseous arsine will form.
The Marsh test was widely used by the end of the 19th century and the start of the 20th;
Toxicology
For the toxicology of other arsenic compounds, see Arsenic, Arsenic trioxide and Arsenicosis.The toxicity of arsine is distinct from that of other arsenic compounds.
The first signs of exposure, which can take several hours to become apparent, are headaches, vertigo and nausea, followed by the symptoms of haemolytic anaemia (high levels of unconjugated bilirubin), haemoglobinuria and nephropathy.
Exposure to arsine concentrations of 250 ppm is rapidly fatal: concentrations of 25–30 ppm are fatal for 30 min exposure, and concentrations of 10 ppm can be fatal at longer exposure times. There is little information on the chronic toxicity of arsine, although it is reasonable to assume that, in common with other arsenic compounds, a long-term exposure could lead to arsenicosis.
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